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LT3972 33V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75A Quiescent Current FEATURES
DESCRIPTION
The LT(R)3972 is an adjustable frequency (200kHz to 2.4MHz) monolithic buck switching regulator that accepts input voltages up to 33V (62V Maximum). A high efficiency 95m switch is included on the die along with a boost Schottky diode and the necessary oscillator, control, and logic circuitry. Current mode topology is used for fast transient response and good loop stability. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping output ripple below 15mV in a typical application. In addition, the LT3972 can further enhance low output current efficiency by drawing bias current from the output when VOUT is above 3V. Shutdown reduces input supply current to less than 1A while a resistor and capacitor on the RUN/SS pin provide a controlled output voltage ramp (soft-start). A power good flag signals when VOUT reaches 91% of the programmed output voltage. The LT3972 is available in 10-Pin MSOP and 3mm x 3mm DFN packages with exposed pads for low thermal resistance.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.

Wide Input Range: Operation from 3.6V to 33V Over-Voltage Lockout Protects Circuits Through 62V Transients 3.5A Maximum Output Current Low Ripple (<15mVP-P) Burst Mode(R) Operation: IQ = 75A at 12VIN to 3.3VOUT Adjustable Switching Frequency: 200kHz to 2.4MHz Low Shutdown Current: IQ < 1A Integrated Boost Diode Synchronizable Between 250kHz to 2MHz Power Good Flag Saturating Switch Design: 95m On-Resistance Output Voltage: 0.79V to 30V Thermal Protection Soft-Start Capability Small 10-Pin Thermally Enhanced MSOP and (3mm x 3mm) DFN Packages
APPLICATIONS

Automotive Battery Regulation Distributed Supply Regulation Industrial Supplies Wall Transformer Regulation
TYPICAL APPLICATION
5V Step-Down Converter
VIN 6.3V TO 33V TRANSIENT TO 62V OFF ON 15k 10F 680pF 63.4k VC RT PG SYNC 536k GND FB 50 100k 47F 0 0.5 2 1.5 1 2.5 OUTPUT CURRENT (A) 3 3.5 LT3972 SW VOUT 5V 3.5A VIN RUN/SS BD BOOST 0.47F 4.7H EFFICIENCY (%) 100 VIN = 12V 90 VIN = 30V 80 VIN = 24V
Efficiency
70
60
VOUT = 5V L = 4.7H f = 600kHz
3680 TA01a 3680 TA01
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LT3972 ABSOLUTE MAXIMUM RATINGS (Note 1)
VIN , RUN/SS Voltage (Note 5)...................................62V BOOST Pin Voltage ...................................................56V BOOST Pin Above SW Pin.........................................30V FB, RT, VC Voltage .......................................................5V PG, BD, SYNC Voltage ..............................................30V Operating Junction Temperature Range (Note 2) LT3972E .............................................- 40C to 125C LT3972I ..............................................- 40C to 125C Storage Temperature Range...................-65C to 150C Lead Temperature (Soldering, 10 sec) (MSE Only) ....................................................... 300C
PIN CONFIGURATION
TOP VIEW BD BOOST SW VIN RUN/SS 1 2 3 4 5 11 10 RT 9 VC 8 FB 7 PG 6 SYNC TOP VIEW BD BOOST SW VIN RUN/SS 1 2 3 4 5 10 9 8 7 6 RT VC FB PG SYNC
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DD PACKAGE 10-LEAD (3mm 3mm) PLASTIC DFN JA = 45C/W, JC = 10C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
MSE PACKAGE 10-LEAD PLASTIC MSOP JA = 45C/W, JC = 10C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LT3972EDD#PBF LT3972IDD#PBF LT3972EMSE#PBF LT3972IMSE#PBF TAPE AND REEL LT3972EDD#TRPBF LT3972IDD#TRPBF LT3972EMSE#TRPBF LT3972IMSE#TRPBF PART MARKING* LDXR LDXR LTDXS LTDXS PACKAGE DESCRIPTION 10-Lead (3mm x 3mm) Plastic DFN 10-Lead (3mm x 3mm) Plastic DFN 10-Lead Plastic MSOP 10-Lead Plastic MSOP TEMPERATURE RANGE -40C to 125C -40C to 125C -40C to 125C -40C to 125C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
PARAMETER Minimum Input Voltage VIN Overvoltage Lockout Quiescent Current from VIN
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise noted. (Note 2)
CONDITIONS

MIN 33
TYP 3 35 0.01 30 120 0.01
MAX 3.6 37 0.5 65 160 0.5
UNITS V V A A A A
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VRUN/SS = 0.2V VBD = 3V, Not Switching VBD = 0, Not Switching
Quiescent Current from BD
VRUN/SS = 0.2V
2
LT3972 ELECTRICAL CHARACTERISTICS
PARAMETER
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise noted. (Note 2)
CONDITIONS VBD = 3V, Not Switching VBD = 0, Not Switching Minimum Bias Voltage (BD Pin) Feedback Voltage

MIN
TYP 90 1 2.7
MAX 130 5 3 800 805 40 0.01
UNITS A A V mV mV nA %/V Mho A A A/V V
780 775
790 790 10 0.002 500 2000 60 60 5.3 2
FB Pin Bias Current (Note 3) FB Voltage Line Regulation Error Amp gm Error Amp Gain VC Source Current VC Sink Current VC Pin to Switch Current Gain VC Clamp Voltage Switching Frequency
VFB = 0.8V, VC = 1.2V 4V < VIN < 33V
RT = 8.66k RT = 29.4k RT = 187k
2.2 1.0 200 4.6
2.45 1.1 230 60 5.4 335 0.02
2.7 1.25 260 150 6.2 2 2 50 8 2.5
MHz MHz kHz nS A mV A V mA A V V mV mV
Minimum Switch Off-Time Switch Current Limit Switch VCESAT Boost Schottky Reverse Leakage Minimum Boost Voltage (Note 4) BOOST Pin Current RUN/SS Pin Current RUN/SS Input Voltage High RUN/SS Input Voltage Low PG Threshold Offset from Feedback Voltage PG Hysteresis PG Leakage PG Sink Current SYNC Low Threshold SYNC High Threshold SYNC Pin Bias Current VSYNC = 0V VPG = 5V VPG = 0.4V VFB Rising ISW = 1A VRUN/SS = 2.5V Duty Cycle = 5% ISW = 3.5A VBD = 0V
1.5 35 5 0.2 65 10 0.1
1
A A V
200 0.5
800 0.7 0.1
V A
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3972E is guaranteed to meet performance specifications from 0C to 125C. Specifications over the -40C to 125C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LT3972I specifications are guaranteed over the -40C to 125C temperature range.
Note 3: Bias current flows out of the FB pin. Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. Note 5: Absolute Maximum at VIN and RUN/SS Pins is 62V for nonrepetitive 1 minute transients, and 40V for continuous operation.
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LT3972 TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency
100 VIN = 12V 90 EFFICIENCY (%) EFFICIENCY (%) VIN = 30V 80 VIN = 24V 90 100
TA = 25C unless otherwise noted. Efficiency
100 3.0
Efficiency
VIN = 12V EFFICIENCY (%)
90
2.5
TOTAL POWER LOSS (W)
80
VIN = 30V VIN = 24V
80
2.0
70
70
70 VIN = 12V VOUT = 5V L = 4.7H f = 600kHz 0 0.5 2 1.5 1 2.5 OUTPUT CURRENT (A) 3 3.5
3680 G03
1.5
60
VOUT = 5V L = 4.7H f = 600kHz 0 0.5 2 1.5 1 2.5 OUTPUT CURRENT (A) 3 3.5
3680 G01
60
VOUT = 3.3V L = 3.3H f = 600kHz 0 0.5 2 1.5 1 2.5 OUTPUT CURRENT (A) 3 3.5
3680 G02
60
1.0
50
50
50
0.5
No Load Supply Current
130 110 SUPPLY CURRENT (A) SUPPLY CURRENT (A) 90 70 50 30 10 0 5 15 10 20 25 INPUT VOLTAGE (V) 30 35
3680 G04
No Load Supply Current
VOUT = 3.3V 400 CATCH DIODE: DIODES, INC. PDS360 350 300 250 200 150 100 50 0 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C)
3680 G05
Maximum Load Current
5.5 TYPICAL
VIN = 12V VOUT = 3.3V LOAD CURRENT (A) INCREASED SUPPLY CURRENT DUE TO CATCH DIODE LEAKAGE AT HIGH TEMPERATURE
5.0 4.5
MINIMUM 4.0 3.5 3.0 2.5 5 10 20 25 15 INPUT VOLTAGE (V) 30
3680 G06
VOUT = 3.3V TA = 25C L = 4.7H f = 600kHz
Maximum Load Current
5.5 TYPICAL 6.0 5.5 5.0 4.5 4.0 3.5 3.0
Switch Current Limit
6.5 6.0 SWITCH CURRENT LIMIT(A)
Switch Current Limit
5.0 LOAD CURRENT (A)
SWITCH CURRENT LIMIT (A)
5.5 5.0 4.5 4.0 3.5 3.0 2.5
DUTY CYCLE = 10 %
4.5 MINIMUM 4.0 VOUT = 5V TA = 25C L = 4.7H f = 600kHz 5 10 25 20 15 INPUT VOLTAGE (V) 30
3680 G07
DUTY CYCLE = 90 %
3.5
3.0
0
20
60 40 DUTY CYCLE (%)
80
100
3680 G08
2.0 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3680 G09
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LT3972 TYPICAL PERFORMANCE CHARACTERISTICS
Switch Voltage Drop
700 600 BOOST PIN CURRENT (mA) VOLTAGE DROP (mV) 500 400 300 200 100 0 0 1 2 4 3 SWITCH CURRENT (A) 5
3680 G10
TA = 25C unless otherwise noted. Feedback Voltage
840
Boost Pin Current
120 105 90 75 60 45 30 15 0 0 1 2 3 SWITCH CURRENT (A) 4 5
3680 G11
FEEDBACK VOLTAGE (mV)
820
800
780
760 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3680 G12
Switching Frequency
1.20 1.15 1.10 FREQUENCY (MHz) 1.05 1.00 0.95 0.90 0.85 0.80 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C)
3680 G13
Frequency Foldback
1200 1000 800 600 400 200 0 0 100 200 300 400 500 600 700 800 900 FB PIN VOLTAGE (mV)
3680 G14
Minimum Switch On-Time
140 MINIMUM SWITCH ON TIME (ns) 120 100 80 60 40 20 0 -50 -25
SWITCHING FREQUENCY (kHz)
0
25 50 75 100 125 150 TEMPERATURE (C)
3680 G15
Soft-Start
7 6 SWITCH CURRENT LIMIT (A) 5 4 3 2 1 0 0 0.5 2 2.5 1.5 RUN/SS PIN VOLTAGE (V) 1 3 3.5
3680 G16
RUN/SS Pin Current
12 10 BOOST DIODE VF (V) 0 20 30 15 25 10 RUN/SS PIN VOLTAGE (V) 35
3680 G17
Boost Diode
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0
RUN/SS PIN CURRENT (A)
8 6 4 2 0 5
0
0.5 1.0 1.5 BOOST DIODE CURRENT (A)
2.0
3680 G18
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LT3972 TYPICAL PERFORMANCE CHARACTERISTICS
Error Amp Output Current
50 40 30 VC PIN CURRENT (A) INPUT VOLTAGE (V) 20 10 0 -10 -20 -30 -40 -50 -200 2.0 -100 100 0 FB PIN ERROR VOLTAGE (mV) 200
3680 G19
TA = 25C unless otherwise noted. Minimum Input Voltage
6.5
Minimum Input Voltage
5.0 4.5 4.0 3.5 3.0 2.5 VOUT = 3.3V TA = 25C L = 4.7H f = 800kHz 1 10 100 1000 LOAD CURRENT (mA) 10000
3680 G20
6.0 INPUT VOLTAGE (V)
5.5
5.0 VOUT = 5V TA = 25 C L = 4.7H f = 800kHz 1 10 100 1000 LOAD CURRENT (mA) 10000
3680 G21
4.5
4.0
VC Voltages
2.50 95
Power Good Threshold
Switching Waveforms; Burst Mode
THRESHOLD VOLTAGE (%)
2.00 CURRENT LIMIT CLAMP VC VOLTAGE (V) 1.50
90
VSW 5V/DIV
85
1.00
IL 0.2A/DIV
SWITCHING THRESHOLD
80
0.50
VOUT 10mV/DIV
0 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3680 G22
75 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
3680 G23
VIN = 12V VOUT = 3.3V ILOAD = 10mA
5s/DIV
3680 G24
Switching Waveforms; Transition from Burst Mode to Full Frequency
VSW 5V/DIV
Switching Waveforms; Full Frequency Continuous Operation
VSW 5V/DIV
IL 0.2A/DIV
IL 0.5A/DIV
VOUT 10mV/DIV
VOUT 10mV/DIV
VIN = 12V VOUT = 3.3V ILOAD = 110mA
1s/DIV
3680 G25
VIN = 12V VOUT = 3.3V ILOAD = 1A
1s/DIV
3680 G26
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LT3972 PIN FUNCTIONS
BD (Pin 1): This pin connects to the anode of the boost Schottky diode. BD also supplies current to the internal regulator. BOOST (Pin 2): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. SW (Pin 3): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor. VIN (Pin 4): The VIN pin supplies current to the LT3972's internal regulator and to the internal power switch. This pin must be locally bypassed. RUN/SS (Pin 5): The RUN/SS pin is used to put the LT3972 in shutdown mode. Tie to ground to shut down the LT3972. Tie to 2.5V or more for normal operation. If the shutdown feature is not used, tie this pin to the VIN pin. RUN/SS also provides a soft-start function; see the Applications Information section. SYNC (Pin 6): This is the external clock synchronization input. Ground this pin for low ripple Burst Mode operation at low output loads. Tie to a clock source for synchronization. Clock edges should have rise and fall times faster than 1s. Tie Pin to GND if not used. See Synchronization section in Applications Information. PG (Pin 7): The PG pin is the open collector output of an internal comparator. PG remains low until the FB pin is within 9% of the final regulation voltage. PG output is valid when VIN is above 3.6V and RUN/SS is high. FB (Pin 8): The LT3972 regulates the FB pin to 0.790V. Connect the feedback resistor divider tap to this pin. VC (Pin 9): The VC pin is the output of the internal error amplifier. The voltage on this pin controls the peak switch current. Tie an RC network from this pin to ground to compensate the control loop. RT (Pin 10): Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the switching frequency. Exposed Pad (Pin 11): Ground. The Exposed Pad must be soldered to PCB.
BLOCK DIAGRAM
VIN C1 INTERNAL 0.79V REF RUN/SS SLOPE COMP SWITCH LATCH R OSCILLATOR 200kHzTO2.4MHz Q S SW 6 SYNC DISABLE BurstMode DETECT 3 D1 C2 L1 VOUT BOOST 2 C3 4 VIN
5
10 RT
RT
SOFT-START 7 PG ERROR AMP
+ -
GND 11
0.73V
+ -
FB 8 R2 R1
+ -
VC CLAMP
BD
1
VC
9 CC RC CF
3680 BD
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LT3972 OPERATION
The LT3972 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, enables an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC . An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC pin. If the error amplifier's output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the VC pin provides current limit. The VC pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented by generating a voltage ramp at the RUN/SS pin using an external resistor and capacitor. An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the VIN pin, but if the BD pin is connected to an external voltage higher than 3V bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. The RUN/SS pin is used to place the LT3972 in shutdown, disconnecting the output and reducing the input current to less than 0.5A. The switch driver operates from either the input or from the BOOST pin. An external capacitor and diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for efficient operation. To further optimize efficiency, the LT3972 automatically switches to Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to 75A in a typical application. The oscillator reduces the LT3972's operating frequency when the voltage at the FB pin is low. This frequency foldback helps to control the output current during startup and overload. The LT3972 contains a power good comparator which trips when the FB pin is at 91% of its regulated value. The PG output is an open-collector transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT3972 is enabled and VIN is above 3.6V. The LT3972 has an overvoltage protection feature which disables switching action when the VIN goes above 35V typical (33V minimum). When switching is disabled, the LT3972 can safely sustain input voltages up to 62V.
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LT3972 APPLICATIONS INFORMATION
FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to: V R1= R2 OUT - 1 0.79 V Reference designators refer to the Block Diagram. Setting the Switching Frequency The LT3972 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.4MHz by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching frequency is in Figure 1.
SWITCHING FREQUENCY (MHz) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 RT VALUE (k) 215 140 100 78.7 63.4 53.6 45.3 39.2 34 26.7 22.1 18.2 15 12.7 10.7 9.09
where VIN is the typical input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V) and VSW is the internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to safely accommodate high VIN/VOUT ratio. Also, as shown in the next section, lower frequency allows a lower dropout voltage. The reason input voltage range depends on the switching frequency is because the LT3972 switch has finite minimum on and off times. The switch can turn on for a minimum of ~150ns and turn off for a minimum of ~150ns. Typical minimum on time at 25C is 80ns. This means that the minimum and maximum duty cycles are: DCMIN = fSW tON(MIN) DCMAX = 1- fSW tOFF(MIN) where fSW is the switching frequency, the tON(MIN) is the minimum switch on time (~150ns), and the tOFF(MIN) is the minimum switch off time (~150ns). These equations show that duty cycle range increases when switching frequency is decreased. A good choice of switching frequency should allow adequate input voltage range (see next section) and keep the inductor and capacitor values small. Input Voltage Range The maximum input voltage for LT3972 applications depends on switching frequency, Absolute Maximum Ratings of the VIN and BOOST pins, and the operating mode. The LT3972 can operate from input voltages of up to 33V, and withstand voltages up to 62V. Note that while VIN is above 35V typical (33V minimum and 37V maximum) the part will keep the switch off and the output will not be in regulation. To safely allow inputs of up to 62V, be sure to choose a Schottky diode, inductor size, and switching frequency to allow safe operation at 37V according to the following discussions. While the output is in start-up, short-circuit, or other overload conditions, the switching frequency should be chosen according to the following equation:
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Figure 1. Switching Frequency vs. RT Value
Operating Frequency Tradeoffs Selection of the operating frequency is a tradeoff between efficiency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efficiency, lower maximum input voltage, and higher dropout voltage. The highest acceptable switching frequency (fSW(MAX)) for a given application can be calculated as follows: fSW(MAX ) = VD + VOUT tON(MIN) ( VD + VIN - VSW )
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LT3972 APPLICATIONS INFORMATION
VIN(MAX ) = VOUT + VD -V +V fSW tON(MIN) D SW the ripple current is: IL = 0.4(IOUT(MAX)) where IOUT(MAX) is the maximum output load current. To guarantee sufficient output current, peak inductor current must be lower than the LT3972's switch current limit (ILIM). The peak inductor current is: IL(PEAK) = IOUT(MAX) + IL/2 where IL(PEAK) is the peak inductor current, IOUT(MAX) is the maximum output load current, and IL is the inductor ripple current. The LT3972's switch current limit (ILIM) is 5.5A at low duty cycles and decreases linearly to 4.5A at DC = 0.8. The maximum output current is a function of the inductor ripple current: IOUT(MAX) = ILIM - IL/2 Be sure to pick an inductor ripple current that provides sufficient maximum output current (IOUT(MAX)). The largest inductor ripple current occurs at the highest VIN . To guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation: V +V V +V L = OUT D 1- OUT D VIN(MAX ) fSW IL where VD is the voltage drop of the catch diode (~0.4V), VIN(MAX) is the maximum input voltage, VOUT is the output voltage, fSW is the switching frequency (set by RT), and L is in the inductor value. The inductor's RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions (start-up or short circuit) and high input voltage (>30V), the saturation current should be above 5A. To keep the efficiency high, the series resistance (DCR) should be less than 0.1, and the core material should be intended for high frequency applications. Table 1 lists several vendors and suitable types.
where VIN(MAX) is the maximum operating input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT), and tON(MIN) is the minimum switch on time (~100ns). Note that a higher switching frequency will depress the maximum operating input voltage. Conversely, a lower switching frequency will be necessary to achieve safe operation at high input voltages. If the output is in regulation and no short-circuit, startup, or overload events are expected, then input voltage transients of up to 33V are acceptable regardless of the switching frequency. In this mode, the LT3972 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation. In this mode the output voltage ripple and inductor current ripple will be higher than in normal operation. The minimum input voltage is determined by either the LT3972's minimum operating voltage of ~3.6V or by its maximum duty cycle (see equation in previous section). The minimum input voltage due to duty cycle is: VIN(MIN) = VOUT + VD -V +V 1- fSW tOFF(MIN) D SW
where VIN(MIN) is the minimum input voltage, and tOFF(MIN) is the minimum switch off time (150ns). Note that higher switching frequency will increase the minimum input voltage. If a lower dropout voltage is desired, a lower switching frequency should be used. Inductor Selection For a given input and output voltage, the inductor value and switching frequency will determine the ripple current. The ripple current IL increases with higher VIN or VOUT and decreases with higher inductance and faster switching frequency. A reasonable starting point for selecting
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LT3972 APPLICATIONS INFORMATION
Table 1. Inductor Vendors
VENDOR Murata TDK Toko Sumida URL www.murata.com www.componenttdk.com www.toko.com www.sumida.com PART SERIES LQH55D SLF10145 D75C D75F CDRH74 CR75 CDRH8D43 MPLC073 MPBI0755 TYPE Open Shielded Shielded Open Shielded Open Shielded Shielded Shielded
NEC
www.nec.com
Of course, such a simple design guide will not always result in the optimum inductor for your application. A larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. If your load is lower than 3.5A, then you can decrease the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. There are several graphs in the Typical Performance Characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. Low inductance may result in discontinuous mode operation, which is okay but further reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Linear Technology Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. See AN19. Input Capacitor Bypass the input of the LT3972 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 10F to 22F ceramic capacitor is adequate to bypass the LT3972 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used. If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a lower performance electrolytic capacitor.
Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3972 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 10F capacitor is capable of this task, but only if it is placed close to the LT3972 and the catch diode (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3972. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3972 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3972's voltage rating. This situation is easily avoided (see the Hot Plugging Safety section). For space sensitive applications, a 4.7F ceramic capacitor can be used for local bypassing of the LT3972 input. However, the lower input capacitance will result in increased input current ripple and input voltage ripple, and may couple noise into other circuitry. Also, the increased voltage ripple will raise the minimum operating voltage of the LT3972 to ~3.7V. Output Capacitor and Output Ripple The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT3972 to produce the DC output. In this role it determines the output ripple, and low impedance at the switching frequency is important. The second function is to store energy in order to satisfy transient loads and stabilize the LT3972's control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: COUT = 100 VOUT fSW
where fSW is in MHz, and COUT is the recommended output capacitance in F. Use X5R or X7R types. This choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value capacitor if the compensation network is also adjusted to maintain the loop bandwidth. A lower
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LT3972 APPLICATIONS INFORMATION
Table 2. Capacitor Vendors
VENDOR Panasonic PHONE (714) 373-7366 URL www.panasonic.com PART SERIES Ceramic, Polymer, Tantalum Ceramic, Tantalum Ceramic, Polymer, Tantalum Ceramic Ceramic, Tantalum Ceramic TPS Series COMMANDS EEF Series
Kemet Sanyo
(864) 963-6300 (408) 749-9714
www.kemet.com www.sanyovideo.com
T494, T495 POSCAP
Murata AVX Taiyo Yuden
(408) 436-1300
www.murata.com www.avxcorp.com
(864) 963-6300
www.taiyo-yuden.com
value of output capacitor can be used to save space and cost but transient performance will suffer. See the Frequency Compensation section to choose an appropriate compensation network. When choosing a capacitor, look carefully through the data sheet to find out what the actual capacitance is under operating conditions (applied voltage and temperature). A physically larger capacitor, or one with a higher voltage rating, may be required. High performance tantalum or electrolytic capacitors can be used for the output capacitor. Low ESR is important, so choose one that is intended for use in switching regulators. The ESR should be specified by the supplier, and should be 0.05 or less. Such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low ESR. Table 2 lists several capacitor vendors. Catch Diode The catch diode conducts current only during switch off time. Average forward current in normal operation can be calculated from: ID(AVG) = IOUT (VIN - VOUT)/VIN where IOUT is the output load current. The only reason to consider a diode with a larger current rating than necessary for nominal operation is for the worst-case condition of shorted output. The diode current will then increase to the
typical peak switch current. Peak reverse voltage is equal to the regulator input voltage. Use a Schottky diode with a reverse voltage rating greater than the input voltage. The overvoltage protection feature in the LT3972 will keep the switch off when VIN > 35V which allows the use of 40V rated Schottky even when VIN ranges up to 62V. Table 3 lists several Schottky diodes and their manufacturers.
Table 3. Diode Vendors
PART NUMBER On Semiconductor MBRA340 Diodes Inc. PDS340 B340A B340LA VR (V) 40 40 40 40 IAVE (A) 3 3 3 3 VF AT 3A (mV) 500 500 500 450
Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT3972 due to their piezoelectric nature. When in Burst Mode operation, the LT3972's switching frequency depends on the load current, and at very light loads the LT3972 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT3972 operates at a lower current limit during Burst Mode operation, the noise is nearly silent to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output.
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12
LT3972 APPLICATIONS INFORMATION
Frequency Compensation The LT3972 uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT3972 does not require the ESR of the output capacitor for stability, so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. Frequency compensation is provided by the components tied to the VC pin, as shown in Figure 2. Generally a capacitor (CC) and a resistor (RC) in series to ground are used. In addition, there may be lower value capacitor in parallel. This capacitor (CF) is not part of the loop compensation but is used to filter noise at the switching frequency, and is required only if a phase-lead capacitor is used or if the output capacitor has high ESR. Loop compensation determines the stability and transient performance. Designing the compensation network is a bit complicated and the best values depend on the application and in particular the type of output capacitor. A practical approach is to start with one of the circuits in this data sheet that is similar to your application and tune the compensation network to optimize the performance. Stability should then be checked across all operating conditions, including load current, input voltage and temperature. The LT1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the stability using a transient load. Figure 2 shows an equivalent circuit for the LT3972 control loop. The error amplifier is a transconductance amplifier with finite output impedance. The power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance amplifier generating an output current proportional to the voltage at the VC pin. Note that the output capacitor integrates this current, and that the capacitor on the VC pin (CC) integrates the error amplifier output current, resulting in two poles in the loop. In most cases a zero is required and comes from either the output capacitor ESR or from a resistor RC in series with CC. This simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. A phase lead capacitor (CPL) across the feedback divider may improve the transient response. Figure 3 shows the transient response when the load current is stepped from 1A to 3A and back to 1A.
LT3972 CURRENT MODE POWER STAGE gm = 5.3mho SW ERROR AMPLIFIER FB ESR gm = 500mho 3M C1 POLYMER OR TANTALUM R2 CERAMIC R1 CPL OUTPUT
VC CF RC CC
Figure 2. Model for Loop Response
VOUT 100mV/DIV
IL 1A/DIV
VIN = 12V VOUT = 3.3V
Figure 3. Transient Load Response of the LT3972 Front Page Application as the Load Current is Stepped from 1A to 3A. VOUT = 5V
- +
0.8V
+
C1
GND
3680 F02
10s/DIV
3680 F03
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13
LT3972 APPLICATIONS INFORMATION
Low-Ripple Burst Mode and Pulse-Skip Mode The LT3972 is capable of operating in either Low-Ripple Burst Mode or Pulse-Skip Mode which are selected using the SYNC pin. See the Synchronization section for details. To enhance efficiency at light loads, the LT3972 can be operated in Low-Ripple Burst Mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode operation, the LT3972 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor. Because the LT3972 delivers power to the output with single, low current pulses, the output ripple is kept below 15mV for a typical application. In addition, VIN and BD quiescent currents are reduced to typically 30A and 80A respectively during the sleep time. As the load current decreases towards a no load condition, the percentage of time that the LT3972 operates in sleep mode increases and the average input current is greatly reduced resulting in high efficiency even at very low loads. See Figure 4. At higher output loads (above 140mA for the front page application) the LT3972 will be running at the frequency programmed by the RT resistor, and will be operating in standard PWM mode. The transition between PWM and Low-Ripple Burst Mode is seamless, and will not disturb the output voltage. If low quiescent current is not required the LT3972 can operate in Pulse-Skip mode. The benefit of this mode is that the LT3972 will enter full frequency standard PWM operation at a lower output load current than when in Burst Mode. The front page application circuit will switch at full frequency at output loads higher than about 60mA. BOOST and BIAS Pin Considerations Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22F capacitor will work well. Figure 2 shows three ways to arrange the boost circuit. The BOOST pin must be more than 2.3V above the SW pin for best efficiency. For outputs of 3V and above, the standard circuit (Figure 5a) is best. For outputs between 2.8V and 3V, use a 1F boost capacitor. A 2.5V output presents a special case because it is marginally adequate to support the boosted drive stage while using the internal boost diode. For reliable BOOST pin operation with 2.5V outputs use a good external Schottky diode (such as the ON Semi MBR0540), and a 1F boost capacitor (see Figure 5b). For lower output voltages the boost diode can be tied to the input (Figure 5c), or to another supply greater than 2.8V. Tying BD to VIN reduces the maximum input voltage to 28V. The circuit in Figure 5a is more efficient because the BOOST pin current and BD pin quiescent current comes from a lower voltage source. You must also be sure that the maximum voltage ratings of the BOOST and BD pins are not exceeded. The minimum operating voltage of an LT3972 application is limited by the minimum input voltage (3.6V) and by the maximum duty cycle as outlined in a previous section. For proper startup, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT3972 is turned on with its RUN/SS pin when the output is already in regulation, then the boost capacitor may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. Figure 6 shows a plot of minimum load to start and to run as a function of input voltage. In many cases the discharged output capacitor will present a load to the switcher, which will allow it to
3972f
VSW 5V/DIV
IL 0.2A/DIV
VOUT 10mV/DIV
VIN = 12V VOUT = 3.3V ILOAD = 10mA
5s/DIV
3680 F04
Figure 4. Burst Mode Operation
14
LT3972 APPLICATIONS INFORMATION
VOUT BD BOOST C3 SW INPUT VOLTAGE (V) VIN VIN LT3972 5.0 4.5 4.0 TO RUN 3.5 3.0 VOUT = 3.3V TA = 25C L = 8.2H f = 700kHz 1 10 100 1000 LOAD CURRENT (mA) 10000 6.0 5.5 TO START (WORST CASE)
4.7F
GND
(5a) For VOUT > 2.8V
VOUT BD BOOST VIN VIN LT3972 SW INPUT VOLTAGE (V) C3 D2
2.5 2.0
8.0 7.0 TO START (WORST CASE)
4.7F
GND
6.0 5.0 4.0 3.0 2.0 1 VOUT = 5V TA = 25C L = 8.2H f = 700kHz 10 100 1000 LOAD CURRENT (mA) 10000
3680 F06
TO RUN
(5b) For 2.5V < VOUT < 2.8V
VOUT BD BOOST VIN VIN LT3972 SW C3
4.7F
GND
Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
3680 FO5
Soft-Start
(5c) For VOUT < 2.5V; VIN(MAX) = 30V Figure 5. Three Circuits For Generating The Boost Voltage
start. The plots show the worst-case situation where VIN is ramping very slowly. For lower start-up voltage, the boost diode can be tied to VIN ; however, this restricts the input range to one-half of the absolute maximum rating of the BOOST pin. At light loads, the inductor current becomes discontinuous and the effective duty cycle can be very high. This reduces the minimum input voltage to approximately 300mV above VOUT. At higher load currents, the inductor current is continuous and the duty cycle is limited by the maximum duty cycle of the LT3972, requiring a higher input voltage to maintain regulation.
The RUN/SS pin can be used to soft-start the LT3972, reducing the maximum input current during start-up. The RUN/SS pin is driven through an external RC filter to create a voltage ramp at this pin. Figure 7 shows the startup and shut-down waveforms with the soft-start circuit. By choosing a large RC time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. Choose the value of the resistor so that it can supply 20A when the RUN/SS pin reaches 2.5V. Synchronization To select Low-Ripple Burst Mode operation, tie the SYNC pin below 0.5V (this can be ground or a logic output).
3972f
15
LT3972 APPLICATIONS INFORMATION
RUN 15k RUN/SS 0.22F GND VRUN/SS 2V/DIV IL 1A/DIV
VOUT 2V/DIV
2ms/DIV
3680 F07
Figure 7. To Soft-Start the LT3972, Add a Resisitor and Capacitor to the RUN/SS Pin
Synchronizing the LT3972 oscillator to an external frequency can be done by connecting a square wave (with 20% to 80% duty cycle) to the SYNC pin. The square wave amplitude should have valleys that are below 0.3V and peaks that are above 0.8V (up to 6V). The LT3972 will not enter Burst Mode at low output loads while synchronized to an external clock, but instead will skip pulses to maintain regulation. The LT3972 may be synchronized over a 250kHz to 2MHz range. The RT resistor should be chosen to set the LT3972 switching frequency 20% below the lowest synchronization input. For example, if the synchronization signal will be 250kHz and higher, the RT should be chosen for 200kHz. To assure reliable and safe operation the LT3972 will only synchronize when the output voltage is near regulation as indicated by the PG flag. It is therefore necessary to choose a large enough inductor value to supply the required output current at the frequency set by the RT resistor. See Inductor Selection section. It is also important to note that slope compensation is set by the RT value: When the sync frequency is much higher than the one set by RT, the slope compensation will be significantly reduced which may require a larger inductor value to prevent subharmonic oscillation. Shorted and Reversed Input Protection If the inductor is chosen so that it won't saturate excessively, an LT3972 buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT3972 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT3972's output. If the VIN pin is allowed to float and the RUN/SS pin is held high (either by a logic signal or because it is tied to VIN), then the LT3972's internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can tolerate a few mA in this state. If you ground the RUN/SS pin, the SW pin current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, then parasitic diodes inside the LT3972 can pull large currents from the output through the SW pin and the VIN pin. Figure 8 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input.
D4 MBRS340 VIN VIN RUN/SS VC GND FB BACKUP BOOST LT3972 SW VOUT
3680 F08
Figure 8. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output. It Also Protects the Circuit from a Reversed Input. The LT3972 Runs Only When the Input is Present
PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 9 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3972's VIN and SW pins, the catch diode (D1) and the input capacitor (C1). The loop formed by these components should be as small as possible. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane below these components. The SW and BOOST nodes should be as small as possible. Finally, keep the FB and VC nodes small so that the ground
3972f
16
LT3972 APPLICATIONS INFORMATION
L1 VOUT C2
RRT
CC
traces will shield them from the SW and BOOST nodes. The Exposed Pad on the bottom of the package must be soldered to ground so that the pad acts as a heat sink. To keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT3972 to additional ground planes within the circuit board and on the bottom side.
R2
RC
Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3972 circuits. However, these capacitors can cause problems if the LT3972 is plugged into a live supply (see Linear Technology Application Note 88 for a complete discussion). The low loss ceramic capacitor, combined with stray inductance in series with the power source, forms an under damped tank circuit, and the
R1
D1
C1 GND RPG
3680 F09
VIAS TO LOCAL GROUND PLANE VIAS TO VOUT VIAS TO SYNC
VIAS TO RUN/SS VIAS TO PG
VIAS TO VIN OUTLINE OF LOCAL GROUND PLANE
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
CLOSING SWITCH SIMULATES HOT PLUG IIN VIN LT3972
DANGER VIN 20V/DIV RINGING VIN MAY EXCEED ABSOLUTE MAXIMUM RATING
+
4.7F
LOW IMPEDANCE ENERGIZED 24V SUPPLY
STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR
IIN 10A/DIV 20s/DIV
(10a)
0.7W LT3972 VIN 20V/DIV
+
0.1F 4.7F IIN 10A/DIV
(10b)
20s/DIV
LT3972
VIN 20V/DIV
+
22F 35V AI.EI.
+
4.7F IIN 10A/DIV
(10c)
20s/DIV
3680 F10
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation when the LT3972 is Connected to a Live Supply
3972f
17
LT3972 APPLICATIONS INFORMATION
voltage at the VIN pin of the LT3972 can ring to twice the nominal input voltage, possibly exceeding the LT3972's rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3972 into an energized supply, the input network should be designed to prevent this overshoot. Figure 10 shows the waveforms that result when an LT3972 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 4.7F ceramic capacitor at the input. The input voltage rings as high as 50V and the input current peaks at 26A. A good solution is shown in Figure 10b. A 0.7 resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1F capacitor improves high frequency filtering. For high input voltages its impact on efficiency is minor, reducing efficiency by 1.5 percent for a 5V output at full load operating from 24V. High Temperature Considerations The PCB must provide heat sinking to keep the LT3972 cool. The Exposed Pad on the bottom of the package must be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the LT3972. Place additional vias can reduce thermal resistance further. With these steps, the thermal resistance from die (or junction) to ambient can be reduced to JA = 35C/W or less. With 100 LFPM airflow, this resistance can fall by another 25%. Further increases in airflow will lead to lower thermal resistance. Because of the large output current capability of the LT3972, it is possible to dissipate enough heat to raise the junction temperature beyond the absolute maximum of 125C. When operating at high ambient temperatures, the maximum load current should be derated as the ambient temperature approaches 125C. Power dissipation within the LT3972 can be estimated by calculating the total power loss from an efficiency measurement and subtracting the catch diode loss and inductor loss. The die temperature is calculated by multiplying the LT3972 power dissipation by the thermal resistance from junction to ambient. Other Linear Technology Publications Application Notes 19, 35 and 44 contain more detailed descriptions and design information for buck regulators and other switching regulators. The LT1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. Design Note 100 shows how to generate a bipolar output supply using a buck regulator.
TYPICAL APPLICATIONS
5V Step-Down Converter
VIN 6.3V TO 33V TRANSIENT TO 62V ON OFF VOUT 5V 3.5A VIN RUN/SS BD BOOST 0.47F 10F 15k 63.4k 680pF f = 600kHz D: ON SEMI MBRA340 L: NEC MPLC0730L4R7
3972f
L 4.7H
VC RT PG SYNC
LT3972
SW D
536k GND FB 100k
3680 TA02
47F
18
LT3972 TYPICAL APPLICATIONS
3.3V Step-Down Converter
VIN 4.4V TO 33V TRANSIENT TO 62V ON OFF VOUT 3.3V 3.5A VIN RUN/SS BD BOOST 0.47F 4.7F 19k 63.4k 680pF f = 600kHz D: ON SEMI MBRA340 L: NEC MPLC0730L3R3 VC RT PG SYNC 316k GND FB 100k
3680 TA03
L 3.3H
LT3972
SW D
22F
2.5V Step-Down Converter
VIN 4V TO 33V TRANSIENT TO 62V ON OFF VOUT 2.5V 3.5A VIN RUN/SS BD BOOST 1F 4.7F 15.4k 63.4k 680pF f = 600kHz D1: ON SEMI MBRA340 D2: MBR0540 L: NEC MPLC0730L3R3 VC RT PG 215k SYNC GND FB 100k
3680 TA04
D2 L 3.3H
LT3972
SW D1
47F
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19
LT3972 TYPICAL APPLICATIONS
5V, 2MHz Step-Down Converter
VIN 8.6V TO 22V TRANSIENT TO 62V ON OFF VOUT 5V 2.5A VIN RUN/SS BD BOOST 0.47F 4.7F 15k 12.7k 680pF f = 2MHz D: ON SEMI MBRA340 L: NEC MPLC0730L2R2 VC RT PG 536k SYNC GND FB 100k
3680 TA05
L 2.2H
LT3972
SW D
22F
12V Step-Down Converter
VIN 15V TO 33V TRANSIENT TO 62V ON OFF VOUT 12V 3.5A VIN RUN/SS BD BOOST 0.47F 10F 17.4k 63.4k 680pF f = 600kHz D: ON SEMI MBRA340 L: NEC MBP107558R2P VC RT PG SYNC GND 715k FB 50k
3680 TA06
L 8.2H
LT3972
SW D
47F
3972f
20
LT3972 TYPICAL APPLICATIONS
1.8V Step-Down Converter
VIN 3.5V TO 27V VIN ON OFF RUN/SS BD BOOST 0.47F 4.7F 16.9k 78.7k 680pF f = 500kHz D: ON SEMI MBRA340 L: NEC MPLC0730L3R3 VC RT PG SYNC 127k GND FB 100k
3680 TA08
VOUT 1.8V 3.5A
L 3.3H
LT3972
SW D
47F
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21
LT3972 PACKAGE DESCRIPTION
DD Package 10-Lead Plastic DFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1699)
0.675
0.05
3.50
0.05 1.65 0.05 2.15 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.38 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 6 0.38 10 0.10
3.00 0.10 (4 SIDES) PIN 1 TOP MARK (SEE NOTE 6)
1.65 0.10 (2 SIDES)
(DD) DFN 1103
5 0.200 REF 0.75 0.05 2.38 0.10 (2 SIDES)
1 0.25 0.05 0.50 BSC
0.00 - 0.05
BOTTOM VIEW--EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
3972f
22
LT3972 PACKAGE DESCRIPTION
MSE Package 10-Lead Plastic MSOP Exposed Die Pad ,
(Reference LTC DWG # 05-08-1664 Rev B)
BOTTOM VIEW OF EXPOSED PAD OPTION
2.794 (.110
0.102 .004)
0.889 (.035
0.127 .005)
1
2.06 0.102 (.081 .004) 1.83 0.102 (.072 .004)
5.23 (.206) MIN
2.083 (.082
0.102 3.20 - 3.45 .004) (.126 - .136)
10
0.50 0.305 0.038 (.0197) (.0120 .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT
3.00 0.102 (.118 .004) (NOTE 3)
10 9 8 7 6
0.497 0.076 (.0196 .003) REF
4.90 0.152 (.193 .006) 0.254 (.010) GAUGE PLANE 0.53 0.152 (.021 .006) DETAIL "A" 0.18 (.007) SEATING PLANE 1.10 (.043) MAX DETAIL "A" 0 - 6 TYP 12345
3.00 0.102 (.118 .004) (NOTE 4)
0.86 (.034) REF
0.17 -0.27 (.007 - .011) TYP
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.50 (.0197) BSC
0.1016 (.004
0.0508 .002)
MSOP (MSE) 0307 REV B
3972f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
23
LT3972 TYPICAL APPLICATION
1.2V Step-Down Converter
VIN 3.6V TO 27V VIN ON OFF RUN/SS BD BOOST 0.47F 4.7F 17k 78.7k 470pF f = 500kHz
3680 TA09
VOUT 1.2V 3.5A
L 3.3H
VC RT PG SYNC
LT3972
SW D
52.3k GND FB 100k 100F
D: ON SEMI MBRA340 L: NEC MPLC0730L3R3
RELATED PARTS
PART NUMBER LT1933 LT1936 LT1940 LT1976/LT1967 LT3434/LT3435 LT3437 LT3480 LT3481 LT3493 LT3505 LT3508 LT3680 LT3684 LT3685 LT3693 DESCRIPTION 500mA (IOUT), 500kHz Step-Down Switching Regulator in SOT-23 36V, 1.4A (IOUT), 500kHz, High Efficiency Step-Down DC/DC Converter Dual 25V, 1.4A (IOUT), 1.1MHz, High Efficiency Step-Down DC/DC Converter 60V, 1.2A (IOUT), 200kHz/500kHz, High Efficiency Step-Down DC/DC Converters with Burst Mode Operation 60V, 2.4A (IOUT), 200kHz/500kHz, High Efficiency Step-Down DC/DC Converters with Burst Mode Operation 60V, 400mA (IOUT), Micropower Step-Down DC/DC Converter with Burst Mode Operation 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation 34V with Transient Protection to 36V, 2A (IOUT), 2.8MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation 36V, 1.4A (IOUT), 750kHz High Efficiency Step-Down DC/DC Converter 36V with Transient Protection to 40V, 1.4A (IOUT), 3MHz, High Efficiency Step-Down DC/DC Converter 36V with Transient Protection to 40V, Dual 1.4A (IOUT), 3MHz, High Efficiency Step-Down DC/DC Converter 36V, 3.5A, 2.4MHz, Low Quiescent Current (<75A) Step-Down DC/DC Converter 34V with Transient Protection to 36V, 2A (IOUT), 2.8MHz, High Efficiency Step-Down DC/DC Converter 36V with Transient Protection to 60V, Dual 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter 36V, 3.5A, 2.4MHz, Step-Down DC/DC Converter COMMENTS VIN: 3.6V to 36V, VOUT(MIN) = 1.2V, IQ = 1.6mA, ISD < 1A, ThinSOTTM Package VIN: 3.6V to 36V, VOUT(MIN) = 1.2V, IQ = 1.9mA, ISD < 1A, MS8E Package VIN: 3.6V to 25V, VOUT(MIN) = 1.2V, IQ = 3.8mA, ISD < 30A, TSSOP16E Package VIN: 3.3V to 60V, VOUT(MIN) = 1.2V, IQ = 100A, ISD < 1A, TSSOP16E Package VIN: 3.3V to 60V, VOUT(MIN) = 1.2V, IQ = 100A, ISD < 1A, TSSOP16 Package VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100A, ISD < 1A, 3mm x 3mm DFN10 and TSSOP16E Packages VIN: 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70A, ISD < 1A, 3mm x 3mm DFN10 and MSOP10E Packages VIN: 3.6V to 34V, VOUT(MIN) = 1.26V, IQ = 50A, ISD < 1A, 3mm x 3mm DFN10 and MSOP10E Packages VIN: 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 1.9mA, ISD < 1A, 2mm x 3mm DFN6 Package VIN: 3.6V to 34V, VOUT(MIN) = 0.78V, IQ = 2mA, ISD = 2A, 3mm x 3mm DFN8 and MSOP8E Packages VIN: 3.7V to 37V, VOUT(MIN) = 0.8V, IQ = 4.6mA, ISD = 1A, 4mm x 4mm QFN24 and TSSOP16E Packages VIN: 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 75A, ISD < 1A, 3mm x 3mm DFN10, MS10E Package VIN: 3.6V to 34V, VOUT(MIN) = 1.26V, IQ = 850A, ISD < 1A, 3mm x 3mm DFN10 and MSOP10E Packages VIN: 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70A, ISD < 1A, 3mm x 3mm DFN10 and MSOP10E Packages VIN: 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 1.3mA, ISD < 1A, 3mm x 3mm DFN10, MS10E Package
3972f LT 0908 * PRINTED IN USA
24 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2008

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