LT1173 Micropower DC/DC Converter Adjustable and Fixed 5V, 12V U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ Operates at Supply Voltages From 2.0V to 30V Consumes Only 110µA 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 SO8 Package The LT1173 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 2.0V to 12V in step-up mode and to 30V in step-down mode. The LT1173 functions equally well in step-up, stepdown or inverting applications. The LT1173 consumes just 110µA supply current at standby, making it ideal for applications where low quiescent current is important. The device can deliver 5V at 80mA from a 3V input in step-up mode or 5V at 200mA from a 12V input in step-down mode. UO APPLICATI ■ ■ ■ ■ ■ ■ ■ ■ Flash Memory Vpp Generators 3V to 5V, 5V to 12V Converters 9V to 5V, 12V to 5V Converters LCD Bias Generators Peripherals and Add-On Cards Battery Backup Supplies Laptop and Palmtop Computers Cellular Telephones Portable Instruments 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, under voltage lockout circuit or error amplifier. For input sources of less than 2V, use the LT1073. and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation. UO ■ S TYPICAL APPLICATI S Logic Controlled Flash Memory VPP Generator L1* 100µH VPP Output 1N5818 12V 100mA 5VIN 47Ω I LIM 10 µ F + VOUT 5V/DIV V IN SW1 1.07M† + LT1173 GND FB SW2 SANYO OS-CON 100 µ F 0V PROGRAM 5V/DIV 124k† 5ms/DIV 1173 TA02 1N4148 PROGRAM LT1173 • TA01 *L1 = GOWANDA GA20-103K COILTRONICS CTX100-4 EFFICIENCY = 81% † = 1% METAL FILM NO OVERSHOOT 1 LT1173 U U RATI GS W W W W AXI U U ABSOLUTE PACKAGE/ORDER I FOR ATIO Supply Voltage (VIN) ................................................ 36V SW1 Pin Voltage (VSW1) .......................................... 50V SW2 Pin Voltage (VSW2) ............................. – 0.5V to VIN Feedback Pin Voltage (LT1173) ................................. 5V Sense Pin Voltage (LT1173, -5, -12) ....................... 36V Maximum Power Dissipation ............................. 500mW Maximum Switch Current ....................................... 1.5A Operating Temperature Range ..................... 0°C to 70°C Storage Temperature Range .................. –65°C to 150°C Lead Temperature, (Soldering, 10 sec.)................ 300°C TOP VIEW ILIM 1 8 FB (SENSE)* VIN 2 7 SET SW1 3 6 AO SW2 4 5 GND LT1173CN8 LT1173CN8-5 LT1173CN8-12 N8 PACKAGE 8-LEAD PLASTIC DIP *FIXED VERSIONS TJMAX = 90°C, θJA = 130°C/W TOP VIEW Consult factory for Industrial and Military grade parts ORDER PART NUMBER ILIM 1 8 FB (SENSE)* VIN 2 7 SET SW1 3 6 AO SW2 4 5 GND LT1173CS8 LT1173CS8-5 LT1173CS8-12 S8 PART MARKING 1173 11735 117312 S8 PACKAGE 8-LEAD PLASTIC SOIC *FIXED VERSIONS TJMAX = 90°C, θJA = 150°C/W ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 3V, unless otherwise noted. SYMBOL PARAMETER IQ Quiescent Current Switch Off IQ Quiescent Current, Boost Mode Configuration No Load Input Voltage Step-Up Mode ● Step-Down Mode ● Comparator Trip Point Voltage LT1173 (Note 1) ● 1.20 Output Sense Voltage LT1173-5 (Note 2) ● 4.75 LT1173-12 (Note 2) ● 11.4 Comparator Hysteresis LT1173 ● 5 10 mV Output Hysteresis LT1173-5 ● 20 40 mV LT1173-12 ● VIN VOUT fOSC tON VOL VSAT CONDITIONS MIN ● MAX 110 150 UNITS µA LT1173-5 135 µA LT1173-12 250 µA Oscillator Frequency 2.0 12.6 V 30 V 1.245 1.30 V 5.00 5.25 V 12.0 12.6 V 50 100 mV ● 18 23 30 kHz % Duty Cycle Full Load ● 43 51 59 Switch ON Time ILIM tied to VIN ● 17 22 32 µs Feedback Pin Bias Current LT1173, VFB = 0V ● 10 50 nA Set Pin Bias Current VSET = VREF ● 20 100 nA Gain Block Output Low ISINK = 100µA, VSET = 1.00V ● 0.15 0.4 V Reference Line Regulation 2.0V ≤ VIN ≤ 5V ● 0.2 0.4 %/V 5V ≤ VIN ≤ 30V ● 0.02 0.075 %/V VIN = 3.0V, ISW = 650mA ● 0.5 0.65 SWSAT Voltage, Step-Up Mode VIN = 5.0V, ISW = 1A 0.8 ● 2 TYP V 1.0 V 1.4 V LT1173 ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 3V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN VSAT SWSAT Voltage, Step-Down Mode VIN = 12V, ISW = 650mA TYP MAX 1.1 ● AV Gain Block Gain RL = 100kΩ (Note 3) Current Limit 220Ω to ILIM to VIN Current Limit Temperature Coeff. VSW2 400 ● ● Switch OFF Leakage Current Measured at SW1 Pin Maximum Excursion Below GND ISW1 ≤ 10µA, Switch Off The ● denotes the specifications which apply over the full operating temperature range. UNITS 1.5 V 1.7 V 1000 V/V 400 mA – 0.3 %/°C 1 10 µA – 400 – 350 mV Note 2: The output voltage waveform will exhibit a sawtooth shape due to the comparator hysteresis. The output voltage on the fixed output versions will always be within the specified range. Note 3: 100kΩ resistor connected between a 5V source and the AO pin. Note 1: This specification guarantees that both the high and low trip points of the comparator fall within the 1.20V to 1.30V range. U W TYPICAL PERFOR A CE CHARACTERISTICS Switch ON Voltage Step-Down Mode (SW1 Pin Connected to VIN) Saturation Voltage Step-Up Mode (SW2 Pin Grounded) 1.2 1.4 1.0 1.3 Maximum Switch Current vs RLIM Step-Up Mode 1200 2V ≤ VIN ≤ 5V VIN = 5.0V VIN= 2.0V 0.6 0.4 0.2 1.2 1.1 1.0 0.9 0 0 0.2 0.4 0.6 0.8 1.0 0.1 0.2 0.3 0.6 0.7 VIN = 24V L = 500µH 600 VIN = 12V L = 250µH 400 300 200 1000 R LIM (Ω) LT1173 • TPC03 Feedback Pin Bias Current vs Temperature 18 VIN = 3V 15 10 5 –50 VIN = 3V 16 14 12 10 8 –25 0 25 50 75 100 125 TEMPERATURE (°C) LT1173 • TPC09 1000 100 R LIM (Ω) 100 0 100 10 0.8 20 VOUT = 5V SET PIN BIAS CURRENT (nA) SWITCH CURRENT (mA) 0.5 Set Pin Bias Current vs Temperature 700 500 0.4 LT1173 • TPC02 Maximum Switch Current vs RLIM Step-Down Mode 800 500 400 ISWITCH (A) LT1173 • TPC01 900 600 100 0 ISWITCH (A) 1000 700 200 0.7 1.2 900 800 300 0.8 FEEDBACK PIN BIAS CURRENT (µA) VCESAT (V) 0.8 1000 SWITCH CURRENT (mA) VIN= 3.0V SWITCH ON VOLTAGE (V) 1100 –50 –25 0 25 50 75 100 125 TEMPERATURE (°C) LT1173 •TPC04 LT1173 •TPC05 3 LT1173 U W TYPICAL PERFOR A CE CHARACTERISTICS Quiescent Current vs Temperature Supply Current vs Switch Current 120 Oscillator Frequency 26.0 50 25.5 VIN = 3V IIN (µA) 110 100 25.0 VIN = 5V 30 FOSC (kHz) SUPPLY CURRENT (mA) 40 20 VIN = 2V 24.5 24.0 23.5 23.0 10 22.5 90 –50 0 –25 0 25 75 50 125 100 22.0 0 TEMPERATURE (°C) 200 400 600 800 1000 0 5 10 SWITCH CURRENT (mA) LT1173 •TPC06 15 20 25 30 VIN(V) LT1173 •TPC07 LT1173 • TPC08 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 100µA. SET (Pin 7): GB input. GB is an op amp with positive input connected to SET pin and negative input connected to 1.245V reference. FB/SENSE (Pin 8): On the LT1173 (adjustable) this pin goes to the comparator input. On the LT1173-5 and LT1173-12, this pin goes to the internal application resistor that sets output voltage. W BLOCK DIAGRA S LT1173 LT1173-5, -12 SET A2 SET A2 AO V IN GAIN BLOCK/ ERROR AMP GAIN BLOCK/ ERROR AMP I LIM SW1 1.245V REFERENCE OSCILLATOR OSCILLATOR DRIVER COMPARATOR FB SW1 COMPARATOR DRIVER GND I LIM 1.245V REFERENCE A1 A1 4 AO V IN SW2 LT1173 • BD01 R1 GND R2 753k Ω SW2 SENSE LT1173-5: R1 = 250k Ω LT1173-12: R1 = 87.4k Ω LT1173 • BD02 LT1173 UO LT1173 OPERATI The LT1173 is a gated oscillator switcher. This type architecture has very low supply current because the switch is cycled only when the feedback pin voltage drops below the reference voltage. Circuit operation can best be understood by referring to the LT1173 block diagram. Comparator A1 compares the feedback pin voltage with the 1.245V reference voltage. When feedback drops below 1.245V, A1 switches on the 24kHz oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch. An adaptive base drive circuit senses switch current and provides just enough base drive to ensure switch saturation without overdriving the switch, resulting in higher efficiency. The switch cycling action raises the output voltage and feedback pin voltage. When the feedback voltage is sufficient to trip A1, the oscillator is gated off. A small amount of hysteresis built into A1 ensures loop stability without external frequency compensation. When the comparator is low the oscillator and all high current circuitry is turned off, lowering device quiescent current to just 110µA, for the reference, A1 and A2. The oscillator is set internally for 23µs ON time and 19µs 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. 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 2µs. 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 LT1173-5 and LT1173-12 are functionally identical to the LT1173. 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. U W U UO APPLICATI A2 is a versatile gain block that can serve as a low battery detector, a linear post regulator, or drive an under voltage lockout circuit. The negative input of A2 is internally connected to the 1.245V reference. A resistor divider from VIN to GND, with the mid-point connected to the SET pin provides the trip voltage in a low battery detector application. The gain block output (AO) can sink 100µA (use a 47k resistor pull-up to + 5V). This line can signal a microcontroller that the battery voltage has dropped below the preset level. S I FOR ATIO Measuring Input Current at Zero or Light Load Obtaining meaningful numbers for quiescent current and efficiency at low output current involves understanding how the LT1173 operates. At very low or zero load current, the device is idling for seconds at a time. When the output voltage falls enough to trip the comparator, the power switch comes on for a few cycles until the output voltage rises sufficiently to overcome the comparator hysteresis. When the power switch is on, inductor current builds up to hundreds of milliamperes. Ordinary digital multimeters are not capable of measuring average current because of bandwidth and dynamic range limitations. A different approach is required to measure the 100µA off-state and 500mA on-state currents of the circuit. Quiescent current can be accurately measured using the circuit in Figure 1. VSET is set to the input voltage of the LT1173. The circuit must be “booted” by shorting V2 to VSET. After the LT1173 output voltage has settled, disconnect the short. Input voltage is V2, and average input current can be calculated by this formula: IIN = V2 − V1 100Ω (01) 5 LT1173 U W U UO APPLICATI S I FOR ATIO 1MΩ the inductive events add to the input voltage to produce the output voltage. Power required from the inductor is determined by +12V 1µF* – 100 Ω LTC1050 V1 V2 + 1000µF V SET *NON-POLARIZED LT1173 CIRCUIT PL = (VOUT + VD – VIN) (IOUT) (02) + LT1173 • TA06 Figure 1. Test Circuit Measures No Load Quiescent Current of LT1073 Converter 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 (03) PL FOSC Inductor Selection in order for the converter to regulate the output. 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 that maximum current ratings of the LT1173 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 LT1173 based designs, small axial leaded units with saturation current ratings in the 300mA to 1A range (depending on application) are adequate. Lastly, the inductor must have sufficiently low DC resistance so that excessive power is not lost as heat in the windings. An additional consideration is Electro-Magnetic 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. When the switch is closed, current in the inductor builds according to Specifying a proper inductor for an application requires first establishing minimum and maximum input voltage, output voltage, and output current. In a step-up converter, 6 –R't V IL t = IN 1 – e L R' () (04) 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 IL t = IN t L (05) 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 LT1173 specification table (typically 23µs) 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 EL = 1 2 Li 2 PEAK (06) 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. LT1173 U W U UO APPLICATI S I FOR ATIO As an example, suppose 9V at 50mA is to be generated from a 3V input. Recalling Equation 02, PL = (9V + 0.5V – 3V) (50mA) = 325mW. (07) Energy required from the inductor is PL FOSC = 325mW = 13.5µJ. 24kHz (08) Picking an inductor value of 100µH with 0.2Ω DCR results in a peak switch current of iPEAK = 3V 1Ω –1Ω •23µ s – e 1 100µ H = 616m A. (09) Substituting iPEAK into Equation 04 results in EL = ( )( ) 2 1 100µH 0.616 A = 19.0µJ. 2 (10) An inductor’s energy storage capability is proportional to its physical size. If the size of the inductor is too large for a particular application, considerable size reduction is possible by using the LT1111. This device is pin compatible with the LT1173 but has a 72kHz oscillator, thereby reducing inductor and capacitor size requirements by a factor of three. For both positive-to-negative (Figure 7) and negative-topositive configurations (Figure 8), all the output power must be generated by the inductor. In these cases (11) In the positive-to-negative case, switch drop can be modeled as a 0.75V voltage source in series with a 0.65Ω resistor so that VL = VIN – 0.75V – IL (0.65Ω). The step-down case is different than the preceeding three in that the inductor current flows through the load in a step-down topology (Figure 6). Current through the switch should be limited to ~650mA in step-down mode. This can be accomplished by using the ILIM pin. With input voltages in the range of 12V to 25V, a 5V output at 300mA can be generated with a 220µH inductor and 100Ω resistor in series with the ILIM pin. With a 20V to 30V input range, a 470µH inductor should be used along with the 100Ω resistor. Capacitor Selection Since 19µJ > 13.5µJ the 100µH inductor will work. This trial-and-error approach can be used to select the optimum inductor. Keep in mind the switch current maximum rating of 1.5A. If the calculated peak current exceeds this, consider using the LT1073. The 70% duty cycle of the LT1073 allows more energy per cycle to be stored in the inductor, resulting in more output power. PL = ( VOUT + VD) (IOUT). In the negative-to-positive case, the switch saturates and the 0.8Ω switch ON resistance value given for Equation 04 can be used. In both cases inductor design proceeds from Equation 03. Selecting the right output capacitor is almost as important as selecting the right inductor. A poor choice for a filter capacitor can result in poor efficiency and/or high output ripple. Ordinary aluminum electrolytics, while inexpensive and readily available, may have unacceptably poor equivalent series resistance (ESR) and ESL (inductance). There are low-ESR aluminum capacitors on the market specifically designed for switch mode DC-DC converters which work much better than general-purpose units. Tantalum 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 2, 3, and 4 show the output voltage of an LT1173 based converter with three 100µF capacitors. The peak switch current is 500mA in all cases. Figure 2 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 240mΩ. Figure 3 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 70mΩ. Figure 4 shows the circuit with a 16V OS-CON unit. ESR is now only 20mΩ. (12) 7 LT1173 W U U UO 50mV/DIV 50mV/DIV S I FOR ATIO 50mV/DIV APPLICATI 5µs/DIV Figure 2. Aluminum 5µs/DIV LT1173 • TA07 Figure 3. Tantalum In very low power applications where every microampere is important, leakage current of the capacitor must be considered. The OS-CON units do have leakage current in the 5µA to 10µA range. If the load is also in the microampere range, a leaky capacitor will noticeably decrease efficiency. In this type application tantalum capacitors are the best choice, with typical leakage currents in the 1µA to 5µA range. Diode Selection Speed, forward drop, and leakage current are the three main considerations in selecting a catch diode for LT1173 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-50µs range. At best, efficiency will be severely compromised when these diodes are used; at worst, the circuit may not work at all. Most LT1173 circuits will be well served by a 1N5818 Schottky diode. 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 LT1173 requirements. At peak switch currents of 100mA or less, a 1N4148 signal diode may be used. This diode has leakage current in the 1nA5nA 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.) In situations where the load is intermittent and the LT1173 is idling most of the time, battery life can sometimes be extended by using a silicon diode such as the 1N4933, which can handle 1A but has leakage current of less than 1µA. Efficiency will decrease somewhat compared to a 1N5818 while delivering power, but the lower idle current may be more important. 8 5µs/DIV LT1173 • TA08 LT1173 • TA09 Figure 4. OS-CON Step-Up (Boost Mode) Operation A step-up DC-DC converter delivers an output voltage higher than the input voltage. Step-up converters are not short circuit protected since there is a DC path from input to output. The usual step-up configuration for the LT1173 is shown in Figure 5. The LT1173 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: i PEAK = VIN L (13) t ON L1 D1 V IN V OUT R3* I LIM V IN SW1 LT1173 GND R2 + C1 FB SW2 R1 * = OPTIONAL LT1173 • TA10 Figure 5. Step-Up Mode Hookup. Refer to Table 1 for Component Values 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 LT1173 to Note 1: This simple expression neglects the effect of switch and coil resistance. This is taken into account in the “Inductor Selection” section. LT1173 W U U UO APPLICATI S I FOR ATIO keep VFB at the internal reference voltage of 1.245V. R1 and R2 set the output voltage according to the formula R2 VOUT = 1 + 1.245V . R1 ( ) (14) Step-Down (Buck Mode) Operation A step-down DC-DC converter converts a higher voltage to a lower voltage. The usual hookup for an LT1173 based step-down converter is shown in Figure 6. VIN R3 100 Ω + C2 I LIM Inverting Configurations V IN SW1 FB LT1173 L1 VOUT SW2 GND R2 D1 1N5818 + C1 R1 LT1173 • TA11 Figure 6. Step-Down Mode Hookup When the switch turns on, SW2 pulls up to VIN – VSW. This puts a voltage across L1 equal to VIN – VSW – VOUT, causing a current to build up in L1. At the end of the switch ON time, the current in L1 is equal to i PEAK = 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 800mA. When using the LT1173 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). VIN − VSW − VOUT L t ON. (15) 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 LT1173. Output voltage is determined by R2 VOUT = 1 + 1.245 V . R1 ( ) (16) The LT1173 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 accomodated as in the prior section. +VIN R3 I LIM V IN + SW1 C2 FB LT1173 L1 SW2 GND R1 D1 1N5818 + C1 R2 –VOUT LT1173 • 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. 9 LT1173 U W L1 D1 U UO APPLICATI S I FOR ATIO +VOUT + C1 I LIM + C2 V IN SW1 R1 IL 2N3906 LT1173 AO GND FB SW2 ON SWITCH OFF R2 VOUT = LT1173 • TA14 ( ) R1 1.245V + 0.6V R2 –VIN Figure 9. No Current Limit Causes Large Inductor Current Build-Up LT1173 • TA13 Figure 8. Negative-to-Positive Converter PROGRAMMED CURRENT LIMIT Using the ILIM Pin The LT1173 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 LT1173 must operate at an 800mA peak switch current with a 2.0V 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 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 1 < . VIN − VSW 1 − DC (17) 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. 10 IL SWITCH ON OFF LT1173 • TA15 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 2µs. The current trip point becomes less accurate for switch ON times less than 4µs. Resistor values programming switch ON time for 2µs 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 LT1173 • TA28 Figure 11. LT1173 Current Limit Circuitry LT1173 W U U UO APPLICATI S I FOR ATIO Using the Gain Block +5V V IN The gain block (GB) on the LT1173 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 1.245V reference. The positive input comes out on the SET pin. LT1173 100k R1 VBAT 1.245V REF – SET + AO R2 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. 100kΩ for R2 is adequate. R3 can be added to introduce a small amount of hysteresis. This will cause the gain block to “snap” when the trip point is reached. Values in the 1M-10M range are optimal. The addition of R3 will change the trip point, however. TO PROCESSOR GND R3 V – 1.245V R1 = LB 11.7µA VLB = BATTERY TRIP POINT R2 = 100kΩ R3 = 4.7MΩ LT1173 • TA16 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 NOTES 2.0-3.1 5 90mA 5 47µH G GA10-472K, C CTX50-1 100µF * 2.0-3.1 5 10mA 5 220µH G GA10-223K, C CTX 22µF 2.0-3.1 12 50mA 5 47µH G GA10-472K, C CTX50-1 47µF 2.0-3.1 12 10mA 5 150µH G GA10-153K 22µF 5 12 90mA 5 120µH G GA10-123K 100µF 5 12 30mA 5 150µH G GA10-153K 47µF 5 15 50mA 5 120µH G GA10-123K C CTX100-4 47µF * ** 5 30 25mA 5 100µH G GA10-103K, C CTX100-4 10µF, 50V 6.5-9.5 5 50mA 6 47µH G GA10-472K, C CTX50-1 100µF ** 12-20 5 300mA 6 220µH G GA20-223K 220µF ** 20-30 5 300mA 6 470µH G GA20-473K 470µF ** 5 –5 75mA 7 100µH G GA10-103K, C CTX100-4 100µF ** 12 –5 250mA 7 470µH G GA40-473K 220µF ** –5 5 150mA 8 100µH G GA10-103K, C CTX100-4 220µF –5 12 75mA 8 100µH G GA10-103K, C CTX100-4 47µF G = Gowanda C = Coiltronics * Add 68Ω from ILIM to VIN ** Add 100Ω from ILIM to VIN 11 LT1173 W U U UO APPLICATI S I FOR ATIO Table 2. Inductor Manufacturers Table 3. Capacitor Manufacturers MANUFACTURER PART NUMBERS MANUFACTURER PART NUMBERS Gowanda Electronics Corporation 1 Industrial Place Gowanda, NY 14070 716-532-2234 GA10 Series GA40 Series Sanyo Video Components 2001 Sanyo Avenue San Diego, CA 92173 619-661-6835 OS-CON Series Caddell-Burns 258 East Second Street Mineola, NY 11501 516-746-2310 7300 Series 6860 Series Nichicon America Corporation 927 East State Parkway Schaumberg, IL 60173 708-843-7500 PL Series Coiltronics International 984 S.W. 13th Court Pompano Beach, FL 33069 305-781-8900 Custom Toroids Surface Mount Sprague Electric Company Lower Main Street Sanford, ME 04073 207-324-4140 150D Solid Tantalums 550D Tantalex Renco Electronics Incorporated 60 Jefryn Boulevard, East Deer Park, NY 11729 800-645-5828 RL1283 RL1284 UO TYPICAL APPLICATI S 3V to –22V LCD Bias Generator L1* 100µH 1N4148 R1 100Ω 2.21M 1% ILIM V IN SW1 2 X 1.5V CELLS 3V LT1173 FB GND SW2 + 4.7µF 0.1µF 118k 1% 1N5818 1N5818 + 22µF * L1 = GOWANDA GA10-103K COILTRONICS CTX100-4 FOR 5V INPUT CHANGE R1 TO 47Ω. CONVERTER WILL DELIVER –22V AT 40mA. 220k –22V OUTPUT 7mA AT 2.0V INPUT 70% EFFICIENCY LT1173 • TA19 12 LT1173 UO TYPICAL APPLICATI S 3V to 5V Step-Up Converter 9V to 5V Step-Down Converter L1* 100 µ H 100 Ω V IN ILIM ILIM V IN 9V BATTERY SW1 2 X 1.5V CELLS LT1173-5 1N5818 LT1173-5 SW2 SENSE 5V OUTPUT 150mA AT 3V INPUT 60mA AT 2V INPUT SENSE GND SW1 + GND SW2 L1* 47µH 100 µ F + 1N5818 * L1 = GOWANDA GA10-103K COILTRONICS CTX100-1 (SURFACE MOUNT) 5V OUTPUT 150mA AT 9V INPUT 50mA AT 6.5V INPUT 100 µ F * L1 = GOWANDA GA10-472K COILTRONICS CTX50-1 FOR HIGHER OUTPUT CURRENTS SEE LT1073 DATASHEET LT1173 • TA17 +5V to –5V Converter LT1173 • TA18 +20V to 5V Step-Down Converter +VIN 5V INPUT +VIN 12V-28V 100 Ω 100 Ω ILIM V IN ILIM V IN SW1 + 22µF SW1 LT1173-5 LT1173-5 SENSE GND SENSE SW2 GND L1* 100µH 100 µ F 1N5818 SW2 L1* 220µH + + 1N5818 5V OUTPUT 300mA 100 µ F –5V OUTPUT 75mA * L1 = GOWANDA GA10-103K COILTRONICS CTX100-1 * L1 = GOWANDA GA20-223K LT1173 • TA20 LT1173 • TA21 Telecom Supply L1* 500µH 44mH ~ + 48V DC 44mH ~ + 47µF 100V – MUR110 220µF 10V 3.6MΩ + +5V 100mA 390kΩ 10k VN2222 12V 10nF *L1 = CTX110077 IQ = 120µA 2N5400 IRF530 100Ω 1N4148 15V ILIM + 1N965B V IN SW1 10µF 16V LT1173 FB GND SW2 110kΩ LT1173 • TA22 13 LT1173 UO TYPICAL APPLICATI S “5 to 5” Step-Up or Step-Down Converter L1* 100µH 1N5818 SI9405DY +5V OUTPUT 56Ω 1 2 V IN ILIM 4 X NICAD OR ALKALINE CELLS + 470µF 470k SW1 7 SET LT1173 AO FB GND 5 75k 3 + 6 470µF 8 + SW2 4 240Ω 470µF 24k *L1 = COILTRONICS CTX100-4 GOWANDA GA20-103K VIN = 2.6V TO 7.2V VOUT = 5V AT 100mA LT1173 • TA23 2V to 5V at 300mA Step-Up Converter with Under Voltage Lockout L1* 20µH, 5A 47k 1N5820 100k 220 ILIM 100k 2N3906 100 SW1 2N4403 LT1173 2.2M 2 X NICAD V IN AO +5V OUTPUT 300mA LOCKOUT AT 1.85V INPUT 301k† SET GND FB SW2 5Ω + MJE200 100k *L1 = COILTRONICS CTX-20-5-52 †1% METAL FILM 14 100k† 100µF OS-CON 47Ω LT1173 • TA24 LT1173 UO TYPICAL APPLICATI S Voltage Controlled Positive-to-Negative Converter 0.22 VIN 5V-12V L1* 50µH, 2.5A MJE210 + 1N5818 V IN 220 –VOUT = –5.13 • VC 2W MAXIMUM OUTPUT ILIM 150 V IN SW1 200k 39k – LT1173 VC (0V TO 5V) LT1006 FB GND 100µF 1N5820 SW2 + * L1 = GOWANDA GT10-101 LT1173 • TA25 High Power, Low Quiescent Current Step-Down Converter 0.22Ω VIN 7V-24V L1* 25µH, 2A MTM20P08 18V 1W 1N5818 2k 51Ω 1N5820 5V 500mA + 470µF 2N3904 V IN 100Ω 1/2W ILIM SW1 1N4148 LT1173 121k FB GND SW2 40.2k * L1 = GOWANDA GT10-100 EFFICIENCY ≥ 80% FOR 10mA ≤ ILOAD ≤ 500mA STANDBY IQ ≤ 150µA OPERATE STANDBY LT1173 • TA26 2 Cell Powered Neon Light Flasher 0.02µF L1* 470µH ILIM 1N4148 1N4148 95V REGULATED V IN SW1 3V 1N4148 0.02µF 0.02µF LT1173 100M FB GND SW2 1.3M 3.3M 0.68µF 200V *TOKO 262LYF-0100K NE-2 BLINKS AT 0.5Hz LT1173 • TA27 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 LT1173 U PACKAGE DESCRIPTIO Dimensions in inches (milimeters) unless otherwise noted. N8 Package 8-Lead Plastic DIP 0.400* (10.160) MAX 8 7 6 5 1 2 3 4 0.255 ± 0.015* (6.477 ± 0.381) 0.300 – 0.325 (7.620 – 8.255) 0.065 (1.651) TYP 0.009 – 0.015 (0.229 – 0.381) ( +0.025 0.325 –0.015 8.255 +0.635 –0.381 0.130 ± 0.005 (3.302 ± 0.127) 0.045 – 0.065 (1.143 – 1.651) 0.125 (3.175) MIN 0.045 ± 0.015 (1.143 ± 0.381) ) 0.018 ± 0.003 (0.457 ± 0.076) 0.100 ± 0.010 (2.540 ± 0.254) 0.015 (0.380) MIN N8 0694 *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTURSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm). S8 Package 8-Lead Plastic SOIC 0.189 – 0.197* (4.801 – 5.004) 8 7 6 5 0.150 – 0.157* (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) 1 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) 2 3 4 0.004 – 0.010 (0.101 – 0.254) 0.050 (1.270) BSC SO8 0294 *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm). 16 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 LT/GP 0894 2K REV B • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 1994