Electronic Design

Simple soft-start circuit for switching regulators

In the early stages of development, one of the many issues circuit designers must evaluate is how the circuit behaves when power is first applied. Unexpected things can happen at startup. All capacitors must be charged and all ICs change from an inactive state to an active state. Frequently, several regulators provide power to the same circuit, and each output must be sequenced at startup. Controlling the slew rate of a regulator’s output voltage at startup lowers the stress on circuit components and allows circuit designers to adjust the startup voltage rate as required by the circuit.

All voltage regulators have some form of current limiting. Without soft-start circuitry, the regulator output current increases to the current-limit value. In addition, the output voltage increases very rapidly when power is first applied. This large startup current also can cause the regulator to “hang up” if the input voltage source has a high impedance, such as an aging alkaline battery pack. If the voltage drop across the source impedance causes the input voltage to drop below the minimum required input voltage of the regulator, the output voltage can “hang up” at a voltage less than the regulator’s minimum input voltage.

The circuit shows a 5-V to 3.3-V, 1.5-A switching regulator with an additional op amp, U1, that forces the output voltage to follow the exponential voltage rise on C1 (see the figure). Consequently, the startup voltage will ramp up in 50 ms. As the output voltage tries to exceed the voltage across C1, the op amp controls the switch duty cycle by injecting current into the feedback node, thus guiding the output voltage into regulation. When the startup voltage reaches 3.3 V, the error amplifier in the LTC1147 takes control of the output. Because C1 continues to charge, the output voltage of U1 goes low, removing its effect on the output. The output voltage ramp time can be changed by adjusting the values of R1 and C1. Although the startup voltage isn’t a linear ramp, using a simple RC network for the ramp generator is sufficient for most applications.

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