AN136| Application Note
Maxim > App Notes > POWER-SUPPLY CIRCUITS Keywords: Three-Input Supply Powers 3.3 Portables
Apr 13, 1995
APPLICATION NOTE 136
Three-Input Supply Powers 3.3 Portables
The single-ended primary-inductance converter (SEPIC) of Figure 1 accepts input voltages ranging from 3V to over 6V, and produces a regulated 3.3V, 200mA output. It accepts an input voltage from three sources: a 5V dc wall cube, a 3-cell AA battery, and a lithium backup battery.
Figure 1. When the wall cube is disconnected, this SEPIC regulator draws power from a 3-cell battery. A lithium battery (optional) backs up the 3.3V output. Unlike conventional boost regulators, whose battery current continues to flow during shutdown (unless you add a cutoff switch), this circuit's output turns fully "off" in response to a shutdown command. And, unlike flybacktransformer regulators and combination step-up/linear regulators, the Figure 1 circuit requires no transformer. Coils L1 and L2 should be the same type and have the same value, but coupling between them is not required. They can be wound on the same core for convenience, but the circuit works equally well if they are completely separate. Capacitor C3 couples energy to the output and requires low ESR to handle the high ripple currents. Conversion efficiency with a low-ESR Sanyo OS-CON capacitor is 85%, which is 3% higher than that available with lessexpensive 1uF ceramic capacitors. Tantalum capacitors are not recommended because high ri
pple currents cause them to self-heat. 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 to provide power. If the 3.3V output drops below 3.0V, a low-battery comparator in IC1 alerts the system by driving LBO low. And for backup, a diode-OR
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