LT1107 Micropower DC/DC Converter Adjustable and Fixed 5V, 12V U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ Operates at Supply Voltages from 2V to 30V Consumes Only 320µA Supply Current Works in Step-Up or Step-Down Mode Only Three External 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 SO-8 Package UO APPLICATI ■ ■ ■ ■ ■ ■ ■ ■ S Palmtop Computers 3V to 5V, 5V to 12V Converters 24V to 5V, 12V to 5V Converters LCD Bias Generators Peripherals and Add-On Cards Battery Backup Supplies Cellular Telephones Portable Instruments The LT1107 is a versatile micropower DC/DC converter. The device requires only three 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 LT1107 functions equally well in step-up, stepdown, or inverting applications. The LT1107 is pin-for-pin compatible with the LT1111, but has a duty cycle of 70%, resulting in increased output current in many applications. The LT1107 can deliver 150mA at 5V from a 2AA cell input and 5V at 300mA from 24V in step-down mode. Quiescent current is just 320µA, making the LT1107 ideal for power-conscious batteryoperated systems. The 63kHz oscillator is optimized to work with surface mount inductors and capacitors. 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. UO TYPICAL APPLICATI Palmtop Computer Logic Supply L1* 33µH Efficiency 82 MBRS120T3 5V 150mA 47Ω 2 × AA ALKALINE CELLS 78 VIN + SW1 47µF LT1107-5 + 100µF SENSE GND SW2 EFFICIENCY (%) ILIM VIN = 3V 80 VIN = 2V 76 VIN = 2.5V 74 72 70 68 66 * SUMIDA CD54-330K COILCRAFT DT3316-473 64 1107 TA01 1 10 100 LOAD CURRENT (mA) 400 1107 TA02 1 LT1107 W W W AXI U U ABSOLUTE RATI GS Supply Voltage (VIN) ............................................... 36V SW1 Pin Voltage (VSW1) ......................................... 50V SW2 Pin Voltage (VSW2) ............................ – 0.5V to VIN Feedback Pin Voltage (LT1107) ................................ 5V Sense Pin Voltage (LT1107-5, LT1107-12) ............ 36V Maximum Power Dissipation ............................ 500mW Set Pin Voltage ...................................................... 5.5V Maximum Switch Current ...................................... 1.5A Operating Temperature Range LT1107C ................................................ 0°C to 70°C LT1107M ........................................ – 55°C to 125°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C U W U PACKAGE/ORDER I FOR ATIO TOP VIEW ILIM 1 8 FB (SENSE)* VIN 2 7 SET SW1 3 6 AO SW2 4 5 GND N8 PACKAGE J8 PACKAGE 8-LEAD CERAMIC DIP 8-LEAD PLASTIC DIP * FIXED VERSIONS ORDER PART NUMBER LT1107CN8 LT1107CN8-5 LT1107CN8-12 LT1107MJ8 LT1107MJ8-5 LT1107MJ8-12 ORDER PART NUMBER TOP VIEW ILIM 1 8 FB(SENSE)* VIN 2 7 SET SW1 3 6 AO SW2 4 5 GND TJMAX = 150°C, θJA = 120°C/W (J) TJMAX = 90°C, θJA = 130°C/W (N) LT1107CS8 LT1107CS8-5 LT1107CS8-12 S8 PACKAGE 8-LEAD PLASTIC SO S8 PART MARKING *FIXED VERSIONS 1107 11075 110712 TJMAX = 90°C, θJA = 150°C/W ELECTRICAL CHARACTERISTICS VIN = 3V, military or commercial version, TA = 25°C, unless otherwise noted. SYMBOL PARAMETER CONDITIONS IQ Quiescent Current Switch OFF Quiescent Current, Step-Up Mode Configuration No Load Input Voltage Step-Up Mode Step-Down Mode ● ● 2 Comparator Trip Point Voltage LT1107 (Note 1) ● 1.2 Output Sense Voltage LT1107-5 (Note 2) LT1107-12 (Note 2) ● ● 4.75 11.40 Comparator Hysteresis LT1107 Output Hysteresis LT1107-5 LT1107-12 VIN VOUT fOSC tON VOL 2 MIN LT1107-5 LT1107-12 TYP MAX UNITS 320 450 µA µA µA 360 550 12.6 30.0 V V 1.25 1.3 V 5 12 5.25 12.60 V V ● 8 12.5 mV ● ● 32 75 50 120 mV mV Oscillator Frequency 50 63 77 kHz Duty Cycle, Step-Up Mode Full Load 64 70 76 % Switch ON Time, Step-Up Mode ILIM Tied to VIN 8.8 11 12.7 µs Feedback Pin Bias Current LT1107, VFB = 0V ● 70 120 nA Set Pin Bias Current VSET = VREF ● 70 300 nA Gain Block Output Low ISINK = 300µA, VSET = 1V ● 0.15 0.4 Reference Line Regulation 5V ≤ VIN ≤ 30V ● 0.02 0.075 V %/V LT1107 ELECTRICAL CHARACTERISTICS VIN = 3V, military or commercial version, TA = 25°C, unless otherwise noted. SYMBOL PARAMETER CONDITIONS AV Gain Block Gain RL = 100k (Note 3) Current Limit 220Ω to ILIM to VIN Current Limit Temperature Coefficient VSW2 ● MIN TYP 1000 6000 V/V 400 mA – 0.3 ● Switch OFF Leakage Current Measured at SW1 Pin, VSW1 = 12V Maximum Excursion Below GND ISW1 ≤ 10µA, Switch OFF MAX UNITS %/°C 1 10 µA – 400 – 350 mV LT1107M TYP MAX UNITS 500 µA VIN = 3V, – 55°C ≤ TA ≤ 125°C, unless otherwise noted. SYMBOL PARAMETER CONDITIONS IQ Quiescent Current Switch OFF MIN ● fOSC Oscillator Frequency ● 40 63 95 kHz DC Duty Cycle Step-Up Mode Step-Down Mode, VIN = 12V ● ● 56 45 69 60 81 73 % % tON Switch ON Time Step-Up Mode Step-Down Mode, VIN = 12V ● ● 7 5 11 9 15 13 µs µs Reference Line Regulation 2V ≤ VIN ≤ 5V, 0°C ≤ TA ≤ 125°C 2.4V ≤ VIN ≤ 5V, TA = – 55°C 0.2 0.4 0.8 %/V %/V Switch Saturation Voltage, Step-Up Mode 0°C ≤ TA ≤ 125°C, ISW = 500mA TA = – 55°C, ISW = 400mA 0.5 0.5 0.65 0.65 V V Switch Saturation Voltage, Step-Down Mode VIN = 12V, ISW = 500mA 0°C ≤ TA ≤ 125°C TA = – 55°C 1.5 2.0 V V VSAT VIN = 3V, 0°C ≤ TA ≤ 70°C, unless otherwise noted. SYMBOL PARAMETER CONDITIONS IQ Quiescent Current Switch OFF MIN LT1107C TYP ● MAX UNITS 450 µA fOSC Oscillator Frequency ● 50 63 88 kHz DC Duty Cycle Step-Up Mode Step-Down Mode, VIN = 12V ● ● 62 50 69 60 78 70 % % tON Switch ON Time Step-Up Mode Step-Down Mode, VIN = 12V ● ● 8 6 11 9 13.5 12.0 µs µs Reference Line Regulation 2V ≤ VIN ≤ 5V ● 0.2 0.7 %/V Switch Saturation Voltage, Step-Up Mode Switch Saturation Voltage, Step-Down Mode VIN = 3V, ISW = 650mA VIN = 12V, ISW = 650mA ● ● 0.5 1.1 0.65 1.5 V V VSAT The ● 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. 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 AO pin. 3 LT1107 U W TYPICAL PERFOR A CE CHARACTERISTICS Saturation Voltage, Step-Up Mode (SW2 Pin Grounded) Switch ON Voltage, Step-Down Mode (SW1 Pin Connected to VIN) 1.2 SWITCH ON VOLTAGE (V) SATURATION VOLTAGE (V) VIN = 3V 0.8 VIN = 2V VIN = 5V 0.6 0.4 0.2 SWITCH CURRENT (A) 1.3 1.0 0 1.2 1.1 1.0 0.9 0.8 0 0.2 0.4 0.6 0.8 SWITCH CURRENT (A) 1.0 0.7 1.2 0 0.1 0.2 0.3 0.4 0.5 0.6 SWITCH CURRENT (A) 1107 G01 0.7 Quiescent Current 150 TA = 25°C 90 360 80 340 320 300 280 260 0 3 6 50 20 –55 –35 –15 9 12 15 18 21 24 27 30 INPUT VOLTAGE (V) Switch ON Time Step-Up Mode 1107 G07 Switch ON Time Step-Down Mode Duty Cycle Step-Up Mode 85 13 15 80 12 75 11 DUTY CYCLE (%) 12 11 10 9 65 60 5 25 45 65 85 105 125 TEMPERATURE (°C) 1107 G08 45 –55 –35 –15 10 9 8 7 6 50 7 4 70 55 8 6 –55 –35 –15 SWITCH ON TIME (µs) 16 13 5 25 45 65 85 105 125 TEMPERATURE (°C) 1107 G06 1107 G05 SWITCH ON TIME (µs) 60 30 200 5 25 45 65 85 105 125 TEMPERATURE (°C) 70 40 240 220 14 1000 Oscillator Frequency FREQUENCY (kHz) QUIESCENT CURRENT (µA) 200 100 RLIM (Ω) 100 380 350 250 STEP-DOWN VIN = 12V 1107 G03 Quiescent Current 300 STEP-UP 2V ≤ VIN ≤ 5V 10 0.8 400 100 –55 –35 –15 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 1107 G02 400 QUIESCENT CURRENT (µA) Maximum Switch Current vs RLIM 1.4 5 25 45 65 85 105 125 TEMPERATURE (°C) 1107 G09 5 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 1107 G10 LT1107 U W TYPICAL PERFOR A CE CHARACTERISTICS Minimum/Maximum Frequency vs ON Time, Step-Up Mode 100 100 90 90 70 60 70 65 50 70 60 TA = 25°C 50 –55°C ≤ TA ≤ 125°C 40 5 6 7 8 9 10 11 12 13 14 ON TIME (µs) 6 7 8 9 1107 G13 LT1107-12 Output Voltage 5.3 LT1107 Feedback Voltage 1.30 12.20 1.29 OUTPUT VOLTAGE (V) 5.0 4.9 4.8 TRIP POINT VOLTAGE (V) 12.15 5.2 5.1 12.10 12.05 12.00 11.95 11.90 5 25 45 65 85 105 125 TEMPERATURE (°C) 11.80 –55 –35 –15 1.28 1.27 1.26 1.25 1.24 1.23 1.22 11.85 4.7 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 1107 G12 LT1107-5 Output Voltage OUTPUT VOLTAGE (V) 35 –55 –35 –15 10 11 12 13 14 15 16 ON TIME (µs) 1107 G11 50 40 30 4 55 45 –55°C ≤ TA ≤ 125°C 40 30 60 80 DUTY CYCLE (%) 0°C ≤ TA ≤ 70°C 80 Duty Cycle Step-Down Mode 0°C ≤ TA ≤ 70°C FREQUENCY (kHz) FREQUENCY (kHz) Minimum/Maximum Frequency vs ON Time, Step-Down Mode 1.21 5 25 45 65 85 105 125 TEMPERATURE (°C) 1107 G16 1107 G17 1.20 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 1107 G18 UO U U PI FU CTI S 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. GND (Pin 5): Ground. AO (Pin 6): Auxiliary Gain Block (GB) Output. Open collector, can sink 300µA. SET (Pin 7): GB Input. GB is an op amp with positive input connected to SET pin and negative input connected to 1.25V reference. FB/SENSE (Pin 8): On the LT1107 (adjustable), this pin goes to the comparator input. On the LT1107-5 and LT1107-12, this pin goes to the internal application resistor that sets output voltage. 5 LT1107 W BLOCK DIAGRA S LT1107 LT1107-5/LT1107-12 SET SET A2 A2 AO VIN GAIN BLOCK/ ERROR AMP ILIM GAIN BLOCK/ ERROR AMP SW1 ILIM SW1 1.25V REFERENCE 1.25V REFERENCE A1 A1 OSCILLATOR FB OSCILLATOR DRIVER DRIVER COMPARATOR COMPARATOR GND AO VIN SW2 1107 BD01 R1 GND R2 220k SW2 SENSE LT1107-5: R1 = 73.5k LT1107-12: R1 = 25.5k 1107 BD02 U OPERATIO The LT1107 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 LT1107 block diagram. Comparator A1 compares the feedback (FB) pin voltage with the 1.25V reference signal. When FB drops below 1.25V, A1 switches on the 63kHz 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 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 output is low, the oscillator and all high current circuitry is turned off, lowering device quiescent current to just 300µA. The oscillator is set internally for 11µs ON time and 5µs OFF time in step-up mode, optimizing the device for converters where VOUT ≈ 3VIN. The combination of high duty cycle and the current limit feature enables continuous mode operation in many applications, increasing available output power. 6 Gain block A2 can serve as a low-battery detector. The negative input of A2 is the 1.25V 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. AO can sink 300µA (use a 22k resistor pull-up to 5V). A resistor connected between the ILIM pin and VIN sets maximum switch current. When the switch current exceeds the set value, the switch cycle is prematurely terminated. If current limit is not used, ILIM should be tied directly to VIN. Propagation delay through the current limit circuitry is approximately 1µs. In step-up mode the switch emitter (SW2) is connected to ground and the switch collector (SW1) drives the inductor; in step-down mode the collector is connected to VIN and the emitter drives the inductor. The LT1107-5 and LT1107-12 are functionally identical to the LT1107. 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. LT1107 U W U UO APPLICATI S I FOR ATIO Inductor Selection –– 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: ( ) PL = VOUT + VD − VIN(MIN) IOUT (01) 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: PL / f OSC (02) 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′ (03) 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: () 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 LT1107 specification table (typically 11µs) 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: 1 2 E L = LI PEAK (05) 2 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 60mA is to be generated from a 3V to 6V input. Recalling equation (01), ( )( ) P L = 12V + 0.5V − 3V 60mA = 570mW (06) Energy required from the inductor is: PL fOSC 570mW = 9.05µJ 63kHz = (07) Picking an inductor value of 33µH with 0.2Ω DCR results in a peak switch current of: I PEAK = −1Ω × 11µs 3V − e 1 33µH = 850mA 1Ω (08) Substituting IPEAK into Equation 04 results in: EL = ( )( ) 1 33µH 0.85A 2 = 11.91µJ 2 (09) Since 11.9µJ > 9.05µJ, the 33µH inductor will work. This trial-and-error approach can be used to select the optimum inductor. A resistor can be added in series with the ILIM 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 VIN increases, peak switch current is held constant, resulting in increasing efficiency. Inductor Selection –– 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 LT1111 and LT1110 data sheets). 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 I OUT DC VOUT + V D V IN − VSW + V D (10) 7 LT1107 U W U UO APPLICATI S I FOR ATIO where DC = duty cycle (0.50 in step-down mode) 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 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 0.65Ω resistor. When the switch closes, current in the inductor builds according to: () 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= VIN (MIN) − VSW − VOUT I PEAK × t ON (11) where tON = switch ON time (7µs). As an example, suppose 5V at 300mA is to be generated from a 12V to 24V input. Recalling Equation (10): ( ) 2 300mA 5 + 0.5 = 600mA (12) 0.50 12 − 1.5 + 0.5 Next, inductor value is calculated using Equation (11): L= 12 − 1.5 − 5 7µs = 64µH 600mA (15) where R′ = 0.65Ω + DCRL VL = VIN – 0.75V As an example, suppose –5V at 50mA is to be generated from a 4.5V to 5.5V input. Recalling Equation (14), ( )( ) PL = −5V + 0.5V 50mA = 275mW (16 ) Energy required from the inductor is: Next, the current limit resistor RLIM is selected to give IPEAK from the Maximum Switch Current vs RLIM curve. The addition of this resistor keeps maximum switch current constant as the input voltage is increased. IPEAK = −R′t V I L t = L 1 − e L R′ (13) PL 275mW = = 4.4µJ fOSC 63kHz (17) Picking an inductor value of 100µH with 0.2Ω DCR results in a peak switch current of: IPEAK = (4.5V − 0.75V) 1 − e (0.65Ω + 0.2Ω) −0.85Ω × 9µs 100µH = 325mA (18) Substituting IPEAK into Equation (04) results in: EL = ( )( 1 100µH 0.325A 2 ) 2 = 5.28µJ (19) Use the next lowest standard value (56µH). Since 5.28µJ > 3.82µJ, the 100µH inductor will work. Then pick RLIM from the curve. For IPEAK = 600mA, RLIM = 56Ω. 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 ~650mA. Inductor Selection –– Positive-to-Negative Converter Figure 4 shows hookup for positive-to-negative conversion. All of the output power must come from the inductor. In this case, ( )( ) PL = VOUT + VD IOUT 8 (14) 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. LT1107 W U U UO APPLICATI S I FOR ATIO The usual step-up configuration for the LT1107 is shown in Figure 1. The LT1107 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 + C2 R3 100Ω ILIM VIN SW1 FB LT1107 V IPEAK = IN t ON L (20) L1 VOUT SW2 GND D1 1N5818 D1 L1 VOUT VIN R2 + C1 R1 R3 R2 VIN ILIM 1107 F02 + SW1 Figure 2. Step-Down Mode Hookup C1 LT1107 FB GND SW2 R1 1107 F01 Figure 1. Step-Up Mode Hookup 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 LT1107 to keep VFB at the internal reference voltage of 1.25V. R1 and R2 set the output voltage according to the formula: R2 VOUT = 1 + 1.25V R1 ( ) (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 LT1107 based step-down converter is shown in Figure 2. 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) Note 1: This simple expression neglects the effects of switch and coil resistance. This is taken into account in the “Inductor Selection” section. 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 dissipation inside the LT1107. Output voltage is determined by: R2 VOUT = 1 + 1.25V R1 ( ) (23) 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 100Ω resistor programs the switch to turn off when the current reaches approximately 700mA. When using the LT1107 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). Inverting Configurations The LT1107 can be configured as a positive-to-negative converter (Figure 3), or a negative-to-positive converter (Figure 4). In Figure 3, 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 9 LT1107 W U U UO APPLICATI S I FOR ATIO VOUTshould be less than 6.2V. More negative output voltages can be accommodated as in the prior section. current will be limited to 800mA, even if the input voltage increases. In Figure 4, 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. Another situation where the ILIM feature is useful occurs when the device goes into continuous mode operation. This occurs in step-up mode when: +VIN + C2 R3 VIN ILIM SW1 FB LT1107 L1 SW2 GND D1 1N5818 R1 + C1 R2 –VOUT 1107 F03 Figure 3. Positive-to-Negative Converter C2 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 5, 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, the switch turns off at the programmed current as shown in Figure 6, keeping output ripple to a minimum. +VOUT + R3 + (24) D1 L1 C1 R1 IL 2N3906 VIN SW1 ILIM VOUT + VDIODE 1 < VIN − VSW 1 − DC LT1107 FB GND –VIN SW2 R2 ( ) VOUT = R1 1.25V + 0.6V R2 SWITCH ON OFF 1107 F05 1107 F04 Figure 4. Negative-to-Positive Converter Figure 5. No Current Limit Causes Large Inductor Current Build-Up Using the ILIM Pin The LT1107 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 LT1107 must operate at an 800mA peak switch current with a 2V input. If VIN rises to 4V, the peak switch 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 10 IL SWITCH PROGRAMMED CURRENT LIMIT ON OFF 1107 F06 Figure 6. Current Limit Keeps Inductor Current Under Control LT1107 U W U UO APPLICATI S I FOR ATIO Figure 7 details current limit circuitry. Sense transistor A1, 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 80Ω resistor R1 and out through the ILIM pin. The value of the external resistor connected between ILIM and VIN sets 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 switch ON times less than 3µs. 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. Using the Gain Block RLIM (EXTERNAL) VIN R1 80Ω (INTERNAL) Q3 SW1 DRIVER Q1 OSCILLATOR Q2 SW2 1107 F07 Figure 7. LT1107 Current Limit Circuitry 5V VIN R1 VBAT The gain block (GB) on the LT1107 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 1.25V reference. The positive input comes out on the SET pin. 1.25V REF – SET + LT1107 47k AO TO PROCESSOR GND R2 R3 ( ) VLB – 1.25V 35.1µA VLB = BATTERY TRIP POINT R2 = 33k 1107 F08 R3 = 1.6M R1 = Arrangement of the gain block as a low-battery detector is straightforward. Figure 8 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 to 10M range are optimal. The addition of R3 will change the trip point, however. Output ripple of the LT1107, normally 50mV at 5VOUT can be reduced significantly by placing the gain block in front of the FB input as shown in Figure 9. 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. 1000µF is a good starting value. C1 should be a low ESR type as well. ILIM Figure 8. Setting Low-Battery Detector Trip Point L1 D1 VOUT R3 270k ILIM VIN AO VBAT R2 + C1 SW1 LT1107 FB GND SET SW2 ( )( ) VOUT = R2 + 1 1.25V R1 R1 1107 F09 Figure 9. Output Ripple Reduction Using Gain Block 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. 11 LT1107 UO TYPICAL APPLICATI 24V-to-5V Step-Down Converter 24VIN 220Ω ILIM VIN SW1 + LT1107-5 22µF SENSE GND SW2 150µH* 1N5818 5V 300mA + 330µF 1107 TA03 *COILTRONICS CTX150-4 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. J8 Package, 8-Lead Ceramic DIP 0.290 – 0.320 (7.366 – 8.128) CORNER LEADS OPTION (4 PLCS) 0.015 – 0.060 (0.381 – 1.524) 0.023 – 0.045 (0.584 – 1.143) HALF LEAD OPTION 0.008 – 0.018 (0.203 – 0.457) 0.045 – 0.068 (1.143 – 1.727) FULL LEAD OPTION 0° – 15° 0.385 ± 0.025 (9.779 ± 0.635) 0.045 – 0.068 (1.143 – 1.727) 0.005 (0.127) MIN 0.405 (10.287) MAX 8 7 6 5 0.025 (0.635) RAD TYP 0.125 3.175 0.100 ± 0.010 MIN (2.540 ± 0.254) 0.014 – 0.026 (0.360 – 0.660) NOTE: LEAD DIMENSIONS APPLY TO SOLDER DIP OR TIN PLATE LEADS. 0.200 (5.080) MAX 0.220 – 0.310 (5.588 – 7.874) 1 2 3 4 N8 Package, 8-Lead Plastic DIP 0.300 – 0.320 (7.620 – 8.128) 0.009 – 0.015 (0.229 – 0.381) ( +0.025 0.325 –0.015 +0.635 8.255 –0.381 ) 0.045 – 0.065 (1.143 – 1.651) 0.400 (10.160) MAX 0.130 ± 0.005 (3.302 ± 0.127) 8 5 0.250 ± 0.010 (6.350 ± 0.254) 0.045 ± 0.015 (1.143 ± 0.381) 0.100 ± 0.010 (2.540 ± 0.254) 0.010 – 0.020 × 45° (0.254 – 0.508) 0.125 (3.175) MIN 0.020 (0.508) MIN 1 2 3 4 0.018 ± 0.003 (0.457 ± 0.076) 0.189 – 0.197 (4.801 – 5.004) 8 0.053 – 0.069 (1.346 – 1.752) 7 6 5 0.004 – 0.010 (0.101 – 0.254) 0°– 8° TYP 0.016 – 0.050 0.406 – 1.270 0.014 – 0.019 (0.355 – 0.483) 0.050 (1.270) BSC 0.228 – 0.244 (5.791 – 6.197) 0.150 – 0.157 (3.810 – 3.988) 1 12 6 0.065 (1.651) TYP S8 Package, 8-Lead Plastic SOIC 0.008 – 0.010 (0.203 – 0.254) 7 Linear Technology Corporation 2 3 4 LTC/GP 0993 10K REV 0 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 LINEAR TECHNOLOGY CORPORATION 1993