The single-ended primary-inductance converter in Fig 1 accepts input voltages ranging from 3 to more than 6V and produces a regulated 3.3V, 200-mA output. The converter accepts an input voltage from three sources: a 5V dc wall cube, a three-cell AA battery, and a lithium backup battery.
Unlike conventional boost regulators whose battery current continues to flow during shutdown (unless you add a cutoff switch), this circuit's output turns off fully in response to a shutdown command. And, unlike flyback-transformer regulators and combination step-up/linear regulators, Fig 1 requires no transformers. Coils L1 and L2 should be the same type and have the same value, but coupling between them isn't required. You can wind the inductors on the same core for convenience, but the circuit works equally well if they are completely separate.
C1 couples energy to the output and must have low equivalent series resistance (ESR) to handle the high ripple currents. Conversion efficiency with a low-ESR Sanyo OS-CON capacitor is 85%, which is 3% higher than with a less-expensive 1-µF ceramic capacitor. Tantalum capacitors are not recommended because they self-heat at high ripple currents.
During normal operation, the ac adapter's 5V output powers the circuit and turns off Q1. Disconnecting the adapter removes 5V, turns on Q1, and allows the three AA cells, B1, to provide power. If the 3.3V output drops below 3V, a low-battery comparator in IC1's step-up dc-dc converter alerts the system by driving LBO low. And for backup, a diode-OR connection allows an optional lithium battery (coin cell B2) to provide load current at the 3.3V output.
As an added twist, D1 provides a supply voltage for IC1 (pin 8) by capturing the switching pulses at LX (pin 7). This voltage, which approximately equals the sum of VIN and VOUT, improves start-up capability under full load and improves low VIN efficiency by boosting gate drive to the internal switching MOSFET. Maximum VIN is limited to about 12V, which easily accommodates the three-cell battery. (DI #1686)