Design Ideas November 21, 1996

Converter generates 3.3V from two or three cells

Robert Badalian, Maxim Integrated Products, Sunnyvale, CA

  Generating 3.3V from two or three cells poses a design challenge. The regulator must step down the voltage when the cells are fresh but step up the voltage 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 V_OUT under varying load conditions.

  However, a single-ended primary-inductance converter (SEPIC) offers simpler circuitry (Figure 1). This circuit generates 3.3V at 400 mA with 78% efficiency. V_IN can range above and below the output, and a capacitor, C₁, couples the output to the switching circuitry. This configuration offers two advantages over flyback-transformer circuits and step-up linear-regulator circuits. First, no I_OUT flows during shutdown, and, second, V_OUT remains well- regulated as V_IN passes through the V_OUT level.

  The two inductors in this circuit can be separate components, or you can wind them on a common core for convenience. They do not work as a transformer, so you can wind them without regard to coupling. C₁, C₂, and C₃ should be low equivalent-series-resistance (ESR) types for best efficiency. C₁'s voltage rating must exceed the maximum V_IN, and the external switch, Q₁, must withstand the sum of V_IN+V_OUT.

  By capturing Q₁'s switching pulses, Schottky diode D₁ boosts the V⁺ voltage (pin 2) to the quantity V_IN+V_OUT. The resulting higher gate drive lowers the losses in Q₁, especially for low V_IN, but this drive also limits V_IN to a maximum of 12V. The circuit can provide 300 mA at a V_IN of 2V and 400 mA at a V_IN of 3V. Figure 2 shows the relationship between V_IN, load current, and efficiency. (DI #1955) 

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