LT1107 Micropower DC/DC Converter Adjustable and Fixed 5V, 12V FEATURES DESCRIPTIOU n Operates at Supply Voltages from 2V to 30V The LT®1107 is a versatile micropower DC/DC converter. n Consumes Only 320m A Supply Current The device requires only three external components to n Works in Step-Up or Step-Down Moden deliver a fixed output of 5V or 12V. Supply voltage ranges Only Three External Components Required from 2V to 12V in step-up mode and to 30V in step-down n Low-Battery Detector Comparator On-Chip mode. The LT1107 functions equally well in step-up, step- n User Adjustable Current Limit down, or inverting applications. n Internal 1A Power Switch The LT1107 is pin-for-pin compatible with the LT1111, but n Fixed or Adjustable Output Voltage Versions has a duty cycle of 70%, resulting in increased output n Space Saving 8-Pin MiniDIP or SO-8 Package current in many applications. The LT1107 can deliver 150mA at 5V from a 2AA cell input and 5V at 300mA from APPLICATIOU S 24V in step-down mode. Quiescent current is just 320m A, making the LT1107 ideal for power-conscious battery- n Palmtop Computers operated systems. The 63kHz oscillator is optimized to n 3V to 5V, 5V to 12V Converters work with surface mount inductors and capacitors. n 24V to 5V, 12V to 5V Converters n LCD Bias Generators Switch current limit can be programmed with a single n Peripherals and Add-On Cards resistor. An auxiliary gain block can be configured as a n low-battery detector, linear post regulator, undervoltage n Cellular Telephones lock-out circuit, or error amplifier. n Portable Instruments , LTC and LT are registered trademarks of Linear Technology Corporation TYPICAL APPLICATIOU Palmtop Computer Logic Supply Efficiency L1* 33m H MBRS120T3 82 5V 80 VIN = 3V 150mA 47W 78 VIN = 2V 2 · AA + ILIM VISNW1 + CY (%) 7764 VIN = 2.5V ALKCAELLINLSE 47m F LT1107-5 100m F FICIEN 72 SENSE EF 70 GND SW2 68 66 64 * SUMIDA CD54-330K 1107 TA01 1 10 100 400 COILCRAFT DT3316-473 LOAD CURRENT (mA) 1107 TA02 1107fa 1

LT1107 ABSOLUTE WAXIWUW RATIUGS (Note 1) Supply Voltage (V )............................................... 36V Maximum Switch Current...................................... 1.5A IN SW1 Pin Voltage (V )......................................... 50V Operating Temperature Range SW1 SW2 Pin Voltage (V )............................ –0.5V to V LT1107C................................................0(cid:176) C to 70(cid:176) C SW2 IN Feedback Pin Voltage (LT1107)................................ 5V LT1107I............................................ –45(cid:176) C to 85(cid:176) C Sense Pin Voltage (LT1107-5, LT1107-12) ............ 36V LT1107M(OBSOLETE) ...............–55(cid:176) C to 125(cid:176) C Maximum Power Dissipation............................ 500mW Storage Temperature Range................. –65(cid:176) C to 150(cid:176) C Set Pin Voltage...................................................... 5.5V Lead Temperature (Soldering, 10 sec)..................300(cid:176) C PACKAGE/ORDER IUFORWATIOU ORDER PART ORDER PART TOP VIEW NUMBER NUMBER ILIM 1 8 FB (SENSE)* VIN 2 7 SET LT1107CN8 TOP VIEW LT1107CS8 LT1107CN8-5 LT1107CS8-5 SW1 3 6 AO ILIM 1 8 FB(SENSE)* LT1107CN8-12 LT1107CS8-12 SW2 4 5 GND VIN 2 7 SET LT1107IS8 N8 PACKAGE SW1 3 6 AO 8-LEAD PLASTIC DIP SW2 4 5 GND * FIXED VERSIONS S8 PART MARKING TJMAX = 90(cid:176)C, q JA = 130(cid:176)C/W (N) S8 PACKAGE 8-LEAD PLASTIC SO J8 PACKAGE LT1107MJ8 *FIXED VERSIONS 1107 TJMAX 8=- L1E5A0D(cid:176)C C, EqRJAA =M 1IC2 0D(cid:176)ICP/W (J) LLTT11110077MMJJ88--512 TJMAX = 90(cid:176)C, q JA = 150(cid:176)C/W 11110077512 1107I OBSOLETE PACKAGE Consider the N8 Package for Alternate Source Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, V = 3V, military or commercial version, T = 25(cid:176) C, unless otherwise noted. IN A SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS I Quiescent Current Switch OFF 320 450 m A Q Quiescent Current, Step-Up Mode Configuration No Load LT1107-5 360 m A LT1107-12 550 m A V Input Voltage Step-Up Mode l 2 12.6 V IN Step-Down Mode l 30.0 V Comparator Trip Point Voltage LT1107 (Note 2) l 1.2 1.25 1.3 V V Output Sense Voltage LT1107-5 (Note 3) l 4.75 5 5.25 V OUT LT1107-12 (Note 3) l 11.40 12 12.60 V Comparator Hysteresis LT1107 l 8 12.5 mV Output Hysteresis LT1107-5 l 32 50 mV LT1107-12 l 75 120 mV f Oscillator Frequency 50 63 77 kHz OSC Duty Cycle, Step-Up Mode Full Load 64 70 76 % t Switch ON Time, Step-Up Mode I Tied to V 8.8 11 12.7 m s ON LIM IN 1107fa 2

LT1107 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, V = 3V, military or commercial version, T = 25(cid:176) C, unless otherwise noted. IN A SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Feedback Pin Bias Current LT1107, V = 0V l 70 120 nA FB Set Pin Bias Current V = V l 70 300 nA SET REF V Gain Block Output Low I = 300m A, V = 1V l 0.15 0.4 V OL SINK SET Reference Line Regulation 5V £ V £ 30V l 0.02 0.075 %/V IN A Gain Block Gain R = 100k (Note 4) l 1000 6000 V/V V L Current Limit 220W to I to V 400 mA LIM IN Current Limit Temperature Coefficient l –0.3 %/(cid:176) C Switch OFF Leakage Current Measured at SW1 Pin, V = 12V 1 10 m A SW1 V Maximum Excursion Below GND I £ 10m A, Switch OFF –400 –350 mV SW2 SW1 The l denotes the specifications which apply over the full operating temperature range, V = 3V, –55(cid:176) C £ T £ 125(cid:176) C, unless IN A otherwise noted. LT1107M SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS I Quiescent Current Switch OFF l 500 m A Q f Oscillator Frequency l 40 63 95 kHz OSC DC Duty Cycle Step-Up Mode l 56 69 81 % Step-Down Mode, V = 12V l 45 60 73 % IN t Switch ON Time Step-Up Mode l 7 11 15 m s ON Step-Down Mode, V = 12V l 5 9 13 m s IN Reference Line Regulation 2V £ V £ 5V, 0(cid:176) C £ T £ 125(cid:176) C 0.2 0.4 %/V IN A 2.4V £ V £ 5V, T = –55(cid:176) C 0.8 %/V IN A V Switch Saturation Voltage, Step-Up Mode 0(cid:176) C £ T £ 125(cid:176) C, I = 500mA 0.5 0.65 V SAT A SW T = –55(cid:176) C, I = 400mA 0.5 0.65 V A SW Switch Saturation Voltage, Step-Down Mode V = 12V, I = 500mA IN SW 0(cid:176) C £ T £ 125(cid:176) C 1.5 V A T = –55(cid:176) C 2.0 V A The l denotes the specifications which apply over the full operating temperature range, V = 3V, 0(cid:176) C £ T £ 70(cid:176) C, unless otherwise IN A noted. LT1107C SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS I Quiescent Current Switch OFF l 450 m A Q f Oscillator Frequency l 50 63 88 kHz OSC DC Duty Cycle Step-Up Mode l 62 69 78 % Step-Down Mode, V = 12V l 50 60 70 % IN t Switch ON Time Step-Up Mode l 8 11 13.5 m s ON Step-Down Mode, V = 12V l 6 9 12.0 m s IN Reference Line Regulation 2V £ V £ 5V l 0.2 0.7 %/V IN V Switch Saturation Voltage, Step-Up Mode V = 3V, I = 650mA l 0.5 0.65 V SAT IN SW Switch Saturation Voltage, Step-Down Mode V = 12V, I = 650mA l 1.1 1.5 V IN SW Note 1: Absolute Maximum Ratings are those values beyond which the life Note 3: The output voltage waveform will exhibit a sawtooth shape due to of a device may be impaired. the comparator hysteresis. The output voltage on the fixed-output versions Note 2: This specification guarantees that both the high and low trip points will always be within the specified range. of the comparator fall within the 1.2V to 1.3V range. Note 4: 100k resistor connected between a 5V source and the AO pin. 1107fa 3

LT1107 TYPICAL PERFORWAUCE CHARACTERISTICS Saturation Voltage, Step-Up Mode Switch ON Voltage, Step-Down (SW2 Pin Grounded) Mode (SW1 Pin Connected to V ) Maximum Switch Current vs R IN LIM 1.2 1.4 1.5 1.4 1.0 1.3 1.3 SATURATION VOLTAGE (V) 000...864 VIN V= I2NV = 3V VIN = 5V SWITCH ON VOLTAGE (V) 1110....2109 SWITCH CURRENT (A) 110000001.........219876540 STEVPIN-D =O 1W2NVS2VTE £ PV-UINP £ 5V 0.2 0.8 0.3 0.2 0 0.7 0.1 0 0.2 0.4 0.6 0.8 1.0 1.2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 10 100 1000 SWITCH CURRENT (A) SWITCH CURRENT (A) RLIM (W ) 1107 G01 1107 G02 1107 G03 Quiescent Current Quiescent Current Oscillator Frequency 400 400 100 380 TA = 25°C 90 350 360 A) A) 80 mNT ( 300 mNT ( 340 Hz) 70 UIESCENT CURRE 225000 UIESCENT CURRE 332220860000 FREQUENCY (k 654000 Q Q 240 150 30 220 100 200 20 –55–35 –15 5 25 45 65 85 105 125 0 3 6 9 12 15 18 21 24 27 30 –55–35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) INPUT VOLTAGE (V) TEMPERATURE (°C) 1107 G05 1107 G06 1107 G07 Switch ON Time Duty Cycle Switch ON Time Step-Up Mode Step-Up Mode Step-Down Mode 16 85 13 15 80 12 14 75 11 s) 13 s) mN TIME ( 1121 YCLE (%) 7605 mN TIME ( 109 O C O TCH 10 UTY 60 TCH 8 WI 9 D WI S 55 S 7 8 50 6 7 6 45 5 –55–35 –15 5 25 45 65 85 105 125 –55–35 –15 5 25 45 65 85 105 125 –55–35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) TEMPERATURE (°C) TEMPERATURE (°C) 1107 G08 1107 G09 1107 G10 1107fa 4

LT1107 TYPICAL PERFORWAUCE CHARACTERISTICS Minimum/Maximum Frequency Minimum/Maximum Frequency Duty Cycle vs ON Time, Step-Down Mode vs ON Time, Step-Up Mode Step-Down Mode 100 100 70 90 90 0°C £ TA £ 70°C 65 80 0°C £ TA £ 70°C 80 60 Hz) Hz) %) Y (k 70 Y (k 70 LE ( 55 C C C N N Y FREQUE 6500 FREQUE 6500 TA = 25°C DUTY C 5405 40 –55°C £ TA £ 125°C 40 –55°C £ TA £ 125°C 40 30 30 35 4 5 6 7 8 9 10 11 12 13 14 6 7 8 9 10 11 12 13 14 15 16 –55–35 –15 5 25 45 65 85 105 125 ON TIME (m s) ON TIME (m s) TEMPERATURE (°C) 1107 G11 1107 G12 1107 G13 LT1107-5 LT1107-12 LT1107 Output Voltage Output Voltage Feedback Voltage 5.3 12.20 1.30 1.29 12.15 5.2 1.28 VOLTAGE (V) 55..10 VOLTAGE (V)111222...100050 T VOLTAGE (V)111...222765 UTPUT 4.9 UTPUT 11.95 P POIN11..2243 O O11.90 TRI 1.22 4.8 11.85 1.21 4.7 11.80 1.20 –55–35 –15 5 25 45 65 85 105 125 –55–35 –15 5 25 45 65 85 105 125 –55–35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) TEMPERATURE (°C) TEMPERATURE (°C) 1107 G16 1107 G17 1107 G18 PIU FUUCTIOUS I (Pin 1): Connect this pin to V for normal use. Where GND (Pin 5): Ground. LIM IN lower current limit is desired, connect a resistor between AO (Pin 6): Auxiliary Gain Block (GB) Output. Open collector, ILIM and VIN. A 220W resistor will limit the switch current can sink 300m A. to approximately 400mA. SET (Pin 7): GB Input. GB is an op amp with positive input V (Pin 2): Input Supply Voltage. IN connected to SET pin and negative input connected to SW1 (Pin 3):Collector of Power Transistor. For step-up 1.25V reference. mode connect to inductor/diode. For step-down mode FB/SENSE (Pin 8): On the LT1107 (adjustable), this pin connect to V . IN goes to the comparator input. On the LT1107-5 and SW2 (Pin 4): Emitter of Power Transistor. For step-up LT1107-12, this pin goes to the internal application resistor mode connect to ground. For step-down mode connect to that sets output voltage. inductor/diode. This pin must never be allowed to go more than a Schottky diode drop below ground. 1107fa 5

LT1107 BLOCK DIAGRAWS LT1107 LT1107-5/LT1107-12 SET SET A2 AO A2 AO VIN VIN GAIN BLOCK/ GAIN BLOCK/ ERROR AMP ILIM SW1 ERROR AMP ILIM SW1 1.25V 1.25V REFERENCE REFERENCE A1 OSCILLATOR A1 OSCILLATOR DRIVER DRIVER COMPARATOR COMPARATOR R2 SW2 GND FB SW2 R1 220k LT1107-5: R1 = 73.5k 1107 BD01 GND SENSE LT1107-12: R1 = 25.5k 1107 BD02 OPERATIOU The LT1107 is a gated oscillator switcher. This type Gain block A2 can serve as a low-battery detector. The architecture has very low supply current because the negative input of A2 is the 1.25V reference. A resistor switch is cycled when the feedback pin voltage drops divider from VIN to GND, with the mid-point connected to below the reference voltage. Circuit operation can best be the SET pin provides the trip voltage in a low-battery understood by referring to the LT1107 block diagram. detector application. AO can sink 300m A (use a 22k Comparator A1 compares the feedback (FB) pin voltage resistor pull-up to 5V). with the 1.25V reference signal. When FB drops below A resistor connected between the I pin and V sets LIM IN 1.25V, A1 switches on the 63kHz oscillator. The driver maximum switch current. When the switch current ex- amplifier boosts the signal level to drive the output NPN ceeds the set value, the switch cycle is prematurely power switch. The switch cycling action raises the output terminated. If current limit is not used, I should be tied LIM voltage and FB pin voltage. When the FB voltage is suffi- directly to V . Propagation delay through the current limit IN cient to trip A1, the oscillator is gated off. A small amount circuitry is approximately 1m s. of hysteresis built into A1 ensures loop stability without In step-up mode the switch emitter (SW2) is connected to external frequency compensation. When the comparator ground and the switch collector (SW1) drives the induc- output is low, the oscillator and all high current circuitry is turned off, lowering device quiescent current to just 300m A. tor; in step-down mode the collector is connected to VIN and the emitter drives the inductor. The oscillator is set internally for 11m s ON time and 5m s The LT1107-5 and LT1107-12 are functionally identical to OFF time in step-up mode, optimizing the device for converters where V » 3V . The combination of high the LT1107. The -5 and -12 versions have on-chip voltage OUT IN setting resistors for fixed 5V or 12V outputs. Pin 8 on the duty cycle and the current limit feature enables continuous fixed versions should be connected to the output. No mode operation in many applications, increasing available external resistors are needed. output power. 1107fa 6

LT1107 APPLICATIOUS IUFORWATIOU Inductor Selection –– Step-Up Converter As an example, suppose 12V at 60mA is to be generated from a 3V to 6V input. Recalling equation (1), In a step-up, or boost converter (Figure 1), power gener- ( )( ) ated by the inductor makes up the difference between P = 12V +0.5V - 3V 60mA = 570mW (6) L input and output. Power required from the inductor is determined by: Energy required from the inductor is: ( ) (cid:230) (cid:246) PL =Ł VOUT +VD - VIN(MIN)ł IOUT (1) PL = 570mW =9.05m J (7) f 63kHz OSC where V is the diode drop (0.5V for a 1N5818 Schottky). D Energy required by the inductor per cycle must be equal or Picking an inductor value of 33m H with 0.2W DCR results greater than: in a peak switch current of: P /f (2) (cid:230) - 1W •11m s(cid:246) L OSC I = 3V (cid:231) 1- e 33m H (cid:247) =850mA (8) in order for the converter to regulate the output. PEAK 1W (cid:231) (cid:247) Ł ł When the switch is closed, current in the inductor builds according to: Substituting IPEAK into Equation 4 results in: ( )( ) (cid:230) - R¢t(cid:246) E = 1 33m H 0.85A 2 =11.9m1 J (9) V L I (t)= IN(cid:231) 1- e L (cid:247) (3) 2 L R¢ (cid:231) (cid:247) Ł ł Since 11.9m J > 9.05m J, the 33m H inductor will work. This where R¢ is the sum of the switch equivalent resistance trial-and-error approach can be used to select the opti- (0.8W typical at 25(cid:176) C) and the inductor DC resistance. mum inductor. When the drop across the switch is small compared to VIN, A resistor can be added in series with the ILIM pin to invoke the simple lossless equation: switch current limit. The resistor should be picked so the I (t)= VIN t (4) calculated IPEAK at minimum VIN is equal to the Maximum L Switch Current (from Typical Performance Characteristic L curves). Then, as V increases, peak switch current is IN can be used. These equations assume that at t = 0, held constant, resulting in increasing efficiency. inductor current is zero. This situation is called “discon- tinuous mode operation” in switching regulator parlance. Inductor Selection –– Step-Down Converter Setting “t” to the switch ON time from the LT1107 speci- The step-down case (Figure 2) differs from the step-up in fication table (typically 11m s) will yield I for a specific PEAK that the inductor current flows through the load during “L” and V . Once I is known, energy in the inductor IN PEAK both the charge and discharge periods of the inductor. at the end of the switch ON time can be calculated as: Current through the switch should be limited to ~650mA 1 E = LI2 (5) in this mode. Higher current can be obtained by using an L 2 PEAK external switch (see LT1111 and LT1110 data sheets). The E must be greater than P /f for the converter to deliver I pin is the key to successful operation over varying L L OSC LIM the required power. For best efficiency I should be inputs. PEAK kept to 1A or less. Higher switch currents will cause After establishing output voltage, output current and input excessive drop across the switch resulting in reduced voltage range, peak switch current can be calculated by the efficiency. In general, switch current should be held to as formula: low a value as possible in order to keep switch, diode and 2I Ø V +V ø inductor losses at a minimum. I = OUT Œ OUT D œ (10) PEAK DC ºŒ V - V + V ßœ IN SW D 1107fa 7

LT1107 APPLICATIOUS IUFORWATIOU where DC = duty cycle (0.50 in step-down mode) In this mode the switch is arranged in common collector V = switch drop in step-down mode or step-down mode. The switch drop can be modeled as SW a 0.75V source in series with a 0.65W resistor. When the V = diode drop (0.5V for a 1N5818) D switch closes, current in the inductor builds according to: I = output current OUT V = output voltage (cid:230) - R¢t(cid:246) OUT ( ) V VIN = minimum input voltage IL t = RL¢ (cid:231)(cid:231) 1- e L (cid:247)(cid:247) (15) Ł ł V is actually a function of switch current which is in turn SW a function of VIN, L, time, and VOUT. To simplify, 1.5V can where R¢ = 0.65W + DCRL be used for VSW as a very conservative value. VL = VIN – 0.75V Once IPEAK is known, inductor value can be derived from: As an example, suppose –5V at 50mA is to be generated from a 4.5V to 5.5V input. Recalling Equation (14), V ( ) - V - V IN MIN SW OUT ( )( ) L= · tON (11) P = - 5V+ 0.5V 50mA =275mW (16) I L PEAK where t = switch ON time (7m s). Energy required from the inductor is: ON Next, the current limit resistor RLIM is selected to give PL = 275mW =4.4m J (17) I from the Maximum Switch Current vs R curve. PEAK LIM f 63kHz OSC The addition of this resistor keeps maximum switch cur- rent constant as the input voltage is increased. Picking an inductor value of 100m H with 0.2W DCR results in a peak switch current of: As an example, suppose 5V at 300mA is to be generated from a 12V to 24V input. Recalling Equation (10): (4.5V- 0.75V) (cid:230) - 0.85W •m9 s(cid:246) 2(300mA) Ø 5+0.5 ø IPEAK = (0.65W +0W.2 ) (cid:231)(cid:231) 1- e 100m H (cid:247)(cid:247) I = Œ œ =600mA (12) Ł ł PEAK 0.50 ºŒ 12- 1.5+ 0.5ßœ =325mA (18) Next, inductor value is calculated using Equation (11): Substituting I into Equation (04) results in: PEAK 12- 1.5- 5 1( )( ) L= 7m s=6m4 H (13) E = 100m H 0.325A 2 =5.2m8 J (19) L 600mA 2 Use the next lowest standard value (56m H). Since 5.28m J > 3.82m J, the 100m H inductor will work. Then pick RLIM from the curve. For IPEAK = 600mA, RLIM With this relatively small input range, RLIM is not usually = 56W . necessary and the ILIM pin can be tied directly to VIN. As in the step-down case, peak switch current should be limited Inductor Selection –– Positive-to-Negative Converter to ~650mA. Figure 4 shows hookup for positive-to-negative conver- Step-Up (Boost Mode) Operation sion. All of the output power must come from the inductor. In this case, A step-up DC/DC converter delivers an output voltage ( )( ) higher than the input voltage. Step-up converters are not P = V +V I (14) short-circuit protected since there is a DC path from input L OUT D OUT to output. 1107fa 8

LT1107 APPLICATIOUS IUFORWATIOU The usual step-up configuration for the LT1107 is shown VIN + R3 in Figure 1. The LT1107 first pulls SW1 low causing VIN – C2 100W V to appear across L1. A current then builds up in L1. CESAT At the end of the switch ON time the current in L1 is1: ILIM VIN SW1 FB LT1107 V L1 I = INt (20) SW2 VOUT PEAK ON L GND R2 + D1 L1 D1 1N5818 C1 VIN VOUT R1 R3 R2 1107 F02 ILIM VIN SW1 + Figure 2. Step-Down Mode Hookup LT1107 C1 FB When the switch turns off, the SW2 pin falls rapidly and GND SW2 actually goes below ground. D1 turns on when SW2 R1 reaches 0.4V below ground. D1 MUST BE A SCHOTTKY DIODE. The voltage at SW2 must never be allowed to go 1107 F01 below –0.5V. A silicon diode such as the 1N4933 will allow Figure 1. Step-Up Mode Hookup SW2 to go to –0.8V, causing potentially destructive power Immediately after switch turn-off, the SW1 voltage pin dissipation inside the LT1107. Output voltage is deter- starts to rise because current cannot instantaneously stop mined by: flowing in L1. When the voltage reaches VOUT + VD, the (cid:230) R2(cid:246) ( ) inductor current flows through D1 into C1, increasing VOUT =(cid:231) 1+ (cid:247) 1.25V (23) Ł R1ł V . This action is repeated as needed by the LT1107 to OUT keep V at the internal reference voltage of 1.25V. R1 and FB R3 programs switch current limit. This is especially im- R2 set the output voltage according to the formula: portant in applications where the input varies over a wide range. Without R3, the switch stays on for a fixed time (cid:230) (cid:246) ( ) R2 VOUT =(cid:231) 1+ (cid:247) 1.25V (21) each cycle. Under certain conditions the current in L1 can Ł R1ł build up to excessive levels, exceeding the switch rating and/or saturating the inductor. The 100W resistor pro- Step-Down (Buck Mode) Operation grams the switch to turn off when the current reaches approximately 700mA. When using the LT1107 in step- A step-down DC/DC converter converts a higher voltage to down mode, output voltage should be limited to 6.2V or a lower voltage. The usual hookup for an LT1107 based less. Higher output voltages can be accommodated by step-down converter is shown in Figure 2. inserting a 1N5818 diode in series with the SW2 pin When the switch turns on, SW2 pulls up to V – V . This IN SW (anode connected to SW2). puts a voltage across L1 equal to V – V – V , IN SW OUT causing a current to build up in L1. At the end of the switch Inverting Configurations ON time, the current in L1 is equal to: The LT1107 can be configured as a positive-to-negative converter (Figure 3), or a negative-to-positive converter I = VIN- VSW- VOUT t (22) (Figure 4). In Figure 3, the arrangement is very similar to PEAK ON L a step-down, except that the high side of the feedback is referred to ground. This level shifts the output negative. As Note 1: This simple expression neglects the effects of switch and coil in the step-down mode, D1 must be a Schottky diode, and resistance. This is taken into account in the “Inductor Selection” section. 1107fa 9

LT1107 APPLICATIOUS IUFORWATIOU ‰ V ‰ should be less than 6.2V. More negative output Using the I Pin OUT LIM voltages can be accommodated as in the prior section. The LT1107 switch can be programmed to turn off at a set In Figure 4, the input is negative while the output is switch current, a feature not found on competing devices. positive. In this configuration, the magnitude of the input This enables the input to vary over a wide range without voltage can be higher or lower than the output voltage. A exceeding the maximum switch rating or saturating the level shift, provided by the PNP transistor, supplies proper inductor. Consider the case where analysis shows the polarity feedback information to the regulator. LT1107 must operate at an 800mA peak switch current with a 2V input. If V rises to 4V, the peak switch current IN +VIN + will rise to 1.6A, exceeding the maximum switch current C2 R3 rating. With the proper resistor selected (see the “Maxi- mum SwitchCurrent vs R ” characteristic), the switch LIM ILIM VIN SW1 current will be limited to 800mA, even if the input voltage FB LT1107 increases. L1 SW2 Another situation where the I feature is useful occurs LIM GND R1 + when the device goes into continuous mode operation. D1 C1 1N5818 This occurs in step-up mode when: R2 –V11O07U FT03 VOUT +VDIODE < 1 (24) Figure 3. Positive-to-Negative Converter V - V 1- DC IN SW When the input and output voltages satisfy this relation- L1 D1 ship, inductor current does not go to zero during the +VOUT + switch OFF time. When the switch turns on again, the R3 C1 R1 current ramp starts from the non-zero current level in the ILIM VIN 2N3906 inductor just prior to switch turn-on. As shown in Figure + SW1 5, the inductor current increases to a high level before the C2 LT1107 comparator turns off the oscillator. This high current can FB ( ) cause excessive output ripple and requires oversizing the GND SW2 R2 VOUT = RR 12 1.25V + 0.6V output capacitor and inductor. With the I feature, the LIM –VIN 1107 F04 switch turns off at the programmed current as shown in Figure 4. Negative-to-Positive Converter Figure 6, keeping output ripple to a minimum. 1107fa 10

LT1107 APPLICATIOUS IUFORWATIOU IL ON SWITCH OFF 1107 F05 Figure 5. No Current Limit Causes Large Inductor Current Build-Up PROGRAMMED CURRENT LIMIT IL ON SWITCH OFF 1107 F06 Figure 6. Current Limit Keeps Inductor Current Under Control 1107fa 11

LT1107 APPLICATIOUS IUFORWATIOU Figure 7 details current limit circuitry. Sense transistor A1, RLIM ILIM (EXTERNAL) whose base and emitter are paralleled with power switch VIN R1 Q2, is ratioed such that approximately 0.5% of Q2’s 80W (INTERNAL) collector current flows in Q1’s collector. This current is Q3 passed through internal 80W resistor R1 and out through DRIVER SW1 the I pin. The value of the external resistor connected Q1 LIM OSCILLATOR Q2 between I and V sets the current limit. When suffi- LIM IN SW2 cient switch current flows to develop a V across R1 + BE 1107 F07 R , Q3 turns on and injects current into the oscillator, LIM Figure 7. LT1107 Current Limit Circuitry turning off the switch. Delay through this circuitry is approximately 800ns. The current trip point becomes less accurate for switch ON times less than 3m s. Resistor 5V values programming switch ON time for 800ns or less will cause spurious response in the switch circuitry although VIN LT1107 47k the device will still maintain output regulation. R1 1.25V – REF Using the Gain Block AO TO VBAT PROCESSOR SET + The gain block (GB) on the LT1107 can be used as an error amplifier, low-battery detector or linear post regulator. GND R2 The gain block itself is a very simple PNP input op amp with R3 an open collector NPN output. The negative input of the ( ) gain block is tied internally to the 1.25V reference. The R1 = VL3B5 –.1 1m.2A5V positive input comes out on the SET pin. VLB = BATTERY TRIP POINT R2 = 33k Arrangement of the gain block as a low-battery detector is R3 = 1.6M 1107 F08 straightforward. Figure 8 shows hookup. R1 and R2 need Figure 8. Setting Low-Battery Detector Trip Point 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 L1 D1 the trip point is reached. Values in the 1M to 10M range are VOUT optimal. The addition of R3 will change the trip point, R3 however. 270k ILIM VIN R2 + C1 Output ripple of the LT1107, normally 50mV at 5V can AO SW1 OUT LT1107 be reduced significantly by placing the gain block in front VBAT FB SET of the FB input as shown in Figure 9. This effectively GND SW2 R1 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 step- VOUT = RR 21 +1 1.25V 1107 F09 down or inverting modes as well. For this technique to be effective, output capacitor C1 must be large, so that each Figure 9. Output Ripple Reduction Using Gain Block switching cycle increases V by only a few millivolts. OUT 1000m F is a good starting value. C1 should be a low ESR type as well. 1107fa 12

LT1107 PACKAGE DESCRIPTIOU J8 Package 8-Lead CERDIP (Narrow .300 Inch, Hermetic) (Reference LTC DWG # 05-08-1110) CORNER LEADS OPTION 0.405 (4 PLCS) (10.287) 0.005 MAX (0.127) MIN 8 7 6 5 0.023 – 0.045 (0.584 – 1.143) HALF LEAD OPTION 0.025 0.220 – 0.310 0.045 – 0.068 (0.635) (5.588 – 7.874) (1.143 – 1.727) RAD TYP FULL LEAD OPTION 1 2 3 4 0.200 0.300 BSC (5.080) (0.762 BSC) MAX 0.015 – 0.060 (0.381 – 1.524) 0.008 – 0.018 0° – 15° (0.203 – 0.457) 0.045 – 0.065 0.125 NOTE: LEAD DIMENSIONS APPLY TO SOLDER DIP/PLATE (1.143 – 1.651) OR TIN PLATE LEADS 3.175 MIN 0.014 – 0.026 0.100 (0.360 – 0.660) (2.54) BSC J8 1298 OBSOLETE PACKAGE 1107fa 13

LT1107 PACKAGE DESCRIPTIOU N8 Package 8-Lead PDIP (Narrow .300 Inch) (Reference LTC DWG # 05-08-1510) .400* (10.160) MAX 8 7 6 5 .255 – .015* (6.477 – 0.381) 1 2 3 4 .300 – .325 .045 – .065 .130 – .005 (7.620 – 8.255) (1.143 – 1.651) (3.302 – 0.127) .065 (1.651) .009 – .015 TYP (0.229 – 0.381) .125 (3.175) .020 +.035 MIN (0.508) .325 ( –.015 ) .100 .018 – .003 MIN 8.255+0.889 (2.54) (0.457 – 0.076) –0.381 BSC N8 0502 NOTE: INCHES 1. DIMENSIONS ARE MILLIMETERS *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm) 1107fa 14

LT1107 PACKAGE DESCRIPTIOU S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610) .189 – .197 .045 – .005 (4.801 – 5.004) .050 BSC NOTE 3 8 7 6 5 N N .245 MIN .160 – .005 .150 – .157 .228 – .244 (3.810 – 3.988) (5.791 – 6.197) NOTE 3 1 2 3 N/2 N/2 .030 – .005 TYP RECOMMENDED SOLDER PAD LAYOUT 1 2 3 4 .010 – .020 · 45(cid:176) .053 – .069 (0.254 – 0.508) (1.346 – 1.752) .004 – .010 .008 – .010 (0.203 – 0.254) 0°– 8° TYP (0.101 – 0.254) .016 – .050 .014 – .019 .050 (0.406 – 1.270) (0.355 – 0.483) (1.270) NOTE: INCHES TYP BSC 1. DIMENSIONS IN (MILLIMETERS) 2. DRAWING NOT TO SCALE 3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm) SO8 0502 1107fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. 15 However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

LT1107 TYPICAL APPLICATIOU 24V-to-5V Step-Down Converter 24VIN 220W ILIM VIN SW1 + 22m F LT1107-5 SENSE GND SW2 150m H* 5V + 300mA 1N5818 330m F 1107 TA03 *COILTRONICS CTX150-4 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1070/LT1070HV 5A I , 40kHz, High Efficiency V = 3V to 40V/60V, V = 65V/75V, I = 6mA, I = <50m A, Can be Used for SW IN OUT Q SD Switching Regulator Buck, Boost, Inverting Applications, TO220-5 Packages LT1071/LT1071HV 2.5A I , 40kHz, High Efficiency V = 3V to 40V/60V, V = 65V/75V, I = 6mA, I = <50m A, Can be Used for SW IN OUT Q SD Switching Regulator Buck, Boost, Inverting Applications, TO220-5 Package LT1072/LT1072HV 1.25A I , 40kHz, High Efficiency V = 3V to 40V/60V, V = 65V/75V, I = 6mA, I = <50m A, Can be Used for SW IN OUT Q SD Switching Regulator Buck, Boost, Inverting Applications, N8, S8, S16, TO220-5 Packages LT1082 1A I , 60kHz, High Efficiency V = 3V to 75V, V = 100V, I = 4.5mA, I = <120m A, Can be Used for SW IN OUT Q SD Switching Regulator Buck, Boost, Inverting Applications, DD, N8, TO220-5 Packages LT1111 1A I , 72kHz, High Efficiency V = 2V to 30V, V = 34V, I = 300m A, Can be Used for SW IN OUT Q Switching Regulator Buck, Boost, Inverting Applications, N8, S8 Packages LT1170/LT1170HV 5A I , 100kHz, High Efficiency V = 3V to 40V/60V, V = 65V/75V, I = 6mA, I = <50m A, Can be Used for SW IN OUT Q SD Switching Regulator Buck, Boost, Inverting Applications, DD, N8, S16, TO220-5 Packages LT1171/LT1171HV 2.5A I , 100kHz, High Efficiency V = 3V to 40V/60V, V = 65V/75V, I = 6mA, I = <50m A, Can be Used for SW IN OUT Q SD Switching Regulator Buck, Boost, Inverting Applications, DD, N8, S16, TO220-5 Packages LT1172/LT1172HV 1.25A I , 100kHz, High Efficiency V = 3V to 40V/60V, V = 65V/75V, I = 6mA, I = <100m A, Can be Used for SW IN OUT Q SD Switching Regulator Buck, Boost, Inverting Applications, N8, S16, DD, TO220-5 Packages LT1307/LT1307B 600mA I , 600kHz, High Efficiency V = 1V to 12V, V = 28V, I = 50m A/1mA, I = <1m A SW IN OUT Q SD Step-Up Switching Regulator Ideal for Single Cell Applications, Low Battery Detect, MS8, N8, S8 Packages LT1317/LT1317B 660mA I , 600kHz, High Efficiency V = 1.5V to 12V, V = 28V, I = 100m A/4.8mA, I = <30m A/28m A SW IN OUT Q SD Step-Up Switching Regulator Low Battery Detect, MS8, S8 Packages LT1370/LT1370HV 6A I , 500kHz, High Efficiency V = 2.7V to 30V, V = 35V/42V, I = 4.5mA, I = <12m A, Can be Used for SW IN OUT Q SD Switching Regulator Buck, Boost, Inverting Applications, DD, TO220-7 Packages LT1371/LT1371HV 3A I , 500kHz, High Efficiency V = 2.7V to 30V, V = 35V/42V, I = 4mA, I = <12m A, Can be Used for SW IN OUT Q SD Switching Regulator Buck, Boost, Inverting Applications, S20, DD, TO220-7 Packages 1107fa 16 Linear Technology Corporation LT/TP 1002 1K REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 l FAX: (408) 434-0507 l www.linear.com ª LINEAR TECHNOLOGY CORPORATION 1993