September 1, 1997

Coilless step-up converter yields dual outputs

Sam Nork, Linear Technology Corp, Burlington, MA

Rapidly proliferating battery-powered systems create a need for dc/dc converters that can accommodate a wide input-supply range. Depending on the battery-pack configuration, the input-supply voltage may be higher or lower than the desired regulated output voltage. The circuit in Figure 1 provides regulated 5 and 3V supplies from a wide input range without the need for inductors. The LTC1514-5 (IC₁) is a switched-capacitor, step-up/step-down dc/dc converter. It produces a regulated 5V±4% supply capable of sourcing as much as 50 mA of output current. The circuit requires only three external capacitors to produce 5V from an input supply of 2.7 to 10V.

IC₁ draws a low, 60-µA-typical quiescent current. For a 3V input, the conversion efficiency for the 5V output ranges from 70% at a 0.5-mA output to 81% at a 50-mA output. For a 6V input, these efficiency figures become 71.5 and 82.5%, respectively. IC₁ contains a low-battery comparator that serves as a feedback comparator to create an auxiliary 3.3V supply (Figure 2). IC₁'s internal reference connects to one end of the feedback comparator. The R₂-R₃ feedback voltage divider establishes the output voltage. The output of the comparator enables the current source comprising Q₁, Q₂, R₁, and R₄.

When the LBO output is low, Q₁ turns on, allowing current to charge output capacitor C₄. Local feedback formed by R₄, Q₁, and Q₂ creates a constant-current source from the 5V output to C₄. R₄ and Q₂'s V_BE set the peak charging current. R₄ and Q₂'s V_BE also provide current limiting in the case of an output short circuit to ground. IC₁ also protects the 5V output against short circuits. R₅ pulls Q₁'s gate high when the 3.3V output is in regulation, thereby turning off the C₄ charging current. C₅ reduces output ripple.

Because the auxiliary regulator implements a hysteretic feedback loop in place of the traditional feedback loop, the circuit needs no compensation for loop stability. Furthermore, the high gain of the comparator provides excellent load regulation and transient response. With the values in Figure 1, the dual regulator delivers a maximum combined output current of 50 mA. (DI #2076)

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