DESIGN IDEAS Battery-Powered Buck-Boost Converter Requires No Magnetics by John Seago One of the problems that designers of portable equipment face is generating a regulated voltage that is between the charged and discharged voltage of a battery pack. As an example, when generating a 3.3V output from a 3cell battery pack, the regulator input voltage changes from about 4.5V at full charge to about 2.7V when discharged. At full charge, the regulator must step down the input voltage, and when the battery voltage drops below 3.3V, the regulator must step up the voltage. The same problem occurs when a 5V output is required from a four-cell input voltage that varies from about 3.6V to 6V. Ordinarily, a flyback or SEPIC configuration is required to solve this problem. The LTC1515 switched capacitor DC/DC converter, can provide this buck-boost function for load currents up to 50mA with only three external capacitors. The circuit shown in Figure 1 will provide a regulated 3.3V output from a three-cell input or a 5V output from a four-cell input. Connecting the 5/3 pin to VIN will program the output to 5V, whereas grounding the 5/3 pin programs the output to 3.3V. The absence of bulky magnetics provides another benefit: this circuit requires only 0.07 square inches of board space in those applications where components can be mounted on both sides of the board. The addition of R1 provides a power-on-reset flag that goes high 200ms after the output reaches 93.5% of its programmed value. The SHDN pin allows the output to be turned on or off with a 3V logic signal. R1 100k LTC1515CS8-3.3/5 1 ON OFF 2 RESET 3 5V 3.3V 7 C2 10µF 16V (1210) 3 OR 4 CELLS VOUT SHDN 4 POR GND 5/3 CI+ VIN 5 CI– 6 C3 10µF 16V (1210) C1 0.1µF (0603) C1 = AVX 0603YC104MAT2A C2, C3 = MURATA GRM235Y5V106Z Figure 1. Battery-powered buck-boost converter PATIENT/CIRCUIT PROTECTION/ISOLATION 3V 0.3Hz HIGHPASS +INPUT C1 R1 R3, 30k RG, 6k R4, 30k R2, 1M 3 8 1 2 7 + – 3V C2, 0.47µF 5 6 LT1167 G = 10 5 + 4 R6 1M – 1 6 7 OUTPUT 1V/mV 4 –3V 2 R7, 10k –3V 1/2 LT1112 R8 100 3 C3, 15nF 1167_05.eps + –INPUT LT1167, continued from page 9 8 1/2 LT1112 + PATIENT GROUND 3.3V/50mA OR 5V/50mA 8 AV = 101 POLE AT 1kHz Figure 8. Medical ECG monitor Conclusion The LT1167 instrumentation amplifier delivers the best precision, lowest noise, highest fault tolerance, plus the ease of use provided by singleresistor gain setting. The LT1167 is 38 offered in 8-pin PDIP and SO packages. The SO uses significantly less board space than discrete designs. With these advantages and the ability to operate over a wide range of supply voltages at a quiescent current of 0.9mA, the LT1167 is appropriate for a wide range of applications. Linear Technology Magazine • May 1998