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LT1110 Micropower DC-DC Converter Adjustable and Fixed 5V, 12V
FEATURES
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DESCRIPTIO
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Operates at Supply Voltages From 1.0V to 30V Works in Step-Up or Step-Down Mode Only Three External Off-the-Shelf Components Required Low-Battery Detector Comparator On-Chip User-Adjustable Current Limit Internal 1A Power Switch Fixed or Adjustable Output Voltage Versions Space-Saving 8-Pin MiniDIP or S8 Package
The LT1110 is a versatile micropower DC-DC converter. The device requires only three external components to deliver a fixed output of 5V or 12V. The very low minimum supply voltage of 1.0V allows the use of the LT1110 in applications where the primary power source is a single cell. An on-chip auxiliary gain block can function as a low battery detector or linear post regulator. The 70kHz oscillator allows the use of surface mount inductors and capacitors in many applications. Quiescent current is just 300A, making the device ideal in remote or battery powered applications where current consumption must be kept to a minimum. The device can easily be configured as a step-up or step-down converter, although for most step-down applications or input sources greater than 3V, the LT1111 is recommended. Switch current limiting is user-adjustable by adding a single external resistor. Unique reverse battery protection circuitry limits reverse current to safe, nondestructive levels at reverse supply voltages up to 1.6V.
APPLICATI
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Pagers Cameras Single-Cell to 5V Converters Battery Backup Supplies Laptop and Palmtop Computers Cellular Telephones Portable Instruments Laser Diode Drivers Hand-Held Inventory Computers
TYPICAL APPLICATI
All Surface Mount Single Cell to 5V Converter
SUMIDA CD54-470K 47H 1 I LIM 1.5V AA CELL* 2 V IN SW1 90 MBRS120T3 5V
EFFICIENCY (%)
85 80 75 70 65 60 15F TANTALUM 55 50 0
LT1110 * TA01
3
VIN = 1.00V
LT1110-5 SENSE 8 SW2 4
GND 5
+
OPERATES WITH CELL VOLTAGE 1.0V *ADD 10F DECOUPLING CAPACITOR IF BATTERY IS MORE THAN 2" AWAY FROM LT1110.
5
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Efficiency
VIN = 1.50V VIN = 1.25V 15 20 25 30 35 40 LOAD CURRENT (mA)
LT1110 * TA02
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1
LT1110 ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW ILIM 1 VIN 2 SW1 3 SW2 4 8 7 6 5 FB (SENSE)* SET A0 GND
Supply Voltage, Step-Up Mode ................................ 15V Supply Voltage, Step-Down Mode ........................... 36V SW1 Pin Voltage ...................................................... 50V SW2 Pin Voltage ......................................... - 0.5V to VIN Feedback Pin Voltage (LT1110) .............................. 5.5V Switch Current ........................................................ 1.5A Maximum Power Dissipation ............................. 500mW Operating Temperature Range ..................... 0C to 70C Storage Temperature Range .................. -65C to 150C Lead Temperature (Soldering, 10 sec.)................. 300C
ORDER PART NUMBER LT1110CN8 LT1110CN8-5 LT1110CN8-12
N8 PACKAGE 8-LEAD PLASTIC DIP *FIXED VERSIONS TJMAX = 90C, JA = 130C/W
TOP VIEW ILIM 1 VIN 2 SW1 3 SW2 4 8 7 6 5 FB (SENSE)* SET A0 GND
LT1110CS8 LT1110CS8-5 LT1110CS8-12 S8 PART MARKING 1110 11105 11012
S8 PACKAGE 8-LEAD PLASTIC SOIC *FIXED VERSIONS TJMAX = 90C, JA = 150C/W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS TA = 25C, VIN = 1.5V, unless otherwise noted.
SYMBOL IQ VIN PARAMETER Quiescent Current Input Voltage CONDITIONS Switch Off Step-Up Mode Step-Down Mode Comparator Trip Point Voltage VOUT Output Sense Voltage Comparator Hysteresis Output Hysteresis fOSC DC tON IFB ISET VAO Oscillator Frequency Duty Cycle Switch ON Time Feedback Pin Bias Current Set Pin Bias Current AO Output Low Reference Line Regulation LT1110, VFB = 0V VSET = VREF IAO = -300A, VSET = 150mV 1.0V VIN 1.5V 1.5V VIN 12V Full Load (VFB < VREF) LT1110 (Note 1) LT1110-5 (Note 2) LT1110-12 (Note 2) LT1110 LT1110-5 LT1110-12
q q q q q q q q q q q q q q q q q
MIN 1.15 1.0 210 4.75 11.4
TYP 300
MAX 12.6 12.6 30
UNITS A V V V mV V V mV mV mV kHz % s nA nA V %/V %/V
220 5.00 12.00 4 90 200
230 5.25 12.6 8 180 400 90 78 12.5 150 300 0.4 1.0 0.1
52 62 7.5
70 69 10 70 100 0.15 0.35 0.05
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LT1110
ELECTRICAL CHARACTERISTICS TA = 25C, VIN = 1.5V, unless otherwise noted.
SYMBOL VCESAT PARAMETER Switch Saturation Voltage Step-Up Mode CONDITIONS VIN = 1.5V, ISW = 400mA
q
MIN
TYP 300 400
MAX 400 600 550 750 1000
UNITS mV mV mV mV mV V/V mA mA %/C
VIN = 1.5V, ISW = 500mA
q
VIN = 5V, ISW = 1A AV IREV ILIM A2 Error Amp Gain Reverse Battery Current Current Limit Current Limit Temperature Coefficient ILEAK VSW2 Switch OFF Leakage Current Maximum Excursion Below GND Measured at SW1 Pin ISW1 10A, Switch Off RL = 100k (Note 3) (Note 4) 220 Between ILIM and VIN
q
700 1000 5000 750 400 - 0.3 1 - 400
10 - 350
A mV
The q denotes the specifications which apply over the full operating temperature range. Note 1: This specification guarantees that both the high and low trip point of the comparator fall within the 210mV to 230mV range. Note 2: This specification guarantees that the output voltage of the fixed versions will always fall within the specified range. The waveform at the sense pin will exhibit a sawtooth shape due to the comparator hysteresis.
Note 3: 100k resistor connected between a 5V source and the AO pin. Note 4: The LT1110 is guaranteed to withstand continuous application of +1.6V applied to the GND and SW2 pins while VIN, ILIM, and SW1 pins are grounded.
TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
100
OSCILLATOR FREQUENCY (KHz) 80 78
90 80 70 60 50 40 -50
FREQUENCY (KHz)
72 70 68 66 64 62 60
ON TIME (s)
3 6 12 15 18 21 24 27 30
LT1110 * TPC02
-25
0
25
50
TEMPERATURE (C)
LT1110 * TPC01
UW
75
Oscillator Frequency
14 13 12 11 10 9 8
Switch On Time
76 74
100
0
9
7 -50
-25
0
25
50
75
100
INPUT VOLTAGE (V)
TEMPERATURE (C)
LT1110 * TPC03
3
LT1110
TYPICAL PERFOR A CE CHARACTERISTICS
Duty Cycle
78 76 74 500 450 400 350
VCESAT (mV) VCESAT (V)
DUTY CYCLE (%)
72 70 68 66 64 62 60 58 -50 -25 0 25 50 75 100
TEMPERATURE (C)
LT1110 * TPC04
Switch On Voltage Step-Down Mode
1.4 VIN = 12V 1.2 1.0
100 95 90 85 80 75 70 65 60 55 50 45 40
OSCILLATOR FREQUENCY (KHz)
QUIESCENT CURRENT (A)
ON VOLTAGE (V)
0.8 0.6 0.4 0.2 0 0 0.2 0.4 0.6 0.8 1.0 ISWITCH (A)
LT1110 * TPC07
Quiescent Current
500 450 QUIESCENT CURRENT (A)
SWITCH CURRENT (A) 1.5 1.3 1.1 0.9 0.7 0.5 0.3 0.1
SWITCH CURRENT (A)
400 350 300 250 200 150 100 -50 -25 0 25 50 75 100
TEMPERATURE (C)
LT1110 * TPC10
4
UW
Switch Saturation Voltage
1.4
VIN = 1.5V ISW = 500mA
Saturation Voltage Step-Up Mode
VIN = 3.0V VIN= 2.0V VIN = 1.5V 1.2 1.0 0.8 V = 1.2V IN 0.6 0.4 0.2 0 VIN = 1.0V VIN = 5.0V
300 250 200 150 100 50 0 -50 - 25 0 25 50 75 100
0
0.2
0.4
0.6
0.8 1.0
1.2
1.4
1.6
TEMPERATURE (C)
LT1110 * TPC05
ISWITCH (A)
LT1110 * TPC06
Minimum/Maximum Frequency vs On Time
400 380 360 340 320 300 280 260 240 220 200
Quiescent Current
0C TA 70C
7
8
9
10
11
12
13
0
3
6
9
12 15 18 21 24 27 30
LT1110 * TPC09
SWITCH ON TIME (s)
LT1110 * TPC08
INPUT VOLTAGE (V)
Maximum Switch Current vs RLIM Step-Up
1.5 1.3 1.1 0.9 0.7 0.5 0.3 0.1
Maximum Switch Current vs RLIM Step-Down
STEP-UP MODE VIN 5V
STEP-DOWN MODE VIN = 12V
10
100 RLIM ()
1000
LT1110 * TPC11
10
100 RLIM ()
1000
LT1110 * TPC12
LT1110
TYPICAL PERFOR A CE CHARACTERISTICS
160 140 120 BIAS CURRENT (nA)
BIAS CURRENT (nA)
Set Pin Bias Current
80 60 40 20 0 -50 -25 0 25 50 75 100
70 60 50 40 30 20 10 0 -50
VREF (mV)
-25
100
TEMPERATURE (C)
LT1110 * TPC13
PI FU CTI
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GND (Pin 5): Ground. AO (Pin 6): Auxiliary Gain Block (GB) output. Open collector, can sink 300A. SET (Pin 7): GB input. GB is an op amp with positive input connected to SET pin and negative input connected to 220mV reference. FB/SENSE (Pin 8): On the LT1110 (adjustable) this pin goes to the comparator input. On the LT1110-5 and LT1110-12, this pin goes to the internal application resistor that sets output voltage.
ILIM (Pin 1): Connect this pin to VIN for normal use. Where lower current limit is desired, connect a resistor between ILIM and VIN. A 220 resistor will limit the switch current to approximately 400mA. VIN (Pin 2): Input supply voltage. SW1 (Pin 3): Collector of power transistor. For step-up mode connect to inductor/diode. For step-down mode connect to VIN. SW2 (Pin 4): Emitter of power transistor. For step-up mode connect to ground. For step-down mode connect to inductor/diode. This pin must never be allowed to go more than a Schottky diode drop below ground.
LT1110 BLOCK DIAGRA
V IN
220mV REFERENCE A1 COMPARATOR GND FB OSCILLATOR DRIVER SW2
LT1110 * BD01
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120 110 100 90 80
FB Pin Bias Current
226 224 222 220 218 216 214
Reference Voltage
0
25
50
75
100
212 -50
-25
0
25
50
75
100
TEMPERATURE (C)
LT1110 * TPC14
TEMPERATURE (C)
LT1110 * TPC15
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SET
+
A2 AO GAIN BLOCK/ERROR AMP I LIM SW1
-
Q1
5
LT1110
LT1110 OPERATI
The LT1110 is a gated oscillator switcher. This type architecture has very low supply current because the switch is cycled only when the feedback pin voltage drops below the reference voltage. Circuit operation can best be understood by referring to the LT1110 block diagram above. Comparator A1 compares the FB pin voltage with the 220mV reference signal. When FB drops below 220mV, A1 switches on the 70kHz oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch Q1. An adaptive base drive circuit senses switch current and provides just enough base drive to ensure switch saturation without overdriving the switch, resulting in higher efficiency. The switch cycling action raises the output voltage and FB pin voltage. When the FB voltage is sufficient to trip A1, the oscillator is gated off. A small amount of hysteresis built into A1 ensures loop stability without external frequency compensation. When the comparator is low the oscillator and all high current circuitry is turned off, lowering device quiescent current to just 300A for the reference, A1 and A2. The oscillator is set internally for 10s ON time and 5s OFF time, optimizing the device for step-up circuits where VOUT 3VIN, e.g., 1.5V to 5V. Other step-up ratios as well as step-down (buck) converters are possible at slight losses in maximum achievable power output.
LT1110-5, -12 BLOCK DIAGRA
SET
-5, LT1110 - -12 OPERATI
+
A2 AO GAIN BLOCK/ERROR AMP I LIM
V IN
-
SW1
220mV REF A1 COMPARATOR R1 R2 300k SENSE LT1110-5: R1 = 13.8k LT1110-12: R2 = 5.6k
LT1110 * BD02
OSCILLATOR DRIVER
Q1
SW2
GND
The LT1110-5 and LT1110-12 fixed output voltage versions have the gain setting resistors on-chip. Only three external components are required to construct a 5V or 12V output converter. 16A flows through R1 and R2 in the LT1110-5, and 39A flows in the LT1110-12. This current represents a load and the converter must cycle from time to time to maintain the proper output voltage. Output ripple, inherently present in gated oscillator designs, will typically run around 90mV for the LT1110-5 and 200mV for the LT1110-12 with the proper inductor/capacitor selection. This output ripple can be reduced considerably by using the gain block amp as a pre-amplifier in front of the FB pin. See the Applications section for details.
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A2 is a versatile gain block that can serve as a low battery detector, a linear post regulator, or drive an under voltage lockout circuit. The negative input of A2 is internally connected to the 220mV reference. An external resistor divider from VIN to GND provides the trip point for A2. The AO output can sink 300A (use a 47k resistor pull up to +5V). This line can signal a microcontroller that the battery voltage has dropped below the preset level. To prevent the gain block from operating in its linear region, a 2M resistor can be connected from AO to SET. This provides positive feedback. A resistor connected between the ILIM pin and VIN adjusts maximum switch current. When the switch current exceeds the set value, the switch is turned off. This feature is especially useful when small inductance values are used with high input voltages. If the internal current limit of 1.5A is desired, ILIM should be tied directly to VIN. Propagation delay through the current limit circuitry is about 700ns. In step-up mode, SW2 is connected to ground and SW1 drives the inductor. In step-down mode, SW1 is connected to VIN and SW2 drives the inductor. Output voltage is set by the following equation in either step-up or stepdown modes where R1 is connected from FB to GND and R2 is connected from VOUT to FB. R2 VOUT = 220mV + 1 . R1
(
)
(01)
LT1110
APPLICATI
S I FOR ATIO
Inductor Selection -- General A DC-DC converter operates by storing energy as magnetic flux in an inductor core, and then switching this energy into the load. Since it is flux, not charge, that is stored, the output voltage can be higher, lower, or opposite in polarity to the input voltage by choosing an appropriate switching topology. To operate as an efficient energy transfer element, the inductor must fulfill three requirements. First, the inductance must be low enough for the inductor to store adequate energy under the worst case condition of minimum input voltage and switch ON time. The inductance must also be high enough so maximum current ratings of the LT1110 and inductor are not exceeded at the other worst case condition of maximum input voltage and ON time. Additionally, the inductor core must be able to store the required flux; i.e., it must not saturate. At power levels generally encountered with LT1110 based designs, small surface mount ferrite core units with saturation current ratings in the 300mA to 1A range and DCR less than 0.4 (depending on application) are adequate. Lastly, the inductor must have sufficiently low DC resistance so excessive power is not lost as heat in the windings. An additional consideration is ElectroMagnetic Interference (EMI). Toroid and pot core type inductors are recommended in applications where EMI must be kept to a minimum; for example, where there are sensitive analog circuitry or transducers nearby. Rod core types are a less expensive choice where EMI is not a problem. Minimum and maximum input voltage, output voltage and output current must be established before an inductor can be selected. Inductor Selection -- Step-Up Converter In a step-up, or boost converter (Figure 4), power generated by the inductor makes up the difference between input and output. Power required from the inductor is determined by
PL = VOUT + V D - VIN MIN IOUT
(
)(
)
where VD is the diode drop (0.5V for a 1N5818 Schottky).
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Energy required by the inductor per cycle must be equal or greater than PL (02) fOSC in order for the converter to regulate the output. When the switch is closed, current in the inductor builds according to
-R't V IL ( t) = IN 1- e L R'
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(03)
where R' is the sum of the switch equivalent resistance (0.8 typical at 25C) and the inductor DC resistance. When the drop across the switch is small compared to VIN, the simple lossless equation
V I L t = IN t (04) L can be used. These equations assume that at t = 0, inductor current is zero. This situation is called "discontinuous mode operation" in switching regulator parlance. Setting "t" to the switch ON time from the LT1110 specification table (typically 10s) will yield IPEAK for a specific "L" and VIN. Once IPEAK is known, energy in the inductor at the end of the switch ON time can be calculated as 12 LI (05) 2 PEAK EL must be greater than PL/fOSC for the converter to deliver the required power. For best efficiency IPEAK should be kept to 1A or less. Higher switch currents will cause excessive drop across the switch resulting in reduced efficiency. In general, switch current should be held to as low a value as possible in order to keep switch, diode and inductor losses at a minimum. EL =
()
As an example, suppose 12V at 120mA is to be generated from a 4.5V to 8V input. Recalling equation (01),
P L = 12 V + 0.5 V - 4.5 V 120mA = 960mW. (06)
(01)
(
)(
)
Energy required from the inductor is 960mW PL = = 13.7J. f OSC 70kHz
(07)
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LT1110
APPLICATI
S I FOR ATIO
Picking an inductor value of 47H with 0.2 DCR results in a peak switch current of
I PEAK =
-1.0 W*10ms 4.5 V 1 - e 47mH = 862mA. 1.0 W
Substituting IPEAK into Equation 05 results in
EL =
1 47H 0.862 A 2 = 17.5J. 2
(
)(
)
Since 17.5J > 13.7J, the 47H inductor will work. This trial-and-error approach can be used to select the optimum inductor. Keep in mind the switch current maximum rating of 1.5A. If the calculated peak current exceeds this, an external power transistor can be used. A resistor can be added in series with the ILIM pin to invoke switch current limit. The resistor should be picked such that the calculated IPEAK at minimum VIN is equal to the Maximum Switch Current (from Typical Performance Characteristic curves). Then, as VIN increases, switch current is held constant, resulting in increasing efficiency. Inductor Selection -- Step-Down Converter The step-down case (Figure 5) differs from the step-up in that the inductor current flows through the load during both the charge and discharge periods of the inductor. Current through the switch should be limited to ~800mA in this mode. Higher current can be obtained by using an external switch (see Figure 6). The ILIM pin is the key to successful operation over varying inputs. After establishing output voltage, output current and input voltage range, peak switch current can be calculated by the formula IPEAK = 2 IOUT V OUT + V D DC V IN - V SW + V D (10)
where DC = duty cycle (0.69) VSW = switch drop in step-down mode VD = diode drop (0.5V for a 1N5818) IOUT = output current
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VOUT = output voltage VIN = minimum input voltage VSW is actually a function of switch current which is in turn a function of VIN, L, time and VOUT. To simplify, 1.5V can be used for VSW as a very conservative value. Once IPEAK is known, inductor value can be derived from
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(08)
(09)
L=
V IN MIN - V SW - V OUT * tON IPEAK
(11)
where tON = switch ON time (10s). Next, the current limit resistor RLIM is selected to give IPEAK from the RLIM Step-Down Mode curve. The addition of this resistor keeps maximum switch current constant as the input voltage is increased. As an example, suppose 5V at 250mA is to be generated from a 9V to 18V input. Recalling Equation (10),
IPEAK = 2 250mA 5 + 0.5 = 498mA . (12) 0.69 9 - 1.5 + 0.5
(
)
Next, inductor value is calculated using Equation (11)
L=
9 - 1.5 - 5 * 10s = 50H. 498mA
(13)
Use the next lowest standard value (47H). Then pick RLIM from the curve. For IPEAK = 500mA, RLIM = 82. Inductor Selection -- Positive-to-Negative Converter Figure 7 shows hookup for positive-to-negative conversion. All of the output power must come from the inductor. In this case,
P L = | VOUT | + V D IOUT .
(
)(
)
(14)
In this mode the switch is arranged in common collector or step-down mode. The switch drop can be modeled as a 0.75V source in series with a 0.65 resistor. When the
LT1110
APPLICATI
S I FOR ATIO
switch closes, current in the inductor builds according to
IL + =
()
VL R'
-R't 1- e L
where R' = 0.65 + DCRL VL = VIN - 0.75V As an example, suppose -5V at 75mA is to be generated from a 4.5V to 5.5V input. Recalling Equation (14),
P L = | -5 V | + 0.5 V 75mA = 413mW.
(
)(
)
Energy required from the inductor is 413mW PL = = 5.9J. 70kHz fOSC
Picking an inductor value of 56H with 0.2 DCR results in a peak switch current of
IPEAK =
(4.5V - 0.75V) 1 - e -0.85 * 10s = 621mA . 56H (0.65 + 0.2)
(18)
Substituting IPEAK into Equation (04) results in
EL =
1 56H 0.621A 2 = 10.8J. 2
(
)(
)
50mV/DIV
Since 10.8J > 5.9J, the 56H inductor will work. With this relatively small input range, RLIM is not usually necessary and the ILIM pin can be tied directly to VIN. As in the step-down case, peak switch current should be limited to ~800mA. Capacitor Selection Selecting the right output capacitor is almost as important as selecting the right inductor. A poor choice for a filter capacitor can result in poor efficiency and/or high output ripple. Ordinary aluminum electrolytics, while inexpensive and readily available, may have unacceptably poor Equivalent Series Resistance (ESR) and ESL (inductance). There are low ESR aluminum capacitors on the market specifically designed for switch mode DC-DC converters which work much better than general-purpose units. Tantalum
50mV/DIV
50mV/DIV
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(15)
(16)
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(17)
capacitors provide still better performance at more expense. We recommend OS-CON capacitors from Sanyo Corporation (San Diego, CA). These units are physically quite small and have extremely low ESR. To illustrate, Figures 1, 2 and 3 show the output voltage of an LT1110 based converter with three 100F capacitors. The peak switch current is 500mA in all cases. Figure 1 shows a Sprague 501D, 25V aluminum capacitor. VOUT jumps by over 120mV when the switch turns off, followed by a drop in voltage as the inductor dumps into the capacitor. This works out to be an ESR of over 240m. Figure 2 shows the same circuit, but with a Sprague 150D, 20V tantalum capacitor replacing the aluminum unit. Output jump is now about 35mV, corresponding to an ESR of 70m. Figure 3 shows the circuit with a 16V OS-CON unit. ESR is now only 20m.
(19)
5s/DIV
LT1110 * TA19
Figure 1. Aluminum
5s/DIV
LT1110 * TA20
Figure 2. Tantalum
5s/DIV
LT1110 * TA21
Figure 3. OS-CON
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LT1110
APPLICATI
Diode Selection
S I FOR ATIO
Speed, forward drop, and leakage current are the three main considerations in selecting a catch diode for LT1110 converters. General purpose rectifiers such as the 1N4001 are unsuitable for use in any switching regulator application. Although they are rated at 1A, the switching time of a 1N4001 is in the 10s-50s range. At best, efficiency will be severely compromised when these diodes are used; at worst, the circuit may not work at all. Most LT1110 circuits will be well served by a 1N5818 Schottky diode, or its surface mount equivalent, the MBRS130T3. The combination of 500mV forward drop at 1A current, fast turn ON and turn OFF time, and 4A to 10A leakage current fit nicely with LT1110 requirements. At peak switch currents of 100mA or less, a 1N4148 signal diode may be used. This diode has leakage current in the 1nA-5nA range at 25C and lower cost than a 1N5818. (You can also use them to get your circuit up and running, but beware of destroying the diode at 1A switch currents.) Step-Up (Boost Mode) Operation A step-up DC-DC converter delivers an output voltage higher than the input voltage. Step-up converters are not short circuit protected since there is a DC path from input to output. The usual step-up configuration for the LT1110 is shown in Figure 4. The LT1110 first pulls SW1 low causing VIN - VCESAT to appear across L1. A current then builds up in L1. At the end of the switch ON time the current in L1 is1: VIN IPEAK = t ON (20) L
L1 V IN R3* I LIM V IN SW1 FB R2 D1 V OUT
+
C1
LT1110
GND
SW2 R1
* = OPTIONAL
Figure 4. Step-Up Mode Hookup.
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Immediately after switch turn off, the SW1 voltage pin starts to rise because current cannot instantaneously stop flowing in L1. When the voltage reaches VOUT + VD, the inductor current flows through D1 into C1, increasing VOUT. This action is repeated as needed by the LT1110 to keep VFB at the internal reference voltage of 220mV. R1 and R2 set the output voltage according to the formula R2 VOUT = 1 + 220mV . R1
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(21)
Step-Down (Buck Mode) Operation A step-down DC-DC converter converts a higher voltage to a lower voltage. The usual hookup for an LT1110 based step-down converter is shown in Figure 5.
VIN R3 220
+
C2
I LIM
V IN
SW1 FB
LT1110 L1 SW2 GND D1 1N5818 R2 VOUT
+
C1 R1
LT1110 * TA15
Figure 5. Step-Down Mode Hookup
When the switch turns on, SW2 pulls up to VIN - VSW. This puts a voltage across L1 equal to VIN - VSW - VOUT, causing a current to build up in L1. At the end of the switch ON time, the current in L1 is equal to
IPEAK = VIN - VSW - VOUT L t ON . (22)
When the switch turns off, the SW2 pin falls rapidly and actually goes below ground. D1 turns on when SW2 reaches 0.4V below ground. D1 MUST BE A SCHOTTKY DIODE. The voltage at SW2 must never be allowed to go below -0.5V. A silicon diode such as the 1N4933 will allow SW2 to go to -0.8V, causing potentially destructive power
Note 1: This simple expression neglects the effects of switch and coil resistance. This is taken into account in the "Inductor Selection" section.
LT1110 * TA14
LT1110
APPLICATI S I FOR ATIO U
Converter" section with the following conservative expression for VSW: (23)
dissipation inside the LT1110. Output voltage is determined by R2 VOUT = 1 + 220mV . R1
(
)
R3 programs switch current limit. This is especially important in applications where the input varies over a wide range. Without R3, the switch stays on for a fixed time each cycle. Under certain conditions the current in L1 can build up to excessive levels, exceeding the switch rating and/or saturating the inductor. The 220 resistor programs the switch to turn off when the current reaches approximately 800mA. When using the LT1110 in stepdown mode, output voltage should be limited to 6.2V or less. Higher output voltages can be accommodated by inserting a 1N5818 diode in series with the SW2 pin (anode connected to SW2). Higher Current Step-Down Operation Output current can be increased by using a discrete PNP pass transistor as shown in Figure 6. R1 serves as a current limit sense. When the voltage drop across R1 equals a VBE, the switch turns off. For temperature compensation a Schottky diode can be inserted in series with the ILIM pin. This also lowers the maximum drop across R1 to VBE - VD, increasing efficiency. As shown, switch current is limited to 2A. Inductor value can be calculated based on formulas in the "Inductor Selection Step-Down
VIN 25V MAX R1 0.3 R2 220 R3 330 D1 1N5821 Q1 MJE210 OR ZETEX ZTX789A L1 VOUT
+
C2
VIN LT1110
IL SW1
R4 FB GND SW2 R5 VOUT = 220mV
Figure 6. Q1 Permits Higher-Current Switching. LT1110 Functions as Controller.
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V SW = V R1 + V SAT 0.9 V . (24) R2 provides a current path to turn off Q1. R3 provides base drive to Q1. R4 and R5 set output voltage.
Inverting Configurations The LT1110 can be configured as a positive-to-negative converter (Figure 7), or a negative-to-positive converter (Figure 8). In Figure 7, the arrangement is very similar to a step-down, except that the high side of the feedback is referred to ground. This level shifts the output negative. As in the step-down mode, D1 must be a Schottky diode, and VOUTshould be less than 6.2V. More negative output voltages can be accommodated as in the prior section.
+VIN R3 I LIM V IN SW1 FB C2 LT1110 L1 SW2 GND D1 1N5818 R1
+
+
C1 R2 -VOUT
LT1110 * TA03
Figure 7. Positive-to-Negative Converter
In Figure 8, the input is negative while the output is positive. In this configuration, the magnitude of the input voltage can be higher or lower than the output voltage. A level shift, provided by the PNP transistor, supplies proper polarity feedback information to the regulator.
L1 D1 +VOUT
+
C1
+
C1 I LIM V IN SW1
R1 2N3906
+
C2
LT1110 AO GND FB SW2 R2 VOUT =
(1 )
R4 + R5
LT1110 * TA16
( R1 ) 220mV + 0.6V R2
LT1110 * TA04
-VIN
Figure 8. Negative-to-Positive Converter
11
LT1110
APPLICATI
S I FOR ATIO
Using the ILIM Pin The LT1110 switch can be programmed to turn off at a set switch current, a feature not found on competing devices. This enables the input to vary over a wide range without exceeding the maximum switch rating or saturating the inductor. Consider the case where analysis shows the LT1110 must operate at an 800mA peak switch current with a 2.0V input. If VIN rises to 4V, peak current will rise to 1.6A, exceeding the maximum switch current rating. With the proper resistor selected (see the "Maximum Switch Current vs RLIM" characteristic), the switch current will be limited to 800mA, even if the input voltage increases. Another situation where the ILIM feature is useful occurs when the device goes into continuous mode operation. This occurs in step-up mode when
VOUT + VDIODE VI N - VSW < 1 . 1 - DC (25)
When the input and output voltages satisfy this relationship, inductor current does not go to zero during the switch OFF time. When the switch turns on again, the current ramp starts from the non-zero current level in the inductor just prior to switch turn on. As shown in Figure 9, the inductor current increases to a high level before the comparator turns off the oscillator. This high current can cause excessive output ripple and requires oversizing the output capacitor and inductor. With the ILIM feature, however, the switch current turns off at a programmed level as shown in Figure 10, keeping output ripple to a minimum. Figure 11 details current limit circuitry. Sense transistor Q1, whose base and emitter are paralleled with power switch Q2, is ratioed such that approximately 0.5% of Q2's collector current flows in Q1's collector. This current is passed through internal 80 resistor R1 and out through the ILIM pin. The value of the external resistor connected between ILIM and VIN set the current limit. When sufficient switch current flows to develop a VBE across R1 + RLIM, Q3 turns on and injects current into the oscillator, turning off the switch. Delay through this circuitry is approximately 800ns. The current trip point becomes less accurate for
12
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switch ON times less than 3s. Resistor values programming switch ON time for 800ns or less will cause spurious response in the switch circuitry although the device will still maintain output regulation.
IL SWITCH ON OFF
LT1110 * TA05
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Figure 9. No Current Limit Causes Large Inductor Current Build-Up
PROGRAMMED CURRENT LIMIT
IL
SWITCH
ON OFF
LT1110 * TA06
Figure 10. Current Limit Keeps Inductor Current Under Control
RLIM (EXTERNAL) VIN Q3 DRIVER OSCILLATOR Q1 ILIM R1 80 (INTERNAL) SW1 Q2 SW2
LT1110 * TA17
Figure 11. LT1110 Current Limit Circuitry
Using the Gain Block The gain block (GB) on the LT1110 can be used as an error amplifier, low battery detector or linear post regulator. The gain block itself is a very simple PNP input op amp with an open collector NPN output. The negative input of the gain block is tied internally to the 220mV reference. The positive input comes out on the SET pin.
LT1110
APPLICATI S I FOR ATIO U
Output ripple of the LT1110, normally 90mV at 5VOUT can be reduced significantly by placing the gain block in front of the FB input as shown in Figure 13. This effectively reduces the comparator hysteresis by the gain of the gain block. Output ripple can be reduced to just a few millivolts using this technique. Ripple reduction works with stepdown or inverting modes as well. For this technique to be effective, output capacitor C1 must be large, so that each switching cycle increases VOUT by only a few millivolts. 1000F is a good starting value.
L1 D1 V OUT
AO TO PROCESSOR
Arrangement of the gain block as a low battery detector is straightforward. Figure 12 shows hookup. R1 and R2 need only be low enough in value so that the bias current of the SET input does not cause large errors. 33k for R2 is adequate. R3 can be added to introduce a small amount of hysteresis. This will cause the gain block to "snap" when the trip point is reached. Values in the 1M-10M range are optimal. The addition of R3 will change the trip point, however.
+5V VIN R1 220mV REF SET LT1110 47k
- +
GND
VBAT
R2 R3 R1 = - 220mV ( VLB4.33A )
VLB = BATTERY TRIP POINT R2 = 33k R3 = 2M
LT1110 * TA07
Figure 12. Setting Low Battery Detector Trip Point Table 1. Inductor Manufacturers
MANUFACTURER Coiltronics International 984 S.W. 13th Court Pompano Beach, FL 33069 305-781-8900 Sumida Electric Co. USA 708-956-0666 PART NUMBERS CTX100-4 Series Surface Mount
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R3 270k
I LIM AO
V IN SW1
R2
+
C1
VBAT
LT1110 FB GND SET SW2 R1
VOUT = R2 + 1 220mV R1
(
)(
)
LT1110 * TA08
Figure 13. Output Ripple Reduction Using Gain Block Table 2. Capacitor Manufacturers
MANUFACTURER Sanyo Video Components 2001 Sanyo Avenue San Diego, CA 92173 619-661-6835 Nichicon America Corporation 927 East State Parkway Schaumberg, IL 60173 708-843-7500 Sprague Electric Company Lower Main Street Sanford, ME 04073 207-324-4140 Matsuo 714-969-2491 PART NUMBERS OS-CON Series
CD54 CDR74 CDR105 Surface Mount
PL Series
150D Solid Tantalums 550D Tantalex
267 Series Surface Mount
Table 3. Transistor Manufacturers
MANUFACTURER Zetex Commack, NY 516-543-7100 PART NUMBERS ZTX Series FZT Series Surface Mount
13
LT1110
TYPICAL APPLICATI
+5V 10%
1 = PROGRAM 0 = SHUTDOWN
22nF
1N4148
1.5V 8
* ADJUST R1
| TOKO 262LYF-0076M
* LASER DIODE CASE COMMON TO +BATTERY TERMINAL * 170mA CURRENT DRAIN FROM 1.5V CELL (50mA DIODE) * NO OVERSHOOT 1.5V Powered Laser Diode Driver
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+
S
All Surface Mount Flash Memory VPP Generator
L1* 47H MMBT4403 MBRS12OT3
10k I LIM 22F 1k GND MMBF170 V IN SW1 VPP 12V 120MA 47F 20V
LT1110CS8-12 SENSE SW2
+
*L1= SUMIDA CD105-470M
LT1110 * TA18
1.5V Powered Laser Diode Driver
TOSHIBA TOLD-9211 4.7k 2N3906 1 I LIM 6 AO 2 V IN 3 SW1 10 220 MJE210 0.22 F CERAMIC
+
C1 100 F OS-CON
2
LT1110 FB GND 5 SET SW2 4 7 1k* R1 1N5818 L1| 2.2 H
FOR CHANGE IN LASER OUTPUT POWER
LT1110 * TA13
LT1110
TYPICAL APPLICATI
All Surface Mount 3V to 5V Step-Up Converter
L1* 47H
220
I LIM 3V 2x AA CELL
V IN SW1
LT1110-5 SENSE SW2
GND
*L1 = COILCRAFT 1812LS-473
LT1110 * TA09
All Surface Mount 1.5V to +10V, +5V Dual Output Step-Up Converter
L1* 82H
I LIM
V IN SW1
1.5V AA OR AAA CELL
LT1110 FB SW2
GND
+
4.7F
= MBRL120 *L1 = COILCRAFT 1812LS-823
LT1110 * TA11
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.
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4.7F
S
All Surface Mount 9V to 5V Step-Down Converter
220 I LIM
MBRL120
V IN SW1
9V
LT1110-5 SENSE SW2
+
5V 40mA 10F
GND
L1* 47H
MBRL120
+
10F
5V 40mA
*L1 = COILCRAFT 1812LS-473
LT1110 * TA10
All Surface Mount 1.5V to 5V Dual Output Step-Up Converter
L1* 82H
+10V 3mA
+
490k +5V 3mA
I LIM
V IN SW1
+
4.7F
1.5V AA OR AAA CELL
LT1110 SENSE SW2 +5V 4mA
GND
+
11k 4.7F
+
4.7F
-5V 4mA 4.7F
+
= MBRL120 *L1 = COILCRAFT 1812LS-823
LT1110 * TA12
15
LT1110
PACKAGE DESCRIPTIO U
Dimensions in inches (millimeters) unless otherwise noted.
N8 Package 8-Lead Plastic DIP
0.400 (10.160) MAX 8 7 6 5
0.300 - 0.320 (7.620 - 8.128)
0.045 - 0.065 (1.143 - 1.651)
0.130 0.005 (3.302 0.127)
0.009 - 0.015 (0.229 - 0.381)
0.065 (1.651) TYP 0.125 (3.175) MIN 0.020 (0.508) MIN
0.250 0.010 (6.350 0.254)
(
+0.025 0.325 -0.015 +0.635 8.255 -0.381
)
0.045 0.015 (1.143 0.381) 0.100 0.010 (2.540 0.254)
1
2
3
4
0.018 0.003 (0.457 0.076)
S8 Package 8-Lead Plastic SOIC
0.189 - 0.197* (4.801 - 5.004) 0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP 0.053 - 0.069 (1.346 - 1.752) 8 0.004 - 0.010 (0.101 - 0.254) 0.228 - 0.244 (5.791 - 6.197) 0.150 - 0.157* (3.810 - 3.988) 7 6 5
0.016 - 0.050 0.406 - 1.270
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) BSC
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
1
2
3
4
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
LT/GP 0594 2K REV B * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1994

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