LT1073

LT1073 Micropower DC/DC Converter Adjustable and Fixed 5V, 12V FEATURES s s s s s DESCRIPTIO s s s s s No Design Required Operates at Supply Voltages from 1V to 30V Consumes Only 95µA Supply Current Works in Step-Up or Step-Down Mode Only Three External Off-the-Shelf 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 PDIP or SO-8 Package The LT®1073 is a versatile micropower DC/DC converter. The device requires only three external components to deliver a fixed output of 5V or 12V. The very low minimum supply voltage of 1V allows the use of the LT1073 in applications where the primary power source is a single cell. An on-chip auxiliary gain block can function as a lowbattery detector or linear post-regulator. Average current drain of the LT1073-5 used as shown in the Typical Application circuit below is just 135µA unloaded, making it ideal for applications where long battery life is important. The circuit shown can deliver 5V at 40mA from an input as low as 1.25V and 5V at 10mA from a 1V input. The device can easily be configured as a step-up or stepdown converter, although for most step-down applications or input sources greater than 3V, the LT1173 is recommended. Switch current limiting is user-adjustable by adding a single external resistor. Unique reversebattery protection circuitry limits reverse current to safe, nondestructive levels at reverse supply voltages up to 1.6V. , LTC and LT are registered trademarks of Linear Technology Corporation. APPLICATIO S s s s s s s s s s s Pagers Cameras Single-Cell to 5V Converters Battery Backup Supplies Laptop and Palmtop Computers Cellular Telephones Portable Instruments 4mA to 20mA Loop Powered Instruments Hand-Held Inventory Computers Battery-Powered α, β, and γ Particle Detectors TYPICAL APPLICATION Single-Cell to 5V Converter CONTINUOUS OPERATION (HOURS) CADDELL-BURNS 7300-12 1N5818 82µH Single Alkaline “AA” Cell Operating Hours vs DC Load Current 1000 5V 40mA 1 ILIM 1.5V AA CELL* 2 VIN SW1 3 100 LT1073-5 SENSE GND 5 SW2 4 8 10 + 100µF SANYO 0S-CON 1 OPERATES WITH CELL VOLTAGE ≥1V *ADD 10µF DECOUPLING CAPACITOR IF BATTERY IS MORE THAN 2" AWAY FROM LT1073 1 1073 TA01 U L = 180µH L = 82µH 10 LOAD CURRENT (mA) 100 LT1073 TA02 U U 1 LT1073 ABSOLUTE (Note 1) AXI U RATI GS PACKAGE/ORDER I FOR ATIO TOP VIEW ILIM 1 VIN 2 SW1 3 SW2 4 N8 PACKAGE 8-LEAD PDIP 8 7 6 5 FB (SENSE)* SET A0 GND Supply Voltage, Step-Up Mode ................................ 15V Supply Voltage, Step-Down Mode ........................... 36V SW1 Pin Voltage ...................................................... 50V SW2 Pin Voltage ........................................... –0.4 to VIN Feedback Pin Voltage (LT1073) ................................. 5V Switch Current ........................................................ 1.5A Maximum Power Dissipation ............................. 500mW Operating Temperature Range ..................... 0°C to 70°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C S8 PACKAGE 8-LEAD PLASTIC SO *FIXED VERSIONS TJMAX = 125°C, θJA = 100°C/W (N8) TJMAX = 125°C, θJA = 120°C/W (S8) ORDER PART NUMBER LT1073CN8 LT1073CN8-5 LT1073CN8-12 LT1073CS8 LT1073CS8-5 LT1073CS8-12 S8 PART MARKING 1073 10735 107312 Consult factory for parts specified with wider operating temperature ranges. The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 1.5V unless otherwise specified. SYMBOL IQ IQ VIN PARAMETER Quiescent Current Quiescent Current, Step-Up Mode Configuration Input Voltage CONDITIONS Switch Off No Load Step-Up Mode Step-Down Mode Comparator Trip Point Voltage VOUT Output Sense Voltage Comparator Hysteresis Output Hysteresis fOSC DC tON IFB ISET VAO Oscillator Frequency Duty Cycle Switch ON Time Feedback Pin Bias Current Set Pin Bias Current AO Output Low Reference Line Regulation VCESAT Switch Saturation Voltage Set-Up Mode LT1073, VFB = 0V VSET = VREF IAO = – 100µA 1V ≤ VIN ≤ 1.5V 1.5V ≤ VIN ≤ 12V VIN = 1.5V, ISW = 400mA q q ELECTRICAL CHARACTERISTICS MIN LT1073-5 LT1073-12 q q q q q q q q q TYP 95 135 250 MAX 130 UNITS µA µA µA 1.15 1.0 202 4.75 11.4 212 5 12 5 125 300 15 65 30 19 72 38 10 60 0.15 0.35 0.05 300 400 12.6 12.6 30 222 5.25 12.6 10 250 600 23 80 50 50 120 0.4 1.0 0.1 400 600 550 750 1000 1500 LT1073 (Note 2) LT1073-5 (Note 3) LT1073-12 (Note 3) LT1073 LT1073-5 LT1073-12 Full Load (VFB = VREF) q q q q q q q VIN = 1.5V, ISW = 500mA q VIN = 5V, ISW = 1A q 700 q AV A2 Error Amp Gain RL = 100kΩ (Note 4) 400 1000 2 U V V V mV V V mV mV mV kHz % µs nA nA V %V %V mV mV mV mV mV mV V/V W U U WW W LT1073 The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 1.5V unless otherwise specified. SYMBOL IREV ILIM ILEAK VSW2 PARAMETER Reverse Battery Current Current Limit Current Limit Temperature Coefficient Switch OFF Leakage Current Maximum Excursion Below GND Measured at SW1 Pin ISW1 ≤ 10µA, Switch Off CONDITIONS (Note 5) 220Ω Between ILIM and VIN MIN TYP 750 400 –0.3 1 –400 10 –350 MAX UNITS mA mA %/°C µA mV ELECTRICAL CHARACTERISTICS 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 point of the comparator fall within the 202mV to 222mV range. Note 3: This specification guarantees that the output voltage of the fixed versions will always fall within the specified range. The waveform at the SENSE pin will exhibit a sawtooth shape due to the comparator hysteresis. Note 4: 100k resistor connected between a 5V source and the AO pin. Note 5: The LT1073 is guaranteed to withstand continuous application of 1.6V applied to the GND and SW2 pins while VIN, ILIM and SW1 pins are grounded. TYPICAL PERFOR A CE CHARACTERISTICS Saturation Voltage Step-Up Mode (SW2 Pin Grounded) 1.2 VIN = 1.5V 1.0 VIN = 1.25V 0.8 VCESAT (V) SWITCH ON VOLTAGE (V) SWITCH CURRENT (mA) VIN = 1V 0.6 0.4 0.2 0 VIN = 3V VIN = 2V 0 0.2 0.4 0.6 ISWITCH (A) 0.8 FB Pin Bias Current vs Temperature 20 18 SET PIN BIAS CURRENT (nA) FB BIAS CURRENT (nA) 16 14 12 10 8 6 4 –50 –25 0 25 50 75 100 125 125 100 75 50 25 0 –50 –25 0 25 50 75 100 125 GB GAIN (V/V) TEMPERATURE (°C) 1073 G04 UW VIN = 5V 1.0 Switch ON Voltage Step-Down Mode (SW1 Pin Connected to VIN) 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 1200 1100 1000 900 800 700 600 500 400 300 200 Maximum Switch Current vs RLIM L = 82µH VIN = 3V VIN = 1.5V 1.2 1073 G01 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 100 10 ISWITCH (A) 1073 G02 100 RLIM (Ω) 1000 1073 G03 SET Pin Bias Current vs Temperature 200 175 150 1800 1600 1400 1200 1000 800 600 400 200 “Gain Block” Gain VIN = 1.5V RL = 100k 0 –50 –25 0 25 50 75 100 125 TEMPERATURE (°C) 1073 G05 TEMPERATURE (°C) 1073 G06 3 LT1073 TYPICAL PERFOR A CE CHARACTERISTICS Supply Current vs Temperature 150 140 130 SUPPLY CURRENT (µA) 120 110 100 90 80 70 60 50 –50 –25 0 25 50 75 100 125 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 INPUT VOLTAGE (V) 4.5 5.0 VIN = 1.5V MINIMUM INDUCTANCE (µH) 200 150 100 50 OUTPUT CURRENT (mA) TEMPERATURE (°C) 1073 G07 PI FU CTIO 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 212mV reference. FB/SENSE (Pin 8): On the LT1073 (adjustable) this pin goes to the comparator input. On the LT1073-5 and LT1073-12, this pin goes to the internal application resistor that sets output voltage. BLOCK DIAGRA S SET LT1073 SET + A2 A0 VIN 212mV REFERENCE – VIN 212mV REFERENCE A1 GND COMPARATOR FB OSCILLATOR DRIVER SW2 1073 BD01 GAIN BLOCK/ERROR AMP ILIM SW1 4 UW Recommended Minimum Inductance Value 300 250 Guaranteed Minimum Output Current at 5V vs VIN 1000 RLIM = 0V 100 FOR VIN > 1.6V A 68Ω RESISTOR MUST BE CONNECTED BETWEEN ILIM AND VIN 10 1.0 1.5 2.0 2.5 VIN (V) 3.0 3.5 1073 G09 1073 G08 W U U U LT1073-5 and LT1073-12 + A2 A0 – GAIN BLOCK/ERROR AMP ILIM A1 OSCILLATOR DRIVER COMPARATOR SW2 SW1 Q1 Q1 R1 GND R2 940k SENSE LT1073-5: R1 = 40k LT1073-12: R2 = 16.3k 1073 BD02 LT1073 OPERATIO LT1073 The LT1073 is 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 LT1073 Block Diagram. Comparator A1 compares the FB pin voltage with the 212mV reference signal. When FB drops below 212mV, A1 switches on the 19kHz oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch Q1. 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 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 is low the oscillator and all high current circuitry is turned off, lowering device quiescent current to just 95µA for the reference, A1 and A2. The oscillator is set internally for 38µs ON time and 15µs OFF time, optimizing the device for step-up circuits where VOUT ≈ 3VIN, e.g., 1.5V to 5V. Other step-up ratios as well as step-down (buck) converters are possible at slight losses in maximum achievable power output. A2 is a versatile gain block that can serve as a low-battery detector, a linear post-regulator, or drive an undervoltage lockout circuit. The negative input of A2 is internally connected to the 212mV reference. An external resistor divider from VIN to GND provides the trip point for A2. The AO output can sink 100µA (use a 56k resistor pull-up to 5V). This line can signal a microcontroller that the battery voltage has dropped below the preset level. U A resistor connected between the ILIM pin and VIN adjusts maximum switch current. When the switch current exceeds the set value, the switch is turned off. This feature is especially useful when small inductance values are used with high input voltages. If the internal current limit of 1.5A is desired, ILIM should be tied directly to VIN. Propagation delay through the current-limit circuitry is about 2µs. In step-up mode, SW2 is connected to ground and SW1 drives the inductor. In step-down mode, SW1 is connected to VIN and SW2 drives the inductor. Output voltage is set by the following equation in either step-up or stepdown modes where R1 is connected from FB to GND and R2 is connected from VOUT to FB.  R2  VOUT = 212mV  + 1  R1  ( ) LT1073-5 and LT1073-12 The LT1073-5 and LT1073-12 fixed output voltage versions have the gain-setting resistor on-chip. Only three external components are required to construct a fixedoutput converter. 5µA flows through R1 and R2 in the LT1073-5, and 12.3µA flows in the LT1073-12. This current represents a load and the converter must cycle from time to time to maintain the proper output voltage. Output ripple, inherently present in gated-oscillator designs, will typically run around 150mV for the LT1073-5 and 350mV for the LT1073-12 with the proper inductor/ capacitor selection. This output ripple can be reduced considerably by using the gain block amp as a preamplifier in front of the FB pin. See the Applications Information section for details. 5 LT1073 APPLICATIO S I FOR ATIO INPUT VOLTAGE (V) 1.55-1.25 1.30-1.05 1.55-1.25 1.30-1.05 3.1-2.1 3.1-2.1 3.3-2.5 3.1-2.1 3.1-2.1 3.3-2.5 4.5-5.5 4.5-5.5 4.5-5.5 G = GOWANDA BATTERY TYPE Single Alkaline Single Ni-Cad Single Alkaline Single Ni-Cad Two Alkaline Two Alkaline Lithium Two Alkaline Two Alkaline Lithium TTL Supply TTL Supply TTL Supply OUTPUT VOLTAGE (V) 3 3 5 5 5 5 5 12 12 12 12 12 24 Table 1. Component Selection for Step-Up Converters OUTPUT CURRENT (MIN) 60mA 20mA 30mA 10mA 80mA 25mA 100mA 25mA 5mA 30mA 90mA 22mA 35mA INDUCTOR VALUE (µH) 82 180 82 180 120 470 150 120 470 150 220 1000 220 CB = CADDELL-BURNS INDUCTOR PART NUMBER G GA10-822K, CB 7300-12 G GA10-183K, CB 7300-16 G GA10-822K, CB 7300-12 G GA10-183K, CB 7300-16 G GA10-123K, CB 7300-14 G GA10-473K, CB 7300-21 G GA40-153K, CB 6860-15 G GA10-123K, CB 7300-14 G GA10-473K, CB 7300-21 G GA10-153K, CB 7300-15 G GA40-223K, CB 6860-17 G GA10-104K, CB 7300-25 G GA40-223K, CB 6860-17 CAPACITOR VALUE (µF) 150 47 100 22 470 150 470 220 100 220 470 100 150 * * * * * * NOTES Measuring Input Current at Zero or Light Load Obtaining meaningful numbers for quiescent current and efficiency at low output current involves understanding how the LT1073 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. 1MΩ 12V 1µF* – LTC1050 100Ω V1 V2 1000µF *NONPOLARIZED LT1073 CIRCUIT + VSET + 1073 F01 Figure 1. Test Circuit Measures No-Load Quiescent Current of LT1073 Converter Quiescent current can be accurately measured using the circuit in Figure 1. VSET is set to the input voltage of the 6 U *Add 68Ω from ILIM to VIN W UU LT1073. The circuit must be “booted” by shorting V2 to VSET. After the LT1073 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 Inductor Selection 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 LT1073 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 LT1073-based designs, small axial-lead units with LT1073 APPLICATIO S I FOR ATIO 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. 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, the inductive events add to the input voltage to produce the output voltage. Power required from the inductor is determined by: PL = (VOUT + VD – VIN)(IOUT) where VD is the diode drop (0.5V for a 1N5818 Schottky). Maximum power in the inductor is ISWITCH (mA) P L = E L• fOSC 2 1 = L iPEAK • fOSC 2 where V  iPEAK =  IN  R  –RtON  1– e  L   R = Switch equivalent resistance (1Ω maximum) added to the DC resistance of the inductor and t ON = ON time of the switch. At maximum VIN and ON time, iPEAK should not be allowed to exceed the maximum switch current shown in Figure 2. Some input/output voltage combinations will cause continuous1 mode operation. In these cases a resistor is needed between ILIM (Pin 1) and VIN (Pin 2) to keep switch current under control. See the “Using the ILIM Pin” section for details. NOTE 1: i.e., inductor current does not go to zero when the switch is off. U 1200 1000 800 600 400 200 0 0 1 2 VIN (V) 1073 F02 W UU 3 4 5 Figure 2. Maximum Switch Current vs Input Voltage Capacitor Selection 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 3, 4, and 5 show the output voltage of an LT1073 based converter with three 100µF capacitors. The peak switch current is 500mA in all cases. Figure 3 shows a Sprague 501D aluminum capacitor. VOUT jumps by over 150mV 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 300mΩ. Figure 4 shows the same circuit, but with a Sprague 150D tantalum capacitor replacing the aluminum unit. Output jump is now about 30mV, corresponding to an ESR of 60mΩ. Figure 5 shows the circuit with an OS-CON unit. ESR is now only 30mΩ. 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 7 LT1073 APPLICATIO S I FOR ATIO 50mV/DIV 50mV/DIV 20µs/DIV Figure 3. Aluminum 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 LT1073 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 and at worst, the circuit may not work at all. Most LT1073 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 LT1073 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.) In situations where the load is intermittent and the LT1073 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. 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 8 U 50mV/DIV 20µs/DIV 20µs/DIV W UU Figure 4. Tantalum Figure 5. OS-CON short-circuit protected since there is a DC path from input to output. The usual step-up configuration for the LT1073 is shown in Figure 6. The LT1073 first pulls SW1 low causing VINVCESAT to appear across L1. A current then builds up in L1. At the end of the switch ON time the current in L1 is2: iPEAK = V IN t ON L NOTE 2: This simple expression neglects the effect of switch and coil resistance. These are taken into account in the “Inductor Selection” section. L1 VIN R3* ILIM VIN SW1 LT1073 FB GND SW2 *= OPTIONAL R1 1073 F06 D1 VOUT R2 + C1 Figure 6. 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 LT1073 to keep VFB at the internal reference voltage of 212mV. R1 and R2 set the output voltage according to the formula: LT1073 APPLICATIO S I FOR ATIO VOUT  R2 =  1 +  212mV  R1 ( ) Step-Down (Buck Mode) Operation A step-down DC/DC converter converts a higher voltage to a lower voltage. It is short-circuit protected because the VIN R3 220Ω ILIM VIN SW1 FB + C2 LT1073 SW2 GND D1 1N5818 L1 VOUT R2 + C1 R1 1073 FO7 Figure 7. Step-Down Mode Hookup switch is in series with the output. Step-down converters are characterized by low output voltage ripple but high input current ripple. The usual hookup for an LT1073based step-down converter is shown in Figure 7. 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 V –V –V = IN SW OUT tON L 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 LT1073. Output voltage is determined by VOUT  R2 =  1 +  212mV  R1 ( ) U 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 220Ω resistor programs the switch to turn off when the current reaches approximately 400mA. When using the LT1073 in stepdown mode, output voltage should be limited to 6.2V or less. Inverting Configurations The LT1073 can be configured as a positive-to-negative converter (Figure 8), or a negative-to-positive converter (Figure 9). In Figure 8, 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 VOUT should be less than 6.2V. +VIN W UU + C2 R3 ILIM VIN SW1 FB LT1073 SW2 GND R1 L1 D1 1N5818 + C1 R2 –VOUT 1073 FO8 Figure 8. Positive-to-Negative Converter L1 D1 +VOUT + C1 ILIM VIN SW1 LT1073 AO GND –VIN VOUT = FB SW2 R2 2N3906 R1 + C2 ( R1 )212mV + 0.6V R2 1073 F09 Figure 9. Negative-to-Positive Converter 9 LT1073 APPLICATIO S I FOR ATIO In Figure 9, 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. Using the ILIM Pin The LT1073 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 LT1073 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 (see the “Maximum Switch Current vs RLIM” characteristic) selected, the switch current will be limited to 800mA, even if the input voltage increases. The LT1073 does this by sampling a small fraction of the switch current and passing this current through the external resistor. When the voltage on the ILIM pin drops a VBE below VIN, the oscillator terminates the cycle. Propagation delay through this loop is about 2µs. Another situation where the ILIM feature is useful is when the device goes into continuous mode operation. This occurs in step-up mode when IL ON SWITCH OFF 1073 F10 Figure 10. No Current Limit Causes Large Inductor Current Build-Up 10 U VOUT + VDIODE 1 < VIN – VSW 1 – DC 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 nonzero current level in the inductor just prior to switch turn-on. As shown in Figure 10, 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 11, keeping output ripple to a minimum. Using the Gain Block The gain block (GB) on the LT1073 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 (–) input of the gain block is tied internally to the 212mV reference. The (+) 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. 100kΩ for R2 is adequate. Output ripple of the LT1073, normally 150mV at 5VOUT, PROGRAMMED CURRENT LIMIT can be reduced significantly by placing the gain block in front of the FB input as shown in Figure 13. This effectively IL reduces the comparator hysteresis by the gain of the gain ON block. Output ripple can be reduced to just a few millivolts SWITCH OFF using this technique. Ripple reduction works with stepdown or inverting modes as well. 1073 F11 W UU Figure 11. Current Limit Keeps Inductor Current Under Control LT1073 APPLICATIO S I FOR ATIO 5V VIN R1 212mV REF SET R2 GND R1 = R2 LB ( 212mV –1) LT1073 100k A0 TO PROCESSOR – + VBAT V VLB = BATTERY TRIP POINT Figure 12. Settling Low Battery Detector Trip Point Table 2. Inductor Manufacturers MANUFACTURER Gowanda Electronics Corporation 1 Industrial Place Gowanda, NY 14070 716-532-2234 Caddell-Burns 258 East Second Street Mineola, NY 11501 516-746-2310 Coiltronics International 984 S.W. 13th Court Pompano Beach, FL 33069 305-781-8900 Toko America Incorporated 1250 Feehanville Drive Mount Prospect, IL 60056 312-297-0070 Renco Electronics Incorporated 60 Jefryn Boulevard, East Deer Park, NY 11729 800-645-5828 PART NUMBERS GA10 Series GA40 Series 7300 Series 6860 Series Custom Toroids Surface Mount Type 8RBS RL1283 RL1284 TYPICAL APPLICATIO S 1.5V to 3V Step-Up Converter L1† 120µH 220Ω ILIM 1.5V CELL VIN SW1 LT1073 FB GND SW2 GND 1N5818 3V OUTPUT 20mA AT VBATTERY = 1V ILIM VIN SW1 LT1073 FB SW2 536k* + 100µF 40.2k* * 1% METAL FILM † L1 = GOWANDA GA10-123k OR CADDELL-BURNS 7300-14 * 1% METAL FILM † L1 = GOWANDA GA10-123k OR CADDELL-BURNS 7300-14 1073 TA03 U L1 R3 680k D1 VOUT ILIM AO VIN SW1 LT1073 FB VBAT GND SET SW2 R1 1073 F13 W U UU R2 + C1 1073 F12 VOUT = R2 + 1 212mV R1 ( )( ) Figure 13. Output Ripple Reduction Using Gain Block Table 3. Capacitor Manufacturers MANUFACTURER Sanyo Video Components 1201 Sanyo Avenue San Diego, CA 92073 619-661-6322 Nichicon America Corporation 927 East State Parkway Schaumberg, IL 60173 708-843-7500 Sprague Electric Company Lower Main Street Stanford, ME 04073 207-324-4140 PART NUMBERS OS-CON Series PL Series 150D Solid Tantalums 550D Tantalex 1.5V to 9V Step-Up Converter L1† 120µH 1N5818 9V OUTPUT 7mA AT VBATTERY = 1V 16mA AT VBATTERY = 1.5V 1M* 1.5V CELL + 47µF 24.3k* 1073 TA04 11 LT1073 TYPICAL APPLICATIO S 1.5V to 12V Step-Up Converter L1† 120µH 1N5818 12V OUTPUT 5mA AT VBATTERY = 1V 16mA AT VBATTERY = 1.5V ILIM 1.5V CELL VIN SW1 LT1073-12 SENSE GND SW2 † L1 = GOWANDA GA10-123k OR CADDELL-BURNS 7300-14 3V to 12V Step-Up Converter L1 68µH 100Ω ILIM TWO 1.5V CELLS VIN SW1 LT1073-12 SENSE GND SW2 † 1N5818 † L1 = GOWANDA GA10-682k OR CADDELL-BURNS 7300-11 5V to 12V Step-Up Converter L1 150µH 5VIN 50Ω ILIM 100µF VIN SW1 LT1073-12 SENSE GND SW2 † 1N5818 12V OUTPUT 130mA AT 4.5VIN + † L1 = GOWANDA GA20-153k OR CADDELL-BURNS 7200-15 12 U + 3V to 5V Step-Up Converter L1† 68µH 100Ω ILIM VIN SW1 LT1073-5 SENSE GND SW2 1N5818 5V OUTPUT 100mA AT VBATTERY = 2V 47µF TWO 1.5V CELLS + 100µF † 1073 TA05 L1 = GOWANDA GA10-682k OR CADDELL-BURNS 7300-11 1073 TA06 3V to 15V Step-Up Converter 12V OUTPUT 35mA AT VBATTERY = 2V L1† 68µH 100Ω 1N5818 15V OUTPUT 27mA AT VBATTERY = 2V 1M* ILIM VIN SW1 LT1073 FB GND SW2 14.3k* + 47µF TWO 1.5V CELLS + 47µF 1073 TA07 * 1% METAL FILM † L1 = GOWANDA GA10-682k OR CADDELL-BURNS 7300-11 1073 TA08 5V to 15V Step-Up Converter L1† 150µH 5VIN 50Ω 1M* 1N5818 15V OUTPUT 100mA AT 4.5VIN + ILIM 100µF VIN SW1 100µF + + LT1073 FB GND SW2 14.3k* 100µF 1073 TA09 * 1% METAL FILM † L1 = GOWANDA GA20-153k OR CADDELL-BURNS 7200-15 1073 TA10 LT1073 TYPICAL APPLICATIO S 1.5V to 5V Step-Up Converter with Logic Shutdown L1† 82µH 1N5818 5V OUTPUT 909k* ILIM 1.5V CELL VIN SW1 LT1073 FB GND SW2 1N4148 40.2k* 74C04 SHUTDOWN OPERATE 1073 TA11 * 1% METAL FILM † L1 = GOWANDA GA10-822k OR CADDELL-BURNS 7300-12 9V to 3V Step-Down Converter 3V OUTPUT 220Ω ILIM VIN SW1 LT1073 9V BATTERY GND FB SW2 L1† 100µH 536k* + 1N5818 * 1% METAL FILM L1 = GOWANDA GA10-103k OR CADDELL-BURNS 7300-13 † 1.5V to 5V Bootstrapped Step-Up Converter L1 47µH † 1N5818 5V OUTPUT 50mA 2N3906 2.2k 1.5V CELL 56Ω ILIM VIN SW1 LT1073-5 SENSE GND SW2 GND † L1 = GOWANDA GA10-123k OR CADDELL-BURNS 7300-14 MINIMUM START-UP VOLTAGE = 1.1V U 1.5V to 5V Step-Up Converter with Low-Battery Detector L1† 82µH 1N5818 5V OUTPUT 100k ILIM SET 100k* AO GND SENSE SW2 LO BAT GOES LOW AT VBATTERY = 1.15V 1073 TA12 442k* 1.5V CELL VIN SW1 + 100µF + LT1073-5 100µF * 1% METAL FILM † L1 = GOWANDA GA10-822k OR CADDELL-BURNS 7300-12 9V to 5V Step-Down Converter 220Ω ILIM VIN SW1 LT1073-5 9V BATTERY GND 40.2k* SENSE SW2 L1† 100µH 5V OUTPUT + 1N5818 † 1073 TA13 100µF 100µF L1 = GOWANDA GA10-103k OR CADDELL-BURNS 7300-13 1073 TA14 Memory Backup Supply 5V MAIN SUPPLY L1† 82µH 5V TO MEMORY, 4.5V WHEN MAIN SUPPLY OPEN 1N5818 + ILIM 100µF 1.5V CELL VIN SW1 806k* + LT1073 FB SW2 40.2k* 100µF** 1073 TA15 * 1% METAL FILM **OPTIONAL † L1 = GOWANDA GA10-822k OR CADDELL-BURNS 7300-12 1073 TA16 13 LT1073 TYPICAL APPLICATIO S 3V to 5V Step-Up Converter with Undervoltage Lockout L1† 68µH 909k* 100k 100k 2N3906 3V 2.2M 100Ω ILIM AO VIN SW1 LT1073 SET 100k* GND FB SW2 40.3k* 1M* 1N5818 5V OUTPUT 100mA LOCKOUT AT 1.8V † * 1% METAL FILM L1 = GOWANDA GA10-682k OR CADDELL-BURNS 7300-11 1.5V to 5V Very Low Noise Step-Up Converter L1 470µH 680k ILIM FB 1.5V AO GND VIN SW1 LT1073 SET SW2 40.2k* 909k* † 1N5818 5V OUTPUT 5mA AT VBATTERY = 1V 10mVP-P RIPPLE * 1% METAL FILM L1 = GOWANDA GA10-473k OR CADDELL-BURNS 7300-21 EFFICIENCY = 83% AT 5mA LOAD † 3V to 6V at 1A Step-Up Converter INPUT 3V TO 6V (2 LITHIUM CELLS) L1† 25µH 6V OUTPUT 1A AT VIN = 3V 1.5V 560k 549k* 1000µF ILIM FB VIN SW1 LT1073 AO GND SET SW2 1N5818 20k* 51Ω MTP3055EL 2N3906 † + * 1% METAL FILM L1 = COILTRONICS CTX25-5-52 LOW IQ (