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LTC3442 Micropower Synchronous Buck-Boost DC/DC Converter with Automatic Burst Mode Operation DESCRIPTIO
The LTC(R)3442 is a highly efficient, fixed frequency, BuckBoost DC/DC converter, which operates from input voltages above, below, and equal to the output voltage. The topology incorporated in the IC provides a continuous transfer function through all operating modes, making the product ideal for a single Lithium-Ion or multicell alkaline applications where the output voltage is within the battery voltage range. The device includes two 0.10 N-channel MOSFET switches and two 0.10 P-channel switches. Operating frequency and average input current limit can each be programmed with an external resistor. Quiescent current is only 35A in Burst Mode operation, maximizing battery life in portable applications. Automatic Burst Mode operation allows the user to program the load current for Burst Mode operation, or to control it manually. Other features include 1A shutdown current, programmable soft-start, peak current limit and thermal shutdown. The LTC3442 is available in a low profile, thermally enhanced 12-lead (4mm x 3mm x 0.75mm) DFN package.
, LTC and LT 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. Protected by U.S. Patents, including 6404251, 6166527.
FEATURES

Regulated Output with Input Voltages Above, Below, or Equal to the Output Single Inductor, No Schottky Diodes Required Manual or Programmable Automatic Burst Mode(R) Operation Programmable Average Input Current Limit Up to 1.2A Continuous Output Current from a Single Lithium-Ion Cell High Efficiency: Up to 95% Output Disconnect in Shutdown 2.4V to 5.5V Input Range 2.4V to 5.25V Output Range 35A Quiescent Current in Burst Mode Operation Programmable Frequency from 300kHz to 2MHz <1A Shutdown Current Small, Thermally Enhanced 12-Lead (4mm x 3mm) DFN Package
APPLICATIO S

PDA/`SMART' Phones Handheld Computers MP3 Players Handheld Instruments Digital Cameras Wireless Handsets USB Peripherals
TYPICAL APPLICATIO
4.7H VIN 2.5V TO 4.2V SW1 VIN 1M
100 300mA LOAD SW2 VOUT VOUT 3.3V 1.2A 90
EFFICIENCY (%)
LTC3442 SHDN/SS FB 15k RLIM VC
340k
2.2k
80
Li-Ion
0.01F
22F 470pF 220pF
70
10F RT 71.5k SGND BURST PGND
60
0.01F 200k 200k
50 2.5
3442 TA01a
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Efficiency vs VIN
1A LOAD
VOUT = 3.3V L = 4.7H F = 600kHz 3.0 3.5 4.0 VIN (V) 4.5 5.0 5.5
3442 * TA01b
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LTC3442
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW SHDN/SS RT SGND SW1 PGND SW2 1 2 3 4 5 6 13 12 FB 11 VC 10 RLIM 9 8 7 VIN VOUT BURST
VIN, VOUT Voltage........................................... - 0.3 to 6V SW1, SW2 Voltage DC ................................................................. - 0.3 to 6V Pulsed <100ns ............................................... - 0.3 to 7V SHDN/SS, BURST Voltage ............................. - 0.3 to 6V Operating Temperature (Note 2) ............. - 40C to 85C Maximum Junction Temperature (Note 4) ............ 125C Storage Temperature Range ................. - 65C to 125C
DE12 PACKAGE 12-LEAD (4mm x 3mm) PLASTIC DFN
TJMAX = 125C, JA = 53C/W 1-Layer Board JA = 43C/W 4-Layer Board, JC = 4.3C/W EXPOSED PAD IS PGND (PIN 13), MUST BE SOLDERED TO PCB
ORDER PART NUMBER LTC3442EDE
DE PART MARKING 3442
Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges.
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = VOUT = 3.6V, RT = 64.9k, unless otherwise noted.
PARAMETER Input Start-Up Voltage Output Voltage Adjust Range Feedback Voltage Feedback Input Current Quiescent Current - Burst Mode Operation Quiescent Current - Shutdown Quiescent Current - Active NMOS Switch Leakage PMOS Switch Leakage NMOS Switch On Resistance PMOS Switch On Resistance Input Current Limit Reverse Current Limit Burst Mode Operation Current Limit Max Duty Cycle Min Duty Cycle Frequency Accuracy Error Amp AVOL Error Amp Source Current Error Amp Sink Current Burst Threshold (Falling) Burst Threshold (Rising) Boost (% Switch C On) Buck (% Switch A In)

ELECTRICAL CHARACTERISTICS
CONDITIONS

MIN 2.4 1.19
TYP 2.3 1.22 1 35 0.1 600 0.1 0.1 0.10 0.10
MAX 2.4 5.25 1.25 50 60 1 1100 2 3
UNITS V V V nA A A A A A A A A % %
VFB = 1.22V VFB = 1.22V, BURST = 0V (Note 3) SHDN = 0V, VOUT = 0V, Not Including Switch Leakage BURST = VIN (Note 3) Switches B and C Switches A and D Switches B and C Switches A and D
2
3 0.5 0.9
70 100 570
88 0 670 90 11 300 0.88 1.12 770
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% kHz dB A A V V
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WW
W
LTC3442
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = VOUT = 3.6V, RT = 64.9k, unless otherwise noted.
PARAMETER Burst Current Ratio Input Current Ratio RLIM Threshold SHDN/SS Threshold SHDN/SS Input Current When IC is Enabled When EA is at Max Boost Duty Cycle VSHDN = 5.5V
ELECTRICAL CHARACTERISTICS
CONDITIONS Ratio of IOUT to IBURST Ratio of IIN to IRLIM, IIN = 0.5A
MIN
TYP 20,000 70,000 0.95
MAX
UNITS
V 1.4 2.4 1 V V A
0.4
0.7 2.2 0.01
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 LTC3442E is guaranteed to meet performance specifications from 0C to 85C. Specifications over -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls.
Note 3: Current Measurements are performed when the outputs are not switching. Note 4: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may result in device degradation or failure.
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load
100 Burst Mode 90 OPERATION V = 5V IN 80 VIN = 3.3V VIN = 3.3V
100 Burst Mode 90 OPERATION
POWER LOSS (mW)
EFFICIENCY (%)
70 60 50 40 30 20 0.1 1 10 100 LOAD (mA) VOUT = 3.3V 600kHz 1000 10000
3442 G01
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 5V VIN = 2.5V FIXED FREQUENCY
Input Current Mirror Linearity
1.00 0.90 0.80
RLIM VOLTAGE (V)
VIN = 3.6V VOUT = 3.3V RLIM = 133k
% CHANGE (NORMALIZED)
0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 0 .05 .10 .15 .20 .25 .30 .35 .40 .45 .50 INPUT CURRENT (A)
3442 G04
4 2 0 -2 -4 -6 -8 2.5 3.0 3.5 VIN (V)
3442 G05
% CHANGE (NORMALIZED)
UW
VIN = 2.5V
(TA = 25C unless otherwise specified). Efficiency vs Frequency
1000
Efficiency and Power Loss vs Load
96 94
100 POWER LOSS 10 FIXED FREQUENCY 1 VIN = 3.6V VOUT = 3.3V 1 10 100 1000 LOAD CURRENT (mA)
80 70 60 50 40 30 20 0.1
92 90 88 86 84 82 VIN = 3.6V VOUT = 3.3V
WITH SCHOTTKY DIODES
WITHOUT SCHOTTKY DIODES
0.1 10000
80 400 600 800 1000 1200 1400 1600 1800 2000 FREQUENCY (kHz)
3442 G03
3442 G02
Average Input Current Limit vs VIN (Normalized)
8 VOUT = 3.3V 6 1MHz
Average Input Current Limit vs Frequency (Normalized)
15 VIN = 5V 10 VOUT SHORTED 5
0
VOUT DROPS 10%
-5
4.0
4.5
5.0
-10 0.50
0.75
1.00 1.25 1.50 FREQUENCY (MHz)
1.75
2.00
3442 TA01b
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LTC3442 TYPICAL PERFOR A CE CHARACTERISTICS
Quiescent Current vs VIN (Fixed Frequency Mode)
4.0
VIN QUIESCENT CURRENT (mA)
3.5 3.0
VIN QUIESCENT CURRENT (A)
2.0 MHz 1.5 MHz
35 30 25 20 15 10 5 0
2.5 2.0 1.5 1.0 0.5 0.0 2.5 3.0 3.5 4.0 VIN (V) 4.5 5.0 5.5
3442 G07
INPUT CURRENT (A)
1.0 MHz 0.5 MHz NO SWITCHING
Automatic Burst Mode Threshold vs RBURST
160 150 140
LOAD CURRENT (mA)
2.30 2.29
LEAVE Burst Mode OPERATION
MINIMUM START VOLTAGE (V)
130 120 110 100 90 80 70 60 150 ENTER Burst Mode OPERATION 175 200 RBURST (k) 225 250
3442 G10
2.26 2.25 2.24 2.23 2.22 2.21 2.20 -55 -25 35 65 5 TEMPERATURE (C) 95 125
3442 G11
CHANGE FROM 25C
Frequency Change vs Temperature (Normalized)
2.0% 1.5% CHANGE FROM 25C
0.5% 0.0% -0.5% -1.0% -1.5% -2.0% -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
3442 G13
CHANGE FROM 25C
1.0%
4
UW
(TA = 25C unless otherwise specified).
Burst Mode Quiescent Current vs VIN
50 45 40
2.5 2.0 1.5 1.0 0.5 3.5 3.0
Peak Current Clamp vs VIN
2.5
3.0
3.5
4.0 VIN (V)
4.5
5.0
5.5
3442 G08
0.0 2.5
3.0
3.5
4.0 VIN (V)
4.5
5.0
5.5
3442 G09
Minimum Start Voltage vs Temperature
5% 4% 3% 2% 1% 0% -1% -2% -3% -4%
Average Input Current Limit vs Temperature (Normalized)
VIN = VOUT = 3.3V
2.28 2.27
-5% -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
3442 G12
Feedback Voltage vs Temperature (Normalized)
1.0% 0.8% 0.6% 0.4% 0.2% 0.0% -2.0% -0.4% -0.6% -0.8% -1.0% -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
3442 G14
Switch Pins Before Entering Boost Mode
SW1 2V/DIV SW2 2V/DIV
VIN = VOUT = 3.3V
50ns/DIV VIN = 2.9V VOUT = 3.3V AT 500mA
3442 G15
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LTC3442 TYPICAL PERFOR A CE CHARACTERISTICS
Switch Pins Entering Buck-Boost Mode
SW1 2V/DIV
VIN = 2.7V VIN = 3.3V
Switch Pins in Buck-Boost Mode
SW1 2V/DIV SW2 2V/DIV
50ns/DIV VIN = 3.3V VOUT = 3.3V AT 500mA
Load Transient Response in Fixed Frequency Mode, No Load to 1A
VOUT 100mV/DIV
LOAD 0.5A/DIV 100s/DIV VIN = 3.6V VOUT = 3.3V COUT = 47F, X5R CERAMIC
3442 G19
Transition from Burst Mode Operation to Fixed Frequency Mode
VOUT 2V/DIV
VOUT 50mV/DIV
INDUCTOR CURRENT 0.5A/DIV 200s/DIV COUT = 100F LOW ESR TANTALUM
3442 G22
UW
(TA = 25C unless otherwise specified).
Output Ripple at 1A Load
SW2 2V/DIV
VIN = 4.2V 1s/DIV
3442 G18
3442 G16
50ns/DIV VIN = 4.2V VOUT = 3.3V AT 500mA
3442 G17
VOUT 20mV/DIV AC COUPLED
Load Transient Response in Auto Burst Mode Operation, No Load to 1A
VOUT 100mV/DIV
Burst Mode Operation
VOUT 50mV/DIV
LOAD 0.5A/DIV 100s/DIV VIN = 3.6V VOUT = 3.3V COUT = 47F, X5R CERAMIC + 100F LOW ESR TANTALUM
3442 G20
INDUCTOR CURRENT 0.5A/DIV 20s/DIV COUT = 100F LOW ESR TANTALUM
3442 G21
Pulsed Overload Using Average Input Current Limit
RLIM PIN 0.5V/DIV INDUCTOR CURRENT 0.5A/DIV 1ms/DIV RLIM = 133k CLIM = .001F
3442 G23
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LTC3442
PI FU CTIO S
SHDN/SS (Pin 1): Combined Soft-Start and Shutdown. Applied voltage <0.4V shuts down the IC. Tie to >1.4V to enable the IC and >2.4V to ensure the error amp is not clamped from soft-start. For Burst Mode operation, this pin must be pulled up to within 0.5V of VIN. An RC network from the shutdown command signal to this pin will provide a soft-start function by limiting the rise time of the VC pin. RT (Pin 2): Programs the Frequency of the Internal Oscillator. Place a resistor from this pin to ground. See the Applications Information section for component value selection. SGND (Pin 3): Signal Ground for the IC. SW1 (Pin 4): Switch Pin Where Internal Switches A and B are Connected. Connect inductor from SW1 to SW2. An optional Schottky diode can be connected from SW1 to ground for a moderate efficiency improvement. Minimize trace length to reduce EMI. PGND (Pin 5, 13): Power Ground for the Internal NMOS Power Switches. The exposed pad must be soldered to PCB ground to provide both electrical contact and a good thermal contact to the PCB. SW2 (Pin 6): Switch Pin Where Internal Switches C and D are Connected. An optional Schottky diode can be connected from SW2 to VOUT for a moderate efficiency improvement. Minimize trace length to reduce EMI. BURST (Pin 7): Used to set the Automatic Burst Mode Operation Threshold. Place a resistor and capacitor in parallel from this pin to ground. See the Applications Information section for component value selection. For manual control, ground the pin to force Burst Mode operation, connect to VOUT to force fixed frequency mode. VOUT (Pin 8): Output of the Synchronous Rectifier. A filter capacitor is placed from VOUT to GND. A ceramic bypass capacitor is recommended as close to the VOUT and GND pins as possible. VIN (Pin 9): Input Supply Pin. Internal VCC for the IC. A 10F ceramic capacitor is recommended as close to VIN and SGND as possible. RLIM (Pin 10): Sets the Average Input Current Limit Threshold. Place a resistor and capacitor in parallel from this pin to ground. See the Applications Information section for component value selection. VC (Pin 11): Error Amp Output. A frequency compensation network is connected from this pin to FB to compensate the loop. During Burst Mode operation, VC is internally connected to a hold circuit. FB (Pin 12): Feedback Pin. Connect resistor divider tap here. The output voltage can be adjusted from 2.4V to 5.25V. The feedback reference voltage is typically 1.22V.
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LTC3442
SI PLIFIED BLOCK DIAGRA
SW1 4
VIN 9
SW A GATE DRIVERS AND ANTICROSS CONDUCTION
REVERSE AMP
RLIM 10
-
AV = 6 1V
+
AVERAGE ILIM 3A
+
5A
-
PWM LOGIC PWM COMPARATORS
VIN
+ -
UVLO
2.3V
SLEEP
RT
2
OSC 7 1.22V VREF THERMAL SHUTDOWN BURST
VIN SHDN/SS 1
SHUTDOWN SOFT-START
VCC SHUTDOWN
2 PGND
-
PEAK CURRENT LIMIT
SS
SGND
+
-
SW B
+
-
+
W
SW2 6 SW D 8 VOUT SW C
W
+
Gm = 1/60k
- +
ERROR AMP 1.22V
-
12 FB
11 VC
AUTOMATIC BURST MODE CONTROL AND VC HOLD
VREF
6
3442 BD
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LTC3442
OPERATIO
The LTC3442 provides high efficiency, low noise power for applications such as portable instrumentation. The LTC proprietary topology allows input voltages above, below or equal to the output voltage by properly phasing the output switches. The error amp output voltage on VC determines the output duty cycle of the switches. Since VC is a filtered signal, it provides rejection of frequencies from well below the switching frequency. The low RDS(ON), low gate charge synchronous switches provide high frequency pulse width modulation control at high efficiency. Schottky diodes across the synchronous switch D and synchronous switch B are not required, but provide a lower voltage drop during the break-before-make time (typically 15ns). Schottky diodes will improve peak efficiency by typically 1% to 2%. High efficiency is achieved at light loads when Burst Mode operation is entered and the IC's quiescent current drops to a low 35A. LOW NOISE FIXED FREQUENCY OPERATION Externally Programmable Current Limit Oscillator The frequency of operation is programmed by an external resistor from RT to ground, according to the following equation:
f (kHz) = 43, 300 RT(k)
Error Amp The error amplifier is a voltage mode amplifier. The loop compensation components are configured around the amplifier (from FB to VC) to obtain stability of the converter. For improved bandwidth, an additional RC feedforward network can be placed across the upper feedback divider resistor. The voltage on SHDN/SS clamps the error amp output, VC, to provide a soft-start function.
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Internal Current Limit There are three different current limit circuits in the LTC3442. Two have internally fixed thresholds which vary inversely with VIN, the third is externally programmable, and does not vary with input voltage. The first circuit is a high speed peak current limit amplifier that will shut off switch A if the current exceeds 5A typical. The delay to output of this amplifier is typically 50ns. A second amplifier will begin to source current into the FB pin to drop the output voltage once the peak input current exceeds 3A typical. This method provides a closed loop means of clamping the input current. During conditions where VOUT is near ground, such as during a short-circuit or during startup, this threshold is cut in half, providing a foldback feature. For this current limit feature to be most effective, the Thevenin resistance from FB to ground should be greater than 100k. The third current limit circuit is programmed by an external resistor on RLIM. This circuit works by mirroring the input current in switch A, averaging it by means of the external RC network on RLIM, and comparing the resulting voltage with an internal reference. If the voltage on RLIM starts to exceed 0.95V, a Gm amplifier will clamp VC, lowering VOUT to maintain control of the input current. This allows the user to program a maximum average input current, for applications such as USB, where the current draw from the bus must be limited to 500mA. The resistor and capacitor values are determined by the following equations:
2 * VIN - VOUT 70 * 0.86 + 40 IIN(AMPS) 0.1 RLIM(k)
(
)

RLIM(k) = CLIM(F)
The programmable current limit feature is disabled in Burst Mode operation.
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LTC3442
OPERATIO
Reverse Current Limit During fixed frequency operation, the LTC3442 operates in forced continuous conduction mode. The reverse current limit amplifier monitors the inductor current from the output through switch D. Once the negative inductor current exceeds 500mA typical, the IC will shut off switch D. Four-Switch Control Figure 1 shows a simplified diagram of how the four internal switches are connected to the inductor, VIN, VOUT and GND. Figure 2 shows the regions of operation for the LTC3442 as a function of the internal control voltage, VCI. Depending on the control voltage, the IC will operate in either buck, buck/boost or boost mode. The VCI voltage is a level shifted voltage from the output of the error amp (VC) (see Figure 5). The four power switches are properly phased so the transfer between operating modes is continuous, smooth and transparent to the user. When VIN approaches VOUT the buck/boost region is reached where
VIN 9
PMOS A
SW1 4
NMOS B
Figure 1. Simplified Diagram of Output Switches
88% DMAX BOOST A ON, B OFF BOOST REGION PWM CD SWITCHES DMIN BOOST DMAX BUCK FOUR SWITCH PWM BUCK/BOOST REGION
D ON, C OFF PWM AB SWITCHES BUCK REGION 0% DUTY CYCLE V1 (0.9V) INTERNAL CONTROL VOLTAGE, VCI
Figure 2. Switch Control vs Internal Control Voltage, VCI
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the conduction time of the four switch region is typically 150ns. Referring to Figures 1 and 2, the various regions of operation will now be described. Buck Region (VIN > VOUT) Switch D is always on and switch C is always off during this mode. When the internal control voltage, VCI, is above voltage V1, output A begins to switch. During the off-time of switch A, synchronous switch B turns on for the remainder of the time. Switches A and B will alternate similar to a typical synchronous buck regulator. As the control voltage increases, the duty cycle of switch A increases until the maximum duty cycle of the converter in buck mode reaches DMAX_BUCK, given by: DMAX_BUCK = 100 - D4SW % where D4SW = duty cycle % of the four switch range. D4SW = (150ns * f) * 100 % where f = operating frequency, Hz. Beyond this point the "four switch," or buck/boost region is reached.
PMOS D
SW2 6
VOUT 8
Buck/Boost or Four Switch (VIN ~ VOUT) When the internal control voltage, VCI, is above voltage V2, switch pair AD remain on for duty cycle DMAX_BUCK, and the switch pair AC begins to phase in. As switch pair AC phases in, switch pair BD phases out accordingly. When the VCI voltage reaches the edge of the buck/boost range, at voltage V3, the AC switch pair completely phase out the BD pair, and the boost phase begins at duty cycle D4SW. The input voltage, VIN, where the four switch region begins is given by:
NMOS C
3442 F01
V4 (2.05V)
V3 (1.65V) V2 (1.55V)
VIN =
VOUT 1 - (150ns * f)
The point at which the four switch region ends is given by: VIN = VOUT(1 - D) = VOUT(1 - 150ns * f) V
3442 F02
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LTC3442
OPERATIO
Boost Region (VIN < VOUT) Switch A is always on and switch B is always off during this mode. When the internal control voltage, VCI, is above voltage V3, switch pair CD will alternately switch to provide a boosted output voltage. This operation is typical to a synchronous boost regulator. The maximum duty cycle of the converter is limited to 88% typical and is reached when VCI is above V4. BURST MODE OPERATION Burst Mode operation occurs when the IC delivers energy to the output until it is regulated and then goes into a sleep mode where the outputs are off and the IC is consuming only 35A of quiescent current from VIN. In this mode the output ripple has a variable frequency component that depends upon load current, and will typically be about 2% peak-to-peak. Burst Mode operation ripple can be reduced slightly by using more output capacitance (47F or greater). Another method of reducing Burst Mode operation ripple is to place a small feed-forward capacitor across the upper resistor in the VOUT feedback divider network (as in Type III compensation). During the period where the device is delivering energy to the output, the peak switch current will be equal to 900mA typical and the inductor current will terminate at zero current for each cycle. In this mode the typical maximum average output current is given by:
SW1 B
L
SW2 C
IINDUCTOR
0.2 * VIN IOUT(MAX)BURST A VOUT + VIN
Note that the peak efficiency during Burst Mode operation is less than the peak efficiency during fixed frequency
VIN 9 A 4 SW1 B dI VIN dt L L D VOUT 8
+
-
SW2 C
IINDUCTOR
5 GND
Figure 3. Inductor Charge Cycle During Burst Mode Operation
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VIN 9 A 4 dI - VOUT L dt D 6 900mA VOUT 8
-
+
0mA T2
3442 F04
5 GND
Figure 4. Inductor Discharge Cycle During Burst Mode Operation
because the part enters full-time 4-switch mode (when servicing the output) with discontinuous inductor current as illustrated in Figures 3 and 4. During Burst Mode operation, the control loop is nonlinear and cannot utilize the control voltage from the error amp to determine the control mode, therefore full-time 4-switch mode is required to maintain the Buck/Boost function. The efficiency below 1mA becomes dominated primarily by the quiescent current. The Burst Mode operation efficiency is given by: EFFICIENCY n * ILOAD 35A + ILOAD
where n is typically 82% during Burst Mode operation. Automatic Burst Mode Operation Control Burst Mode operation can be automatic or manually controlled with a single pin. In automatic mode, the IC will enter Burst Mode operation at light load and return to fixed frequency operation at heavier loads. The load current at which the mode transition occurs is programmed using a single external resistor from the BURST pin to ground, according to the following equations: 17.6 RBURST 22.4 Leave Burst Mode: I = RBURST Enter Burst Mode: I = where RBURST is in k and IBURST is the load transition
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900mA
0mA T1
3442 F03
LTC3442
OPERATIO
current in Amps. Do not use values of RBURST greater than 250k. For automatic operation, a filter capacitor should also be connected from BURST to ground to prevent ripple on BURST from causing the IC to oscillate in and out of Burst Mode operation. The equation for the minimum capacitor value is:
CBURST(MIN) COUT * VOUT 60, 000
where CBURST(MIN) and COUT are in F In the event that a load transient causes the feedback pin to drop by more than 4% from the regulation value while in Burst Mode operation, the IC will immediately switch to fixed frequency mode and an internal pull-up will be momentarily applied to BURST, rapidly charging the BURST cap. This prevents the IC from immediately reentering Burst Mode operation once the output achieves regulation. Manual Burst Mode Operation For manual control of Burst Mode operation, the RC network connected to BURST can be eliminated. To force fixed frequency mode, BURST should be connected to VOUT. To force Burst Mode operation, BURST should be grounded. When commanding Burst Mode operation manually, the circuit connected to BURST should be able to sink up to 2mA.
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For optimum transient response with large dynamic loads, the operating mode should be controlled manually by the host. By commanding fixed frequency operation prior to a sudden increase in load, output voltage droop can be minimized. Note that if the load current applied during forced Burst Mode operation (BURST pin is grounded) exceeds the current that can be supplied, the output voltage will start to droop and the IC will automatically come out of Burst Mode operation and enter fixed frequency mode, raising VOUT. Once regulation is achieved, the IC will then enter Burst Mode operation once again, and the cycle will repeat, resulting in about 4% output ripple. Note that Burst Mode operation is inhibited during soft-start. Burst Mode Operation to Fixed Frequency Transient Response In Burst Mode operation, the compensation network is not used and VC is disconnected from the error amplifier. During long periods of Burst mode operation, leakage currents in the external components or on the PC board could cause the compensation capacitor to charge (or discharge), which could result in a large output transient when returning to fixed frequency mode of operation, even at the same load current. To prevent this, the LTC3442 incorporates an active clamp circuit that holds the voltage on VC at an optimal voltage during Burst Mode operation. This minimizes any output transient when returning to fixed frequency mode operation. For optimum transient
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LTC3442
OPERATIO
response, Type 3 compensation is also recommended to broad band the control loop and roll off past the two pole response of the output LC filter. (See Closing the Feedback Loop.) Soft-Start The soft-start function is combined with shutdown. When the SHDN/SS pin is brought above 1V typical, the IC is enabled but the EA duty cycle is clamped from VC. A
TO PWM COMPARATORS
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detailed diagram of this function is shown in Figure 5. The components RSS and CSS provide a slow ramping voltage on SHDN/SS to provide a soft-start function. To ensure that VC is not being clamped, SHDN/SS must be raised above 2.4V. To enable Burst Mode operation, SHDN/SS must be raised to within 0.5V of VIN.
VIN 14A ERROR AMP VOUT FB R1
+ -
SOFT-START CLAMP VCI
1.22V
12 VC 11 CP1 R2
SHDN/SS 1 CSS
RSS ENABLE SIGNAL
3442 F05
+
CHIP ENABLE
-
1V
Figure 5. Soft-Start Circuitry
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LTC3442
APPLICATIO S I FOR ATIO
COMPONENT SELECTION
VIN 1 SHDN/SS 2 RT 3 SGND 4 SW1 5 PGND 6 SW2 FB 12 VC 11 RLIM 10 VIN 9 VOUT 8 BURST 7
VIN
GND RT
MULTIPLE VIAS
3442 F06
Figure 6. Recommended Component Placement. Traces Carrying High Current Should be Short and Wide. Trace Area at FB and VC Pins are Kept Low. Lead Length to Battery Should be Kept Short. VOUT and VIN Ceramic Capacitors Close to the IC Pins.
Inductor Selection The high frequency operation of the LTC3442 allows the use of small surface mount inductors. The inductor ripple current is typically set to 20% to 40% of the maximum inductor current. For a given ripple the inductance terms are given as follows:
L BOOST > L BUCK >
VIN(MIN) * ( VOUT - VIN(MIN) ) f * IL * VOUT * ( VIN(MAX ) - VOUT )
H
VOUT
f * IL * VIN(MAX )
H
Table 1. Inductor Vendor Information
SUPPLIER Coilcraft CoEv Magnetics Murata Sumida TDK TOKO PHONE (847) 639-6400 (800) 227-7040 (814) 237-1431 (800) 831-9172 USA: (847) 956-0666 Japan: 81(3) 3607-5111 (847) 803-6100 (847) 297-0070 FAX (847) 639-1469 (650) 361-2508 (814) 238-0490 USA: (847) 956-0702 Japan: 81(3) 3607-5144 (847) 803-6296 (847) 699-7864 WEB SITE www.coilcraft.com www.circuitprotection.com/magnetics.asp www.murata.com www.sumida.com www.component.tdk.com www.tokoam.com
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where f = operating frequency, Hz IL = maximum allowable inductor ripple current, A VIN(MIN) = minimum input voltage, V VIN(MAX) = maximum input voltage, V VOUT = output voltage, V
VOUT
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IOUT(MAX) = maximum output load current For high efficiency, choose a ferrite inductor with a high frequency core material to reduce core loses. The inductor should have low ESR (equivalent series resistance) to reduce the I2R losses, and must be able to handle the peak inductor current without saturating. Molded chokes or chip inductors usually do not have enough core to support the peak inductor currents in the 1A to 2A region. To minimize radiated noise, use a shielded inductor. See Table 1 for a suggested list of inductor suppliers. Output Capacitor Selection The bulk value of the output filter capacitor is set to reduce the ripple due to charge into the capacitor each cycle. The steady state ripple due to charge is given by: % RIPPLE_BOOST = IOUT(MAX) * (VOUT - VIN(MIN) ) *100 COUT * VOUT 2 * f % RIPPLE_BUCK = %
IOUT(MAX) * (VIN(MAX) - VOUT ) *100 COUT * VIN(MAX) * VOUT * f
%
where COUT = output filter capacitor in Farads and f = switching frequency in Hz.
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LTC3442
APPLICATIO S I FOR ATIO
The output capacitance is usually many times larger than the minimum value in order to handle the transient response requirements of the converter. For a rule of thumb, the ratio of the operating frequency to the unity-gain bandwidth of the converter is the amount the output capacitance will have to increase from the above calculations in order to maintain the desired transient response. The other component of ripple is due to the ESR (equivalent series resistance) of the output capacitor. Low ESR capacitors should be used to minimize output voltage ripple. For surface mount applications, Taiyo Yuden or TDK ceramic capacitors, AVX TPS series tantalum capacitors or Sanyo POSCAP are recommended. See Table 2 for contact information. Input Capacitor Selection Since VIN is the supply voltage for the IC, as well as the input to the power stage of the converter, it is recommended to place at least a 4.7F, low ESR ceramic bypass capacitor close to the VIN and SGND pins. It is also important to minimize any stray resistance from the converter to the battery or other power source. Optional Schottky Diodes The Schottky diodes across the synchronous switches B and D are not required (VOUT < 4.3V), but provide a lower drop during the break-before-make time (typically 15ns) improving efficiency. Use a surface mount Schottky diode such as an MBRM120T3 or equivalent. Do not use ordinary rectifier diodes, since the slow recovery times will compromise efficiency. For applications with an output voltage above 4.3V, a Schottky diode is required from SW2 to VOUT. Output Voltage < 2.4V The LTC3442 can operate as a buck converter with output
Table 2. Capacitor Vendor Information
SUPPLIER AVX Murata Sanyo Taiyo Yuden TDK PHONE (803) 448-9411 (814) 237-1431 (800) 831-9172 (619) 661-6322 (408) 573-4150 (847) 803-6100 FAX
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voltages as low as 0.4V. The part is specified at 2.4V minimum to allow operation without the requirement of a Schottky diode. Synchronous switch D is powered from VOUT and the RDS(ON) will increase at low output voltages, therefore a Schottky diode is required from SW2 to VOUT to provide the conduction path to the output. Note that Burst Mode operation is inhibited at output voltages below 1.6V typical. Output Voltage > 4.3V A Schottky diode from SW2 to VOUT is required for output voltages over 4.3V. The diode must be located as close to the pins as possible in order to reduce the peak voltage on SW2 due to the parasitic lead and trace inductance. Input Voltage > 4.5V For applications with input voltages above 4.5V which could exhibit an overload or short-circuit condition, a 2/1nF series snubber is required between SW1 and GND. A Schottky diode from SW1 to VIN should also be added as close to the pins as possible. For the higher input voltages, VIN bypassing becomes more critical; therefore, a ceramic bypass capacitor as close to the VIN and SGND pins as possible is also required. Operating Frequency Selection Higher operating frequencies allow the use of a smaller inductor and smaller input and output filter capacitors, thus reducing board area and component height. However, higher operating frequencies also increase the IC's total quiescent current due to the gate charge of the four switches, as given by: Buck: Boost: Iq = (0.8 * VIN * f) mA Iq = [0.4 * (VIN + VOUT) * f] mA Buck/Boost: Iq = [f * (1.2 * VIN + 0.4 * VOUT)] mA
WEB SITE www.avxcorp.com www.murata.com www.sanyovideo.com www.t-yuden.com www.component.tdk.com
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(803) 448-1943 (814) 238-0490 (619) 661-1055 (408) 573-4159 (847) 803-6296
LTC3442
APPLICATIO S I FOR ATIO
where f = switching frequency in MHz. Therefore frequency selection is a compromise between the optimal efficiency and the smallest solution size. Closing the Feedback Loop The LTC3442 incorporates voltage mode PWM control. The control to output gain varies with operation region (buck, boost, buck/boost), but is usually no greater than 15. The output filter exhibits a double pole response, as given by: f FILTER--POLE = (in buck mode) f FILTER--POLE = VIN 2 * VOUT * * L * COUT Hz 1 2 * * L * COUT Hz
Figure 7. Error Amplifier with Type I Compensation
(in boost mode) where L is in henries and COUT is in farads. The output filter zero is given by: Hz 2 * * RESR * COUT where RESR is the equivalent series resistance of the output cap. A troublesome feature in boost mode is the right-half plane zero (RHP), given by:
f FILTER-- ZERO = 1
VIN f RHPZ = Hz 2 * * IOUT * L * VOUT
The loop gain is typically rolled off before the RHP zero frequency. A simple Type I compensation network can be incorporated to stabilize the loop, but at a cost of reduced bandwidth and slower transient response. To ensure proper phase margin using Type I compensation, the loop must be crossed over a decade before the LC double pole. The unity-gain frequency of the error amplifier with the Type I compensation is given by:
2
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fUG = 1 Hz 2 * * R1 * CP1
referring to Figure 7.
VOUT
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+
ERROR AMP
1.22V FB 12 VC 11 CP1 R2
3442 F07
R1
-
Most applications demand an improved transient response to allow a smaller output filter capacitor. To achieve a higher bandwidth, Type III compensation is required, providing two zeros to compensate for the double-pole response of the output filter. Referring to Figure 8, the location of the poles and zeros are given by: Hz 2 * * 32e3 * R1 * CP1 (which is extremely close to DC) 1 Hz 2 * * RZ * CP1 1 fZERO2 = Hz 2 * * R1 * CZ1 1 fPOLE2 = Hz 2 * * RZ * CP2 fZERO1 = where resistance is in ohms and capacitance is in farads.
VOUT
fPOLE1
1
+
ERROR AMP
1.22V FB 12 VC 11 CP2
3442 F08
R1
CZ1
-
RZ
CP1
R2
Figure 8. Error Amplifier with Type III Compensation
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LTC3442
TYPICAL APPLICATIO S
1MHz Li-Ion to 3.3V at 1.2A Converter with Manual Mode Control (and Peak Current Limit Only)
L1 3.3H
2.5V TO 4.2V
CIN 10F
Li-Ion
Multi-Input 3.3V at 600mA Boost Converter for Portable Applications with Automatic Burst Mode Operation and Average Input Current Limit for USB Powered Devices
1nF L1 4.7H
Li-Ion USB/5V CIN 10F 143k 2N7002 USB PRESENT 0.01F RSNUB** 1 1nF 143k 64.9k D1 MBRM120T3 2.5V TO 5.5V 1M SW1 VIN SW2 LTC3442 VOUT FB VC BURST PGND 200k 0.01F 15k 340k 2.2k 220pF 470pF 200k COUT 22F VOUT 3.3V 600mA
**A SNUBBER RESISTOR IS REQUIRED TO PREVENT CIN: TAIYO YUDEN JMK212BJ106MG RINGING IF THERE IS SIGNIFICANT INPUT INDUCTANCE, COUT: TAIYO YUDEN JMK325BJ476MM L1: TDK RLF7030T-4R7M3R4 SUCH AS FROM A USB CABLE
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SW2 LTC3442 VOUT VIN SW1 1M SHDN/SS RLIM RT 0.01F 43.2k SGND FB VC BURST PGND 15k 340k 2.2k 220pF 470pF 200k
VOUT 3.3V 1.2A COUT 22F
BURST FIXED FREQ
CIN: TAIYO YUDEN JMK212BJ106MG COUT: TAIYO YUDEN JMK325BJ226MM L1: TDK RLF7030T-3R3M4R
3442 TA02
2
SHDN/SS RLIM RT SGND
3442 TA03
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LTC3442
TYPICAL APPLICATIO S
High Efficiency Li-Ion Powered Constant Current LED Driver with Open-LED Protection
R5 7.87k VIN 2.5V TO 4.2V *OFF ON 10F RLIM VC SW1 VIN 3.3H
OPEN LED VOLTAGE LIMIT = (R4+R5)*0.95/R4
* NOTE: THE SHDN/SS VOLTAGE MUST BE NO MORE THAN 0.5V BELOW VIN WHEN ENABLED.
EFFICIENCY (%)
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SW2 VOUT LTC3442
VOUT ILED = 500mA
SHDN/SS
FB 1nF LHXL-PW03 R2 200k 4.7F
RT R4 2k SGND 57.6k
BURST PGND R3 95.3k 47pF R1 301k
3442 TA04a
R2 = R1/1.5 ILED = 24 * (R1+R2+R3)/(R1 * R3) AMPS
LED Driver Efficiency vs LED Current
100 98 96 94 92 90 88 86 84 82 80 0.1 LED CURRENT (A)
3442 TA04b
VIN = 3.6V 750kHz
1.0
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LTC3442
TYPICAL APPLICATIO S
High Current LED Driver with Low/High Current Range for Pulsed Applications; LED Current is 0.5A with 1.5A Pulse
R5 7.87k VIN 2.7V TO 4.2V *OFF ON 10F 6.3V RLIM SW1 VIN
R4 2k
OPEN LED VOLTAGE LIMIT = (R4+R5) * 0.95/R4 LOW HI
* NOTE: THE SHDN/SS VOLTAGE MUST BE NO MORE THAN 0.5V BELOW VIN WHEN ENABLED.
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3.3H
SW2 VOUT LTC3442
VOUT ILED = 500mA/1.5A
SHDN/SS
FB 1nF VC R2 200k R2 20k 10F 6.3V LHXL-PW03
RT 57.6k SGND
BURST PGND 95.3k 1nF R1 301k
40.2k
2N7002
R2 = R1/1.5 ILED = 24 * (R1+R2+R3)/(R1 * R3) AMPS (OR: ILED = 40/R3 + .08)
3442 TA05
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LTC3442
PACKAGE DESCRIPTIO
3.50 0.05 1.70 0.05 2.20 0.05 (2 SIDES)
0.25 0.05 3.30 0.05 (2 SIDES) 0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 4.00 0.10 (2 SIDES) R = 0.20 TYP 3.00 0.10 (2 SIDES) 1.70 0.10 (2 SIDES) PIN 1 NOTCH
(UE12/DE12) DFN 0603
PIN 1 TOP MARK (NOTE 6)
0.200 REF
NOTE: 1. DRAWING PROPOSED TO BE A VARIATION OF VERSION (WGED) IN JEDEC PACKAGE OUTLINE M0-229 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
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.
U
UE/DE Package 12-Lead Plastic DFN (4mm x 3mm)
(Reference LTC DWG # 05-08-1695)
0.65 0.05 PACKAGE OUTLINE 7 R = 0.115 TYP 0.38 0.10 12 0.75 0.05 6 0.25 0.05 3.30 0.10 (2 SIDES) 1 0.50 BSC 0.00 - 0.05 BOTTOM VIEW--EXPOSED PAD
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LTC3442
RELATED PARTS
PART NUMBER LT 1613 LT1618 LT1930/LT1930A LT1935 LT1946/LT1946A LT1961
(R)
DESCRIPTION 550mA (ISW), 1.4MHz, High Efficiency Step-Up DC/DC Converter 1.5A (ISW), 1.25MHz, High Efficiency Step-Up DC/DC Converter 1A (ISW), 1.2MHz/2.2MHz, High Efficiency Step-Up DC/DC Converter 2A (ISW), 1.2MHz, 38V Step-Up DC/DC Converter 1.5A (ISW), 1.2MHz/2.7MHz, High Efficiency Step-Up DC/DC Converter 1.5A (ISW), 1.25MHz, High Efficiency Step-Up DC/DC Converter
COMMENTS VIN: 0.9V to 10V, VOUT(MAX) = 34V, IQ = 3mA, ISD < 1A, ThinSOTTM Package VIN: 1.6V to 18V, VOUT(MAX) = 35V, IQ = 1.8mA, ISD < 1A, MS10 Package VIN: 2.6V to 16V, VOUT(MAX) = 34V, IQ = 4.2mA/5.5mA, ISD < 1A, ThinSOT Package VIN: 2.3V to 16V, VOUT(MAX) = 38V, IQ = 3mA, ISD < 1A, ThinSOT Package VIN: 2.45V to 16V, VOUT(MAX) = 34V, IQ = 3.2mA, ISD < 1A, MS8 Package VIN: 3V to 25V, VOUT(MAX) = 35V, IQ = 0.9mA, ISD = 6A, MS8E Package VIN: 0.85V to 5V, VOUT(MAX) = 5V, IQ = 19A/300A, ISD < 1A, ThinSOT Package VIN: 0.5V to 5V, VOUT(MAX) = 6V, IQ = 38A, ISD < 1A, MS Package VIN: 2.7V to 6V, VOUT(MIN) = 0.8V, IQ = 20A, ISD 1A, MS10 Package VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20A, ISD 1A, ThinSOT Package VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40A, ISD 1A, MS Package VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60A, ISD 1A, MS Package VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60A, ISD 1A, TSSOP16E Package VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12A, ISD < 1A, QFN Package VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12A, ISD < 1A, QFN Package VIN: 0.5V to 4.4V, VOUT(MIN) = 5V, IQ = 20A, ISD < 1A, QFN Package VIN: 3V to 25V, VOUT(MAX) = 34V, IQ = 0.9mA, ISD < 6A, TSSOP-16E Package VIN: 2.5V to 5.5V, VOUT(MIN) = 5.5V, IQ = 25A, ISD < 1A, MS, DFN Packages VIN: 2.5V to 5.5V, VOUT(MIN) = 5.5V, IQ = 25A, ISD < 1A, DFN Package VIN: 2.4V to 5.5V, VOUT(MIN) = 5.25V, IQ = 28A, ISD < 1A, MS Package VIN: 2.6V to 16V, VOUT(MAX) = 40V, IQ = 1.2mA, ISD < 1A, ThinSOT Package
LTC3400/LTC3400B 600mA (ISW), 1.2MHz Synchronous Step-Up DC/DC Converter LTC3401/LTC3402 1A/2A (ISW), 3MHz Synchronous Step-Up DC/DC Converter
LTC3405/LTC3405A 300mA (IOUT), 1.5MHz Synchronous Step-Down DC/DC Converter LTC3406/LTC3406B 600mA (IOUT), 1.5MHz Synchronous Step-Down DC/DC Converter LTC3407 LTC3411 LTC3412 LTC3421 LTC3425 LTC3429 LTC3436 LTC3440 LTC3441 LTC3443 LT3467 600mA (IOUT), 1.5MHz Dual Synchronous Step-Down DC/DC Converter 1.25A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter 3A (ISW), 3MHz Synchronous Step-Up DC/DC Converter 5A (ISW), 8MHz Multiphase Synchronous Step-Up DC/DC Converter 600mA (ISW), 500kHz Synchronous Step-Up DC/DC Converter 3A (ISW), 1MHz, 34V Step-Up DC/DC Converter 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter 1.2A (IOUT), 600kHz Synchronous Buck-Boost DC/DC Converter 1.1A (ISW), 1.3MHz, High Efficiency Step-Up DC/DC Converter
ThinSOT is a trademark of Linear Technology Corporation.
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT 0306 REV A * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2004

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