The usual way to step down from a low voltage to an even lower one is with a low-dropout (LDO) linear regulator. But in battery-powered systems, the LDO probably won't deliver the maximum energy available. A cell count chosen for near-dropout operation when the battery is empty applies too much voltage over most of the battery's discharge, and a cell count chosen for maximum efficiency over that range allows dropout well before the battery is empty.
One solution to this problem is the highly efficient buck DC-DC converter (Figure 1). This circuit can step down inputs as low as 2V to outputs as low as 1.25V, with efficiencies as high as 80% (Figure 2). Like an LDO, it works well at low input voltages. Unlike an LDO, its efficiency remains fairly high with inputs up to the allowable maximum (6.5V).
Figure 1. These external components enable a boost-controller IC to implement a low-voltage buck-regulator circuit.
Figure 2. The conversion efficiency of the circuit in Figure 1 varies with output current as shown.
A step-up switching regulator (IC1) is made to step down with the addition of an external switching transistor (Q1). Via LX (pin 8), Q1 is driven by the IC's internal switching transistor: an open-drain, n-channel power MOSFET connected to ground. R2 limits the Q1 base current, and R1 turns Q1 off when LX floats. The R1 and R2 values are chosen for maximum efficiency at light loads (1mA to 10mA), which limits the maximum available output current. Lower values for R1 and R2 allow higher output current, but cause the circuit to draw higher levels of quiescent current.
R3 and R4 determine output voltage, as shown in the following equation:
VOUT = VREF (R3 + R4) / R4
where VREF = 1.25V.
The minimum output voltage is 1.25V (with R3 = 0 and R4 absent). R5 and R6 determine the threshold for low battery voltage in a similar manner. Input and output capacitors can be inexpensive electrolytic or tantalum types. For greatest efficiency, the inductor should be rated in excess of the desired output current, and it should have a reasonably low series resistance. Diode D1 should be a Schottky type, because losses are proportional to the diode's forward voltage, and this voltage is a substantial fraction of the output voltage.
A related idea appeared in the 6/5/97 issue of EDN.
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