August 1, 1997

Add trimmable current limit to dc/dc supply

Terry Millward, Maxim Integrated Products UK Ltd, Theale, UK

You can add a simple, two-transistor circuit to a standard, step-down dc/dc converter to provide an adjustable limit for the output current (Figure 1). You can apply this method to most step-down converters that provide access to the feedback node. The circuit uses a PWM switch-mode regulator, IC₁, to generate 9V from an input of 12 to 30V (a range that satisfies automotive applications). R₄ senses the output current; the current limit is 0.65/R₄, which in this case is 440 mA. The limiting current produces sufficient voltage across R₄ to turn on Q₂.

The current through R₂ then turns on Q₁, which throttles the output current by sourcing current to the feedback node on Pin 8. R₃ protects Q₂'s base-emitter junction in the event of an output short circuit. Because R₅ and R₆ sense the output voltage downstream from the current-sense resistor, V_OUT stays in regulation until the current reaches the limit. This regulation chara cteristic is useful in battery-charging applications. R₅ and R₆ set the regulated output level according to

(DI #2062)

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