advertisement Micropower Buck/Boost Circuits, Part 1: Converting Three Cells to 3.3V* – Design Note 109 Mitchell Lee Two combinations of cell count and output voltage are to be strictly avoided: three cells converted to 3.3V and four cells converted to 5V. These combinations are troublesome because no ordinary regulator (boost, buck or linear) can accommodate a situation where the input voltage range overlaps the desired output voltage. the topologies. The LT1372 500kHz converter is used for compact layouts at higher current levels. You can expect 200mA output from LT1303 based circuits and 300mA from the LT1372 circuit without modification. All of the circuits feature output disconnect; in shutdown the outputs fall to 0V. The input range of LT1303 based converters extends well beyond the 3-cell source shown. These function at 1.8V, and although not fully characterized for efficiency, can accept inputs of up to 10V. The LT1372 converter operates from 2.7V to 10V. This design note presents four circuits capable of solving the 3-cell dilemma. Design Note 110 will discuss 4-cell, 5V circuits. The LT®1303 and LT1372 high efficiency DC/DC converters are used throughout, giving a fair comparison of each topology’s efficiency. The LT1303 is optimized for battery operation and includes a lowbattery detector which is required to implement one of L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. *For 4-cell to 5V buck/boost circuits, see Design Note 110. 95 330μF* IOUT = 200mA 90 VOUT 3.3V 200mA 85 EFFICIENCY (%) 20μH** 33μF* MBRS130T3 1/2 CTX20-1 VIN = 2V TO 6V • 6 7 + + VIN SW 2 22μF* 169k LBO 1% + 4 3 OFF LT1303 FB CELLS • 100k 3 5 SHDN LBI 20μH** 1% 1/2 CTX20-1 SGND PGND ON 1 8 80 75 70 65 60 55 *AVX TPS SERIES TANTALUM OR SANYO OS-CON **COILTRONICS 50 1 2 3 4 5 6 INPUT VOLTAGE (V) 7 8 DN109 • F01 Figure 1. 3-Cell to 3.3V SEPIC + 10μF* 35V ON 3 CELLS 4 47μF* 16V 5μH** 1/2 CTX5-1 • 5 8 VIN VSW LT1372 + 2 16.9k 1% 10k 1% SHDN VC OFF FB 95 MBRS130T3 + • 5μH** 1/2 CTX5-1 47μF* 16V GND 1 6, 7 75 70 65 55 50 4.7nF 1 *AVX TPS SERIES TANTALUM **COILTRONICS 2 3 4 5 6 INPUT VOLTAGE (V) 7 DN109 • F02 Figure 2 08/95/109_conv 85 80 60 47nF 2k IOUT = 300mA 90 VOUT 3.3V 300mA EFFICIENCY (%) VIN = 2.7V TO 6V 8 The circuits in Figures 1 and 2 are based on the SEPIC (Single-Ended Primary Inductance Converter) topology. Although not stellar, the efficiency is quite consistent over a wide input voltage range. Peculiar to the SEPIC topology is its use of two inductors. These, however, are wound together on a single core and consume no more space than a simple 2-terminal inductor of similar rating. A wide selection of stock 2-winding, 4-terminal inductors are available from Coiltronics and other magnetics vendors. the transistor serves as a linear post regulator, cascoding the output of the boost converter and dissipating power as would any linear regulator. Highest peak efficiency is obtained with the circuit in Figure 4 using a MOSFET buck/boost converter. For VIN < VOUT, the circuit operates as a boost converter and the MOSFET, driven by the LT1303’s low-battery detector/amplifier, is held 100% ON. The output voltage is developed and controlled by the boost converter. For VIN > VOUT, the boost function can no longer control the output voltage and it begins to rise. Staggered feedback (R3, R4, R5) allows the low-battery detector/ amplifier to take control using the MOSFET as a linear pass element. Because the MOSFET requires no base drive, and because it has such a low ON resistance, the efficiency peaks at well over 90%. Furthermore, the efficiency peak occurs in the vicinity of a NiCd’s nominal terminal voltage of 3 × 1.25 = 3.75V, right where the efficiency is needed most. Peak efficiency improves in Figure 3 using a bipolar buck/boost topology. This circuit is essentially a boost converter with a linear post regulator. For VIN < VOUT, the LT1303 boosts the input driving the bipolar emitter just high enough to maintain the desired output voltage—the transistor is saturated. For VIN > VOUT, the LT1303 drives the emitter to a value just higher than the input voltage sufficient to develop the base current necessary to support any load current. In this condition 95 510Ω 6 + VIN 33μF* 3 CELLS LBO OFF LT1303 3 FB SHDN LBI SGND ON 2 5 85 169k 1% + 4 220μF* + 33μF* PGND 1 VOUT 3.3V 200mA ZTX788B 7 SW EFFICIENCY (%) VIN = 2V TO 6V IOUT = 200mA 90 10μH** CD54-100MC MBRS130T3 100k 1% 80 75 70 65 60 55 8 50 *AVX TPS SERIES TANTALUM OR SANYO OS-CON **SUMIDA 2 1 3 4 5 6 INPUT VOLTAGE (V) 7 8 DN109 • F03 Figure 3. 3-Cell to 3.3V Bipolar Buck/Boost 95 VIN = 2V TO 6V + 7 SW 6 + 33μF* 3 CELLS VIN LBO OFF 3 LT1303 SHDN SGND ON 1 FB LBI 33μF* 100k 100Ω 2 4 5 200k 1% PGND 121k 1% 8 + 330μF* 1.96k 1% IOUT = 200mA 90 VOUT 3.3V 200mA Si9405 85 EFFICIENCY (%) 10μH** CD54-100MC MBRS130T3 80 75 70 65 60 55 50 1 *AVX TPS SERIES TANTALUM OR SANYO OS-CON **SUMIDA 2 3 4 5 6 INPUT VOLTAGE (V) 7 8 DN109 • F04 Figure 4. 3-Cell to 3.3V MOSFET Buck/Boost Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn109f_conv LT/GP 0895 190K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1995