Step-Up/Step-Down Converter Generates 3.3V From Two Or Three Cells
Generating 3.3V from two or three cells poses a challenge to design engineers. The regulator must step down when the cells are fresh, but it must also step up when the cells are semidischarged and weak. One solution to this problem is a flyback-transformer design, which requires that you select transformer ratios to ensure a constant output voltage under varying load conditions.
Alternatively, a single-ended primary-inductance converter (SEPIC) offers simpler circuitry (Figure 1). This circuit generates 3.3V at 400mA with 78% efficiency. Input voltage can range above and below the output, and a capacitor (C2) couples the output to the switching circuitry. This configuration offers two advantages over flyback-transformer circuits and step-up linear-regulator circuits: no output current flows during shutdown, and VOUT remains well regulated as VIN passes through the VOUT level.
Figure 1. This SEPIC switching regulator maintains a 3.3 output for VIN that ranges above and below 3.3V ( as does the voltage from a 2-cell or 3-cell battery, for example.
The two inductors in this circuit can be separate components or can be wound on a common core for convenience. They don't work as a transformer, so you can wind them without regard to coupling. Capacitors C2, C3, and C5 should have low equivalent series resistance for best efficiency. C2's voltage rating must exceed the maximum input voltage, and the external switch (Q1) must withstand the sum of (VIN + VOUT).
By capturing Q1's switching pulses, Schottky diode D2 boosts the V+ voltage to (VIN + VOUT). The resulting higher gate drive lowers the losses in Q1, especially for low input voltage, but it also limits VIN to 12V (max). Output capabilities are 300mA at VIN = 2V and 400mA at VIN = 3V, with efficiency versus load current as shown in Figure 2.
Figure 2. Efficiency for the Figure 1 circuit approaches 80%.
A similar idea appeared in the 11/21/96 issue of EDN.
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