LT1108CN8-12

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Adjustable and Fixed 5V, 12V D UESCRIPTIOSFEATURE The LT1108 is a versatile micropower DC/DC converter. The device requires only four external components to deliver a fixed output of 5V or 12V. Supply voltage ranges from 2V to 12V in step-up mode and to 30V in step-down mode. The LT1108 functions equally well in step-up, step- down, or inverting applications. The LT1108 is pin-for-pin compatible with the LT1173, but has a duty cycle of 70%, resulting in increased output current in many applications. The LT1108 can deliver 150mA at 5V from a 2 AA cell input and 5V at 300mA from 9V in step-down mode. Quiescent current is just 110µA, making the LT1108 ideal for power conscious battery- operated systems. Switch current limit can be programmed with a single resistor. An auxiliary gain block can be configured as a low battery detector, linear post regulator, undervoltage lock- out circuit, or error amplifier. n Operates at Supply Voltages from 2V to 30V n Consumes Only 110µA Supply Current n Works in Step-Up or Step-Down Mode n Only Four External Components Required n Low Battery Detector Comparator On-Chip n User Adjustable Current Limit n Internal 1A Power Switch n Fixed or Adjustable Output Voltage Versions n Space Saving 8-Pin MiniDIP or S8 Package USA OPPLICATI n Palmtop Computers n 3V to 5V, 5V to 12V Converters n 9V to 5V, 12V to 5V Converters n LCD Bias Generators n Peripherals and Add-On Cards n Battery Backup Supplies n Cellular Telephones n Portable Instruments UA OPPLICATITYPICAL LOAD CURRENT (mA) EFFICIENCY (%) 10 100 LT1108 • TA02 VIN = 3V VIN = 2.5V VIN = 2V AVX TPS 330µF 6.3V L1* 100µH LT1108 • TA01 1N5817 150mA GND SW2 SW1 ILIM VIN LT1108-5 SENSE 100µF *L1 = +2 × AA CELLS GOWANDA GA20-103K COILTRONICS CTX100-4 SUMIDA CD105-101K 47Ω EfficiencyPalmtop Computer Logic Supply

A UGWA WU WARBSOLUTEX I T I S WU UPACKAGE/ORDER I FOR ATIO ORDER PART NUMBER LT1108CS8 LT1108CS8-5 LT1108CS8-12 S8 PART MARKING ORDER PART NUMBER LT1108CN8 LT1108CN8-5 LT1108CN8-12 TJMAX = 90°C, θJA = 150°C/WTJMAX = 90°C, θJA = 130°C/W TOP VIEW ILIM VIN SW1 SW2 FB (SENSE*) SET GND N8 PACKAGE 8-LEAD PLASTIC DIP *FIXED VERSIONS TOP VIEW ILIM VIN SW1 SW2 FB (SENSE*) SET GND S8 PACKAGE 8-LEAD PLASTIC SOIC *FIXED VERSIONS ELECTRICAL C CHARA TERISTICS TA = 25°C, VIN = 3V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS IQ Quiescent Current Switch OFF l 110 150 µA Quiescent Current, Boost Mode Configuration No Load LT1108-5 135 µA LT1108-12 250 µA VIN Input Voltage Step-Up Mode l 2 12.6 V Step-Down Mode l 30.0 V Comparator Trip Point Voltage LT1108 (Note 1) l 1.2 1.245 1.3 V VOUT Output Sense Voltage LT1108-5 (Note 2) l 4.75 5 5.25 V LT1108-12 (Note 2) l 11.4 12 12.6 V Comparator Hysteresis LT1108 l 51 0 m V Output Hysteresis LT1108-5 l 20 40 mV LT1108-12 l 50 100 mV fOSC Oscillator Frequency l 14 19 25 kHz Duty Cycle Full Load, Step-Up Mode l 63 70 78 % tON Switch-ON Time I LIM Tied to VIN, Step-Up Mode l 28 36 48 µs Feedback Pin Bias Current LT1108, V FB = 0V l 10 50 nA Set Pin Bias Current V SET = VREF l 20 100 nA VOL Gain Block Output Low I SINK = 100µA, VSET = 1V l 0.15 0.4 V Reference Line Regulation 2V ≤ VIN ≤ 5V l 0.20 0.400 %/V 5V ≤ VIN ≤ 30V l 0.02 0.075 %/V VSAT SWSAT Voltage, Step-Up Mode V IN = 3V, ISW = 650mA l 0.5 0.65 V VIN = 5V, ISW = 1A 0.8 1.00 V 1108 10805 10812

TA = 25°C, VIN = 3V, unless otherwise noted.ELECTRICAL C CHARA TERISTICS SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS VSAT SWSAT Voltage, Step-Down Mode V IN = 12V, ISW = 650mA 1.1 1.5 V l 1.7 V AV Gain Block Gain R L = 100k (Note 3) l 400 1000 V/ V Current Limit 220 Ω from ILIM to VIN 400 mA Current Limit Temperature Coefficient l – 0.3 %/ °C Switch OFF Leakage Current Measured at SW1 Pin 1 10 µA VSW2 Maximum Excursion Below GND I SW1 ≤ 10µA, Switch OFF –400 –350 mV Note 2: The output voltage waveform will exhibit a sawtooth shape due to the comparator hysteresis. The output voltage on the fixed output versions will always be within the specified range. Note 3: 100k resistor connected between a 5V source and the A0 pin. The l denotes specifications which apply over the full operating temperature range. Note 1: This specification guarantees that both the high and low trip points of the comparator fall within the 1.2V to 1.3V range. CCHARA TERISTICSUWATYPICALP E RFOR CE RLIM (Ω ) SWITCH CURRENT (mA) 100 1000 LT1108 • TPC03 1000 1200 1100 900 800 700 600 500 400 300 200 100 2V ≤ VIN ≤ 5V Saturation Voltage Step-Up Mode (SW2 Pin Grounded) Switch ON Voltage Step-Down Mode (SW1 Pin Connected to VIN) Maximum Switch Current vs RLIM Saturation Voltage Step-Up Mode (SW2 Pin Grounded) RLIM (Ω ) 100 SWITCH CURRENT (mA) 200 400 600 800 1000 LT1108 • TPC04 100 300 500 700 900 1000 VOUT = 5V VIN = 24V L = 500µH VIN = 12V L = 250µH Supply Current vs Switch Current Quiescent Current TEMPERATURE (°C) –50 QUIESCENT CURRENT (µA) 100 105 25 75 LT1108 • TPC06 –25 0 50 110 115 120 100 SWITCH CURRENT (A) VCESAT (V) 0.2 0.4 0.6 0.8 1.2 0.2 0.4 0.6 0.8 LT1108 • TPC01 1.0 1.2 1.0 VIN = 2V VIN = 5V VIN = 3V SWITCH CURRENT (A) SWITCH ON VOLTAGE (V) 1.1 1.2 1.3 0.6 LT1108 • TPC02 1.0 0.9 0.2 0.4 0.8 0.8 0.7 1.4 0.50.1 0.3 0.7 SWITCH CURRENT (mA) SUPPLY CURRENT (mA) 800 LTC1108 • TPC05 200 400 600 1000 VIN = 5V VIN = 2V

CCHARA TERISTICSUWATYPICALP E RFOR CE GND (Pin 5): Ground. AO (Pin 6): Auxiliary gain block (GB) output. Open collector, can sink 100µA. SET (Pin 7): GB input. GB is an op amp with positive input connected to SET pin and negative input connected to 1.245V reference. FB/SENSE (Pin 8): On the LT1108 (adjustable) this pin goes to the comparator input. On the LT1108-5 and LT1108-12, this pin goes to the internal application resistor that sets output voltage. Oscillator Frequency Duty Cycle Switch-ON Time TEMPERATURE (°C) FREQUENCY (kHz) LT1108 • TPC07 –50 25 75–25 0 50 100 TEMPERATURE (°C) DUTY CYCLE (%) LT1108 • TPC08 –50 25 75–25 0 50 100 Minimum/Maximum Frequency Switch Saturation Voltage Switch Saturation Voltage vs ON-Time Step-Up Mode Step-Down Mode PI UFU UC USOTI 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 220Ω 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. TEMPERATURE (˚C) –50 25 75 LT1108 • TPC09 –25 0 50 100 SWITCH-ON TIME (µs) TEMPERATURE (°C) –50 VCESAT (V) 0.3 0.4 0.5 25 75 LT1108 • TPC11 0.2 0.1 –25 0 50 0.6 0.7 0.8 100 ISW = 650mA TEMPERATURE (°C) –50 0.8 VSAT (V) 1.0 1.1 1.2 1.3 1.4 1.5 –25 0 25 50 LT1108 • TPC12 1.6 1.7 1.8 0.9 100 ISW = 650mA ON-TIME (µs) FREQUENCY (kHz) LT1108 • TPC10 30 35 40 50

negative input of A2 is the 1.245V reference. A resistor divider from VIN to GND, with the mid-point connected to the SET pin provides the trip voltage in a low battery detector application. A0 can sink 100µA (use a 47k resis- tor pull-up to 5V). A resistor connected between the I LIM pin and V IN sets maximum switch current. When the switch current ex- ceeds the set value, the switch cycle is prematurely terminated. If current limit is not used, ILIM should be tied directly to V IN. Propagation delay through the current- limit circuitry is approximately 2µs. In step-up mode the switch emitter (SW2) is connected to ground and the switch collector (SW1) drives the induc- tor; in step-down mode the collector is connected to V IN and the emitter drives the inductor. The LT1108-5 and LT1108-12 are functionally identical to the LT1108. The -5 and -12 versions have on-chip voltage setting resistors for fixed 5V or 12V outputs. Pin 8 on the fixed versions should be connected to the output. No external resistors are needed. The LT1108 is a gated oscillator switcher. This type architecture has very low supply current because the switch is cycled when the feedback pin voltage drops below the reference voltage. Circuit operation can best be understood by referring to the LT1108 block diagram. Comparator A1 compares the feedback (FB) pin voltage with the 1.245V reference signal. When FB drops below 1.245V, A1 switches on the 19kHz oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch. The switch cycling action raises the output voltage and FB pin voltage. When the FB voltage is suffi- cient 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 output is low, the oscillator and all high current circuitry is turned off, lowering device quiescent current to just 110µA. The oscillator is set internally for 36µs ON-time and 17µs OFF-time, allowing continuous mode operation in many cases such as 2V to 5V converters. Continuous mode greatly increases available output power. Gain block A2 can serve as a low battery detector. The WIDA G R ABLOCK S LT1108 LT1108-5/LT1108-12 ILIM VIN GND FB SET GAIN BLOCK/ ERROR AMP COMPARATOR DRIVER SW1 SW2 1.245V REFERENCE OSCILLATOR LT1108 • BD ILIM VIN GND SET GAIN BLOCK/ ERROR AMP COMPARATOR DRIVER SW1 SW2 1.245V REFERENCE OSCILLATOR LT1108-5 • BD SENSE 753k LT1108-5: R1 = 250k LT1108-12: R1 = 87.4k

A DC/DC converter operates by storing energy as mag- netic 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 oppo- site in polarity to the input voltage by choosing an appro- priate switching topology. To operate as an efficient energy transfer element, the inductor must fulfill three requirements. First, the induc- tance 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 LT1108 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 LT1108 based designs, small surface mount ferrite core units with saturation current ratings in the 300mA to 1A range and DCR less than 0.4Ω (depending on application) are adequate. Lastly, the inductor must have sufficiently low DC resis- tance so excessive power is not lost as heat in the windings. An additional consideration is Electro-Magnetic Interfer- ence (EMI). Toroid and pot core type inductors are recom- mended 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. Step-Up Converter In a step-up, or boost converter (Figure 1), power generated by the inductor makes up the difference between input and output. Power required from the inductor is determined by PV V V IL OUT D IN MIN OUT=+() ( ) ±( ) 01 USA OPPLICATI WU UI FOR ATIO where VD is the diode drop (0.5V for a 1N5818 Schottky). Energy required by the inductor per cycle must be equal or greater than P fL OSC/( ) 02 in order for the converter to regulate the output. When the switch is closed, current in the inductor builds according to It V R eL IN Rt L() © ±( ) 10 3 where R' is the sum of the switch equivalent resistance (0.8Ω typical at 25 °C) and the inductor DC resistance. When the drop across the switch is small compared to VIN, the simple lossless equation It V L tL IN() = ()04 can be used. These equations assume that at t = 0, inductor current is zero. This situation is called “discontinu- ous mode operation” in switching regulator parlance. Setting “t” to the switch-ON time from the LT1108 specifi- cation table (typically 36µs) will yield I PEAK 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 EL IL PEAK= 1 2 052 () 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. As an example, suppose 12V at 30mA is to be generated from a 2V to 3V input. Recalling equation (01), P V V V mA mWL =+() ( ) =12 0 5 2 30 315 06.± ( )

USA OPPLICATI WU UI FOR ATIO Energy required from the inductor is P f mW kHz JL OSC ==315 19 16 6 07.( )µ Picking an inductor value of 100µH with 0.2Ω DCR results in a peak switch current of I V em APEAK s  = 10 1 605 08 10 3 6 100. ±( ) Ω Ωµ µ Substituting IPEAK into Equation 04 results in EH A JL = () ( ) =1 2 100 6 605 18 3 092µµ .. ( ) Since 18.3µJ > 16.6µJ, the 100µH 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 I LIM pin to invoke switch current limit. The resistor should be picked so the calculated IPEAK at minimum VIN is equal to the Maximum Switch Current (from Typical Performance Characteristic curves). Then, as V IN increases, switch current is held constant, resulting in increasing efficiency. Step-Down Converter The step-down case (Figure 2) 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 ~650mA in this mode. Higher current can be obtained by using an external switch (see Figure 3). The I LIM 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: I I DC VV VV VPEAK OUT OUT D IN SW D = + 2 10± () where DC = duty cycle (0.60) VSW = switch drop in step-down mode VD = diode drop (0.5V for a 1N5818) IOUT = output current 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 L VV V I tIN MIN SW OUT PEAK ON= −− × ()11 where tON = switch-ON time (36µs). 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 300mA is to be generated from a 12V to 24V input. Recalling Equation (10), I mA mAPEAK = () +  = 2 300 06 0 50 5 1 2 15 05 500 12. ±. . () Next, inductor value is calculated using Equation (11) L mA sH==12 1 5 5 500 36 396 13±.± ()µµ Use the next lowest standard value (330µH). Then pick RLIM from the curve. For IPEAK = 500mA, RLIM = 220Ω . Positive-to-Negative Converter Figure 4 shows hookup for positive-to-negative conver- sion. All of the output power must come from the inductor. In this case, PL = (VOUT+ VD)(IOUT) (14)

still maintain output regulation. Figure 9. Setting Low Battery Detector Trip Point Table 1. Inductor Manufacturers Table 2. Capacitor Manufacturers

1201 Sanyo Avenue

927 East State Parkway

Table 3. Transistor Manufacturers

87 Modular Avenue ZTX 849 (NPN)

Information furnished by Linear Technology Corporation is believed to be accurate and reliable. tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. input comes out on the SET pin. Figure 8. LT1108 Current Limit Circuitry

6.5V-20V to 5V Step-Down Converter5V to –5V Converter 8-Lead Plastic SOIC N8 03930.045 ± 0.015 (1.143 ± 0.381) 0.100 ± 0.010 (2.540 ± 0.254) 0.065 (1.651) TYP 0.045 – 0.065 (1.143 – 1.651) 0.130 ± 0.005 (3.302 ± 0.127) 0.020 (0.508) MIN 0.018 ± 0.003 (0.457 ± 0.076) 0.125 (3.175) MIN 12 3 4 87 6 5 0.250 ± 0.010 (6.350 ± 0.254) 0.400 (10.160) MAX 0.009 – 0.015 (0.229 – 0.381) 0.300 – 0.320 (7.620 – 8.128) 0.325 +0.025 –0.015 +0.635 –0.3818.255() 8-Lead Plastic DIP Dimensions in inches (millimeters) unless otherwise noted. UPACKAGE DESCRIPTIO LT/GP 0493 10K REV 0  LINEAR TECHNOLOGY CORPORA TION 1993 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 l FAX: (408) 434-0507 l TELEX: 499-3977 1 2 3 4 0.150 – 0.157 (3.810 – 3.988) 8 7 6 5 0.189 – 0.197 (4.801 – 5.004) 0.228 – 0.244 0.406 – 1.270 0.010 – 0.020 0°– 8° TYP 0.008 – 0.010 (0.203 – 0.254) SO8 0393 0.053 – 0.069 (1.346 – 1.752) 0.014 – 0.019 (0.355 – 0.483) 0.004 – 0.010 (0.101 – 0.254) 0.050 (1.270) BSC SW2 SW1 LT1108 • TA03 ILIM SENSE GND 220Ω LT1108-5 MBRS130T3 330µF L1* 300µH –5V OUTPUT 150mA 33pF VIN 5V INPUT * L1 = COILTRONICS CTX300-4 VIN * L1 = COILTRONICS CTX100-4 100Ω 100Ω 220Ω ZETEX ZTX-949 0.22Ω SW2 SW1 ILIM SENSE GND LT1108-5 VIN 47µF 1N5818 L1* 100µH 330µF VIN 6.5V TO 20V 5VOUT 200mA AT 6.5VIN 500mA AT 8VIN LT1108 • TA04