LT1111 Micropower DC/DC Converter Adjustable and Fixed 5V, 12V U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO Operates at Supply Voltages from 2V to 30V Works with Surface Mount Inductors 72kHz Oscillator Only Three External Components Required Step-Up or Step-Down Mode 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 The LT®1111 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 LT1111 functions equally well in step-up, stepdown, or inverting applications. The LT1111 oscillator is set at 72kHz, optimizing the device to work with off-the-shelf surface mount inductors. The device can deliver 5V at 100mA from a 3V input in step-up mode or 5V at 200mA from a 12V input in stepdown mode. U APPLICATIO S ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Switch current limit can be programmed with a single resistor. An auxiliary open-collector gain block can be configured as a low-battery detector, linear post regulator, undervoltage lock-out circuit, or error amplifier. 3V to 5V, 5V to 12V Converters 9V to 5V, 12V to 5V Converters Remote Controls Peripherals and Add-On Cards Battery Backup Supplies Uninterruptible Supplies Laptop and Palmtop Computers Cellular Telephones Portable Instruments Flash Memory VPP Generators For input sources of less than 2V use the LT1110. , LTC and LT are registered trademarks of Linear Technology Corporation U TYPICAL APPLICATIO Typical Load Regulation All Surface Mount 3V to 5V Step-Up Converter SUMIDA CD54-220M MBRS120T3 22µH 5V 100mA + 10 µ F* V IN SW1 LT1111CS8-5 + 33 µ F SENSE GND OUTPUT VOLTAGE (V) 5 3V INPUT I LIM 6 VIN = 2V 4 2.2 2.4 2.6 2.8 3V 3 2 1 SW2 0 0 25 50 75 100 125 150 175 200 LOAD CURRENT (mA) *OPTIONAL LT1111 • TA01 LT1111 • TA02 1111fd 1 LT1111 W W W AXI U U ABSOLUTE RATI GS (Note 1) Supply Voltage (VIN) ............................................... 36V SW1 Pin Voltage (VSW1) ......................................... 50V SW2 Pin Voltage (VSW2) ............................ – 0.5V to VIN Feedback Pin Voltage (LT1111) ............................. 5.5V Switch Current ....................................................... 1.5A Maximum Power Dissipation ............................ 500mW Operating Temperature Range LT1111C ............................................... 0°C to 70°C LT1111I ......................................... – 40°C to 105°C LT1111M (OBSOLETE) ............. – 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 A0 SW2 4 5 GND ORDER PART NUMBER LT1111CN8 LT1111CN8-5 LT1111CN8-12 N8 PACKAGE 8-LEAD PLASTIC DIP TOP VIEW ILIM 1 8 FB (SENSE)* VIN 2 7 SET SW1 3 6 A0 SW2 4 5 GND *FIXED VERSIONS TJMAX = 90°C, θJA = 130°C/W (N) J8 PACKAGE 8-LEAD CERAMIC DIP TJMAX = 150°C, θJA = 120°C/W (J) ORDER PART NUMBER LT1111CS8 LT1111CS8-5 LT1111CS8-12 LT1111IS8 S8 PART MARKING S8 PACKAGE 8-LEAD PLASTIC SO LT1111MJ8 LT1111MJ8-5 LT1111MJ8-12 1111 11115 11111 1111I *FIXED VERSION TJMAX = 90°C, θJA = 150°C/W OBSOLETE PACKAGE Consider the N8 Package for Alternate Source Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.VIN = 3V, Military or Commercial Version SYMBOL PARAMETER CONDITIONS IQ Quiescent Current Switch OFF VIN Input Voltage Step-Up Mode Step-Down Mode ● ● 2.0 Comparator Trip Point Voltage LT1111 (Note 2) ● 1.20 Output Sense Voltage LT1111-5 (Note 3 LT1111-12 (Note 3) ● ● 4.75 11.40 Comparator Hysteresis LT1111 Output Hysteresis LT1111-5 LT1111-12 VOUT MIN TYP MAX UNITS 300 400 µA 12.6 30.0 V V 1.25 1.30 V 5.00 12.00 5.25 12.60 V V ● 8 12.5 mV ● ● 32 75 50 120 mV mV 54 72 88 kHz fOSC Oscillator Frequency DC Duty Cycle: Step-Up Mode Step-Down Mode Full Load 43 24 50 34 59 50 % % tON Switch ON Time: Step-Up Mode Step-Down Mode ILIM Tied to VIN VOUT, = 5V, VIN = 12V 5 3.3 7 5 9 7.8 µs µs VSAT SW Saturation Voltage, Step-Up Mode VIN = 3.0V, ISW = 650mA VIN = 5.0V, ISW = 1A 0.5 0.8 0.65 1.0 V V SW Saturation Voltage, Step-Down Mode VIN = 12V, ISW = 650mA 1.1 1.5 V 1111fd 2 LT1111 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.VIN = 3V, Military or Commercial Version SYMBOL PARAMETER CONDITIONS TYP MAX UNITS IFB Feedback Pin Bias Current LT1111, VFB = 0V ● MIN 70 120 nA ISET Set Pin Bias Current VSET = VREF ● 70 300 nA VOL Gain Block Output Low ISINK = 300µA, VSET = 1.00V ● 0.15 0.4 V Reference Line Regulation 5V ≤ VIN ≤ 30V 2V ≤ VIN ≤ 5V ● 0.02 0.20 0.075 0.400 AV Gain Block Gain RL = 100k (Note 4) ● ILIM Current Limit 220Ω from ILIM to VIN Current Limit Temperature Coefficient 1000 Measured at SW1 Pin, VSW1 = 12V Maximum Excursion Below GND ISW1≤ 10µA, Switch OFF 6000 V/V 400 mA %/°C – 0.3 ● Switch OFF Leakage Current %/V %/V 1 10 µA – 400 – 350 mV The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 3V, 0°C ≤ TA ≤ 70°C unless otherwise noted. SYMBOL PARAMETER CONDITIONS IQ Quiescent Current Switch OFF fOSC Oscillator Frequency DC Duty Cycle: Step-Up Mode Step-Down Mode tON Switch ON Time: Step-Up Mode Step-Down Mode VSAT MIN LT1111C TYP MAX UNITS ● 300 450 µA ● 54 72 95 kH Full Load ● ● 43 24 50 34 59 50 % % ILIM Tied to VIN VOUT = 5V, VIN = 12V ● ● 5.0 3.3 7 5 9.0 7.8 µs µs Reference Line Regulation 2V ≤ VIN ≤ 5V ● 0.2 0.7 %/V SW Saturation Voltage, Step-Up Mode SW Saturation Voltage, Step-Down Mode VIN = 3V, ISW = 650mA VIN = 12V, ISW = 650mA ● ● 0.5 1.1 0.65 1.50 V V Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: This specification guarantees that both the high and low trip points of the comparator fall within the 1.20V to 1.30V range. Note 3: 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 4: 100k resistor connected between a 5V source and the A0 pin. 1111fd 3 LT1111 U W TYPICAL PERFOR A CE CHARACTERISTICS Oscillator Frequency Oscillator Frequency 90 10 74 9.5 9.0 70 60 8.5 72 ON TIME (µs) 80 71 70 7.0 5.5 67 –25 25 75 0 50 TEMPERATURE (°C) 100 125 3 0 6 5.0 –50 9 12 15 18 21 24 27 30 INPUT VOLTAGE (V) 56 0.8 54 52 50 48 46 44 VIN = 3V 1.2 0.7 0.6 0.5 0.4 0.3 0.2 100 0 –50 125 VIN = 2V 1.0 0.8 VIN = 5V 0.6 0.4 0.2 0.1 42 – 25 50 75 0 25 TEMPERATURE (°C) LT1111 • TPC04 100 0 125 0 0.2 0.4 0.6 0.8 1.0 1.2 SWITCH CURRENT (A) LT1111 • TPC05 Switch ON Voltage Step-Down Mode 1.4 1.6 LT1111 • TPC06 Switch ON Voltage Step-Down Mode 100 1.4 2.00 125 1.4 VIN = 3V ISW = 650mA SATURATION VOLTAGE (V) 0.9 SATURATION VOLTAGE (V) 58 50 75 0 25 TEMPERATURE (°C) 100 Saturation Voltage Step-Up Mode 1.0 –25 50 75 0 25 TEMPERATURE (°C) LT111 • TPC03 Saturation Voltage Step-Up Mode 60 40 –50 –25 LT1111 • TPC02 Duty Cycle Minimum/Maximum Frequency vs ON Time VIN = 12V VIN = 12V ISW = 650mA OSCILLATOR FREQUENCY (KHz) 1.2 1.0 1.50 ON VOLTAGE (V) ON VOLTAGE (V) 7.5 6.0 68 LT1111 • TPC01 1.75 8.0 6.5 69 50 40 –50 DUTY CYCLE (%) Switch ON Time 75 73 FREQUENCY (KHz) OSCILLATOR FREQUENCY (KHz) 100 1.25 1.00 0.8 0.6 0.4 0.75 0.2 90 0°C ≤ TA ≤ 70°C 80 70 60 50 –55°C ≤ TA ≤ 125°C 0.50 –50 0 –25 50 75 0 25 TEMPERATURE (°C) 100 125 LT1111 • TPC07 0 0.2 0.8 0.4 0.6 SWITCH CURRENT (A) 1.0 LT1111 • TPC08 40 4 5 8 9 6 7 10 SWITCH ON TIME (µs) 11 12 LT1111 • TPC09 1111fd 4 LT1111 U W TYPICAL PERFOR A CE CHARACTERISTICS Quiescent Current Quiescent Current 500 400 450 340 320 300 280 260 240 400 SWITCH CURRENT (A) 360 QUIESCENT CURRENT (µA) QUIESCENT CURRENT (µA) 380 350 300 250 200 150 220 200 0 3 6 9 12 15 18 21 24 27 30 INPUT VOLTAGE (V) 100 –50 –25 0 25 50 75 TEMPERATURE (°C) LT1111 • TPC10 100 125 Set Pin Bias Current 80 70 70 BIAS CURRENT (nA) 90 80 BIAS CURRENT (nA) 90 60 50 40 30 10 100 RLIM (Ω) 1000 LT1111 • TPC12 60 50 40 30 20 20 10 10 0 25 50 75 TEMPERATURE (°C) STEP-DOWN VIN = 12V Feedback Bias Current 100 –25 STEP-UP 2V ≤ VIN ≤ 5V LT1111 • TPC11 100 0 –50 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 Maximum Switch Current vs RLIM 100 125 LT1111 • TPC13 0 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 125 LT1111 • TPC14 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. A0 (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 LT1111 (adjustable) this pin goes to the comparator input. On the LT1111-5 and LT1111-12, this pin goes to the internal application resistor that sets output voltage. 1111fd 5 LT1111 W BLOCK DIAGRA S LT1111 LT1111-5/LT1111-12 SET + SET A2 A2 A0 – V IN I LIM GAIN BLOCK/ ERROR AMP 1.25V REFERENCE – – DRIVER OSCILLATOR DRIVER COMPARATOR SW2 FB SW1 + A1 OSCILLATOR COMPARATOR GND I LIM SW1 + A1 A0 – V IN GAIN BLOCK/ ERROR AMP 1.25V REFERENCE + R1 SW2 R2 220k SENSE LT1111 • BD01 GND LT1111-5: R1 = 73.5k LT1111-12: R1 = 25.5k LT1111 • BD02 U OPERATIO The LT1111 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 LT1111 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 72kHz 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 7µs ON time and 7µs OFF time, optimizing the device for circuits where VOUT and VIN differ by roughly a factor of 2. Examples include a 3V to 5V step-up converter or a 9V to 5V step-down converter. 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 LT1111-5 and LT1111-12 are functionally identical to the LT1111. 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. 1111fd 6 LT1111 U W U UO APPLICATI S I FOR ATIO Inductor Selection — General PL / f OSC 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 LT1111 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 LT1111 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 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 (1) 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: (2) in order for the converter to regulate the output. When the switch is closed, current in the inductor builds according to: –R′ t VIN 1– e L IL (t) = R′ (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: () IL t = VIN t (4) 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 LT1111 specification table (typically 7µs) 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 LI PEAK (5) 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. EL = As an example, suppose 12V at 60mA is to be generated from a 4.5V to 8V input. Recalling equation (1), ( )( ) PL = 12V + 0.5V – 4.5V 60mA = 480mW (6) Energy required from the inductor is PL f OSC = 480mW = 6.7µJ 72kHz (7) 1111fd 7 LT1111 U W U UO APPLICATI S I FOR ATIO Picking an inductor value of 47µH with 0.2Ω DCR results in a peak switch current of: I PEAK –1.0 Ω • 7 µs 4.5V 1 – e 47µH = 623mA . = 1.0Ω (8) Substituting IPEAK into Equation 4 results in: ( )( 1 E L = 47µH 0.623A 2 ) 2 = 9.1µJ 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: (9) L= Since 9.1µJ > 6.7µJ, the 47µH 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, consider using the LT1110. The 70% duty cycle of the LT1110 allows more energy per cycle to be stored in the inductor, resulting in more output power. VIN MIN − VSW − VOUT I PEAK • tON (11) where tON = switch-on time (7µs). 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. 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, switch current is held constant, resulting in increasing efficiency. As an example, suppose 5V at 300mA is to be generated from a 12V to 24V input. Recalling Equation (10), Inductor Selection — Step-Down Converter Next, inductor value is calculated using Equation (11): 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 ~650mA 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: I PEAK = 2I OUT DC V OUT + V D V IN – V SW + V D where DC = duty cycle (0.50) VSW = switch drop in step-down mode (10) I PEAK = L= ( ) 2 300mA 5 + 0.5 = 600mA 0.50 12 – 1.5 + 0.5 12 – 1.5 – 5 7µs = 64µH. 600mA (12) (13) Use the next lowest standard value (56µH). Then pick RLIM from the curve. For IPEAK = 600mA, RLIM = 56Ω. 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, PL = (VOUT+ VD)(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 0.65Ω resistor. When the 1111fd 8 LT1111 U W U UO APPLICATI S I FOR ATIO switch closes, current in the inductor builds according to IL VL 1 – e t = R′ () –R′ t L (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: PL 275mW = = 3.8µJ. fOSC 72kHz (17) specifically designed for switch mode DC/DC converters which work much better than general-purpose units. Tantalum 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 LT1111 based converter with three 100µF 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 0.24Ω. 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 0.07Ω. Figure 3 shows the circuit with a 16V OS-CON unit. ESR is now only 0.02Ω. – 0.85Ω × 7 µs 4.5V – 0.75V ) ( = 445mA . 56µH IPEAK = 1– e (0.65Ω + 0.2Ω) (18) 50mV/DIV Picking an inductor value of 56µH with 0.2Ω DCR results in a peak switch current of: 5µs/DIV Substituting IPEAK into Equation (4) results in: ( )( 1 56µH 0.445A 2 ) 2 = 5.54µJ. Figure 1. Aluminum (19) Since 5.54µJ > 3.82µJ, the 56µH 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 ~650mA. 50mV/DIV EL = LT1111 • F01 5µs/DIV LT1111 • F02 Figure 2. Tantalum 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 50mV/DIV Capacitor Selection 5µs/DIV LT1111 • F01 Figure 3. OS-CON 1111fd 9 LT1111 W U U UO APPLICATI S I FOR ATIO At the end of the switch ON time the current in L1 is1: Diode Selection Speed, forward drop, and leakage current are the three main considerations in selecting a catch diode for LT1111 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 10µs to 50µs range. At best, efficiency will be severely compromised when these diodes are used; at worst, the circuit may not work at all. Most LT1111 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 4µA to 10µA leakage current fit nicely with LT1111 requirements. At peak switch currents of 100mA or less, a 1N4148 signal diode may be used. This diode has leakage current in the 1nA to 5nA range at 25°C 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. I PEAK = L tON (20) 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 LT1111 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 LT1111 based step-down converter is shown in Figure 5. VIN R3 100 Ω + C2 I LIM V IN SW1 FB The usual step-up configuration for the LT1111 is shown in Figure 4. The LT1111 first pulls SW1 low causing VIN – VCESAT to appear across L1. A current then builds up in L1. L1 VI N LT1111 L1 VOUT SW2 GND R2 D1 1N5818 D1 V IN + C1 R1 V OUT R3* I LIM V IN SW1 LT1111 GND Figure 5. Step-Down Mode Hookup + FB SW2 LT1111 • F05 R2 C1 R1 *OPTIONAL 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: LT1111 • F04 Figure 4. Step-Up Mode Hookup. Refer to Table 1 for Component Values. I PEAK = VIN − VSW − VOUT L t ON (22) Note 1: This simple expression neglects the effect of switch and coil resistance. This is taken into account in the “Inductor Selection” section. 1111fd 10 LT1111 U W U UO APPLICATI S I FOR ATIO 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 LT1111. Output voltage is determined by: VOUT R2 = 1 + 1.25V R1 ( ) VIN 30V MAX L1 VOUT R2 220 D1 1N5821 R3 330 IL VIN + SW1 C2 + C1 LT1111 R4 FB SW2 GND ( R5 R4 VOUT = 1.25V 1 + R5 (23) ) LT1111 • TA08 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 LT1111 in step-down 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). Figure 6. Q1 Permits Higher Current Switching. LT1111 Functions as Controller. Inverting Configurations The LT1111 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 Higher Current Step-Down Operation + C2 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 — StepDown Converter” section with the following conservative expression for VSW: VSW = V R1 + V Q1SAT ≈ 1.0V Q1 MJE210 OR ZETEX ZTX749 R1 0.3Ω (24) R2 provides a current path to turn off Q1. R3 provides base drive to Q1. R4 and R5 set output voltage. A PMOS FET can be used in place of Q1 when VIN is between 10V and 20V. R3 I LIM V IN SW1 FB LT1111 L1 SW2 GND D1 1N5818 R1 + C1 R2 –VOUT LT1111 • F07 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. 1111fd 11 LT1111 W U U UO APPLICATI S I FOR ATIO D1 L1 VOUT + C1 I LIM + C2 VIN SW1 R1 IL 2N3906 LT1111 A0 GND FB SW2 R2 –VIN SWITCH LT1111 • F08 ON OFF ( ) VOUT = R1 1.25V + 0.6V R2 LT1111 • F09 Figure 9. No Current Limit Causes Large Inductor Current Build-Up Figure 8. Negative-to-Positive Converter PROGRAMMED CURRENT LIMIT Using the ILIM Pin The LT1111 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 wh ere analysis shows the LT1111 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 ILIM” 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 VIN − 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. 12 IL SWITCH ON OFF LT1111 • F10 Figure 10. Current Limit Keeps Inductor Current Under Control 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 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 1µs. The current trip point becomes less accurate for switch ON times less than 3µs. Resistor values programming switch ON time for 1µs or less will cause spurious response in the switch circuitry although the device will still maintain output regulation. RLIM (EXTERNAL) VIN ILIM R1 80Ω (INTERNAL) Q3 SW1 DRIVER OSCILLATOR Q1 Q2 SW2 LT1111 • F11 Figure 11. LT1111 Current Limit Circuitry 1111fd LT1111 W U U UO APPLICATI S I FOR ATIO Using the Gain Block The gain block (GB) on the LT1111 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. 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 to 10M range are optimal. However, the addition of R3 will change the trip point. 5V V IN LT1111 R1 VBAT 1.25V REF – SET + 47k A0 VLB – 1.25V 35.1µA VLB = BATTERY TRIP POINT R2 = 33k R3 = 1.6M R1 = GND R2 TO PROCESSOR R3 LT1111 • F12 Figure 12. Setting Low-Battery Detector Trip Point Table 1. Component Selection for Common Converters INPUT VOLTAGE OUTPUT VOLTAGE OUTPUT CURRENT (MIN) CIRCUIT FIGURE INDUCTOR VALUE INDUCTOR PART NUMBER CAPACITOR VALUE 2 to 3.1 2 to 3.1 2 to 3.1 2 to 3.1 5 5 6.5 to 11 12 to 20 20 to 30 5 12 5 5 12 12 12 12 5 5 5 –5 –5 90mA 10mA 30mA 10mA 90mA 30mA 50mA 300mA 300mA 75mA 250mA 4 4 4 4 4 4 5 5 5 6 6 15µH 47µH 15µH 47µH 33µH 47µH 15µH 56µH 120µH 56µH 120µH S CD75-750K S CD54-470K, C CTX50-1 S CD75-150K S CD54-470K, C CTX50-1 S CD75-330K S CD75-470K, C CTX50-1 S CD54-150K S CD105-560K, C CTX50-4 S CD105-121K, C CTX100-4 S CD75-560K, C CTX50-4 S CD105-121K, C CTX100-4 33µF 10µF 22µF 10µF 22µF 15µF 47µF 47µF 47µF 47µF 100µF S = Sumida C = Coiltronics NOTES * ** ** ** ** * Add 47Ω from ILIM to VIN ** Add 220Ω from ILIM to VIN Table 3. Capacitor Manufacturers Table 2. Inductor Manufacturers MANUFACTURER PART NUMBERS MANUFACTURER PART NUMBERS Coiltronics Incorporated 6000 Park of Commerce Blvd. Boca Raton, FL 33487 407-241-7876 CTX100-4 Series Surface Mount Sanyo Video Components 1201 Sanyo Avenue San Diego, CA 92073 619-661-6322 OS-CON Series Toko America Incorporated 1250 Feehanville Drive Mount Prospect, IL 60056 312-297-0070 Type 8RBS Nichicon America Corporation 927 East State Parkway Schaumberg, IL 60173 708-843-7500 PL Series Sumida Electric Co. USA 708-956-0666 CD54 CDR74 CDR105 Surface Mount Sprague Electric Company Lower Main Street Sanford, ME 04073 207-324-4140 150D Solid Tantalums 550D Tantalex Matsuo 714-969-2491 267 Series Surface Mount 1111fd 13 LT1111 UO TYPICAL APPLICATI S 3V to – 22V LCD Bias Generator L1* 27µH 1N4148 R1 100Ω ILIM 732k 1% V IN SW1 2 × 1.5V CELLS LT1111 3V 0.1µF FB GND SW2 + 4.7µF 39.2k 1% MBRS130T3 MBRS130T3 + 22µF 220k * L1 = SUMIDA CD54-270K FOR 5V INPUT CHANGE R1 TO 47Ω. CONVERTER WILL DELIVER –22V AT 40mA. –22V OUTPUT 7mA AT 2V INPUT LT1111 • TA03 9V to 5V Step-Down Converter 20V to 5V Step-Down Converter VIN 12V TO 28V 100 Ω 100 Ω ILIM V IN ILIM V IN SW1 SW1 9V BATTERY LT1111-5 LT1111-5 SENSE SENSE GND SW2 GND L1* 15µH L1* 68µH 5V OUTPUT 150mA AT 9V INPUT 50mA AT 6.5V INPUT + MBRS130T3 SW2 5V OUTPUT 300mA + 22µF 47µF MBRS130T3 * L1 = SUMIDA CD54-150K * L1 = SUMIDA CD74-680M LT1111 • TA04 5V to –5V Converter Voltage Controlled Positive-to-Negative Converter VIN 5V INPUT + BAT54 V IN MBRD320 220Ω SW1 SW2 220Ω V IN LT1111 L1* 33µH FB GND MBRS130T3 200k – SENSE GND ILIM SW2 39k VC (0V TO 5V) LT1006 + V IN LT1111-5 47µF –VOUT = –5.13 × VC 2W MAXIMUM OUTPUT SW1 + 22µF L1* 20µH, 3A ZETEX† ZTX788A 0.22Ω VIN 5V TO 12V 100 Ω ILIM LT1111 • TA06 + 33µF –5V OUTPUT 75mA * L1 = SUMIDA CD54-330K LT1111 • TA05 * L1 = COILTRONICS CTX20-4 † ZETEX INC. 516-543-7100 LT1111 • TA07 1111fd 14 LT1111 U PACKAGE DESCRIPTIO J8 Package 8-Lead CERDIP (Narrow .300 Inch, Hermetic) (Reference LTC DWG # 05-08-1110) 0.300 BSC (0.762 BSC) 0.200 (5.080) MAX CORNER LEADS OPTION (4 PLCS) 0.008 – 0.018 (0.203 – 0.457) 0° – 15° 0.015 – 0.060 (0.381 – 1.524) 0.023 – 0.045 (0.584 – 1.143) HALF LEAD OPTION 0.045 – 0.068 (1.143 – 1.727) FULL LEAD OPTION 0.405 (10.287) MAX 0.005 (0.127) MIN 8 NOTE: LEAD DIMENSIONS APPLY TO SOLDER DIP/PLATE OR TIN PLATE LEADS 0.014 – 0.026 (0.360 – 0.660) 0.100 (2.54) BSC 5 0.025 (0.635) RAD TYP 0.220 – 0.310 (5.588 – 7.874) 1 0.045 – 0.065 (1.143 – 1.651) 6 7 2 3 4 0.125 3.175 MIN J8 1298 OBSOLETE PACKAGE N8 Package 8-Lead PDIP (Narrow .300 Inch) (Reference LTC DWG # 05-08-1510) 0.300 – 0.325 (7.620 – 8.255) 0.009 – 0.015 (0.229 – 0.381) ( 0.045 – 0.065 (1.143 – 1.651) 0.130 ± 0.005 (3.302 ± 0.127) 0.065 (1.651) TYP 8 7 6 5 1 2 3 4 0.255 ± 0.015* (6.477 ± 0.381) +0.035 0.325 –0.015 +0.889 8.255 –0.381 0.400* (10.160) MAX ) 0.100 (2.54) BSC 0.125 (3.175) 0.020 MIN (0.508) MIN 0.018 ± 0.003 (0.457 ± 0.076) N8 1098 *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm) S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610) 0.189 – 0.197* (4.801 – 5.004) 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 0.053 – 0.069 (1.346 – 1.752) 0°– 8° TYP 0.016 – 0.050 (0.406 – 1.270) 0.014 – 0.019 (0.355 – 0.483) TYP *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 8 7 6 5 0.004 – 0.010 (0.101 – 0.254) 0.050 (1.270) BSC 0.150 – 0.157** (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) SO8 1298 1 2 3 4 1111fd 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. 15 LT1111 UO TYPICAL APPLICATI S High Power, Low Quiescent Current Step-Down Converter 0.22Ω VIN 8V TO 18V MTM20P08 BAT54 2k 51Ω L1* 10µH, 3A MBRD320 5V 500mA + 220µF 2N3904 V IN ILIM SW1 1N4148 LT1111 121k FB GND SW2 40.2k * L1 = SUMIDA CDR105-100M OPERATE STANDBY LT1111 • TA20 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1070/LT1070HV 5A ISW, 40kHz, High Efficiency Switching Regulator VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <50µA, Can be Used for Buck, Boost, Inverting Applications, TO220-5 Package LT1071/LT1071HV 2.5A ISW, 40kHz, High Efficiency Switching Regulator VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <50µA, Can be Used for Buck, Boost, Inverting Applications, N8, S8, S16, TO220-5 Packages LT1072/LT1072HV 1.25A ISW, 40kHz, High Efficiency Switching Regulator VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <50µA, Can be Used for Buck, Boost, Inverting Applications, N8, S8, S16, TO220-5 Packages LT1082 1A ISW, 60kHz, High Efficiency Switching Regulator VIN = 3V to 75V, VOUT = 100V, IQ = 4.5mA, ISD = ≤120µA, Can be Used for Buck, Boost, Inverting Applications, DD, N8, TO220-5 Packages LT1107 1A ISW, 63kHz, High Efficiency Switching Regulator VIN = 2V to 30V, VOUT = 34V, IQ = 320µA, Can be Used for Buck, Boost, Inverting Applications, N8, S8 Packages LT1170/LT1170HV 5A ISW, 100kHz, High Efficiency Switching Regulator VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <50µA, Can be Used for Buck, Boost, Inverting Applications, DD, N8, S16, TO220-5 Packages LT1171/LT1171HV 2.5A ISW, 100kHz, High Efficiency Switching Regulator VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <50µA, Can be Used for Buck, Boost, Inverting Applications, DD, N8, S16, TO220-5 Packages LT1172/LT1172HV 1.25A ISW, 100kHz, High Efficiency Switching Regulator VIN = 3V to 40V/60V, VOUT = 65V/75V, IQ = 6mA, ISD = <100µA, Can be Used for Buck, Boost, Inverting Applications, DD, N8, S16, TO220-5 Packages LT1307/LT1307B 600mA ISW, 600kHz, High Efficiency Step-Up Switching Regulator VIN = 1V to 12V, VOUT = 28V, IQ = 50µA/1mA, ISD = ≤1µA Ideal for Single Cell Applications, Low Battery Detect, MS8, N8, S8 Packages LT1317/LT1317B 660mA ISW, 600kHz, High Efficiency Step-Up Switching Regulator VIN = 1.5V to 12V, VOUT = 28V, IQ = 100µA/4.8mA, ISD = ≤30µA/28µA Low Battery Detect, MS8, S8 Packages LT1370/LT1370HV 6A ISW, 500kHz, High Efficiency Switching Regulator VIN = 2.7V to 30V, VOUT = 35V/42V, IQ = 4.5mA, ISD = <12µA, Can be Used for Buck, Boost, Inverting Applications, DD, TO220-7 Packages LT1371/LT1371HV 3A ISW, 500kHz, High Efficiency Switching Regulator VIN = 2.7V to 30V, VOUT = 35V/42V, IQ = 4mA, ISD = <12µA, Can be Used for Buck, Boost, Inverting Applications, DD, S20, TO220-7 Packages 1111fd 16 Linear Technology Corporation LT/TP 0103 1K REV D • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 1994