Supplement-Protective Function: Output Pre-bias Protection


Points of this article

・Protection is provided for a pre-bias state in which a voltage already appears at the output upon startup of a synchronous rectifying step-down switching regulator.

・A function is provided to prevent the possibility of breakdown of the low-side switch due to drawing of an excessive current when the switch is turned on while in a pre-bias state.

This article is a supplement to ” Protective and Sequencing Functions” in ” Switching Regulator Basics”. In recent years, an increasing number of DC-DC converters have been provided with this protection function.

Output Pre-bias Protection

The voltage Vout when a DC-DC is started is assumed to be essentially 0 volts. However, due to the circuit configuration, restarting after a short time interval, and other factors, a voltage Vout may exist prior to startup, that is, the voltage may not be zero. In many cases, charge remains on the Cout connected to Vout, or on a Cin of the load (the IC being fed) or the like; or, it may be due to a leakage current via another route, or to a bias from a pull-up resistor or the like.

In a state with this pre-bias, when the low-side switch (MOSFET) of a synchronous rectifying step-down converter is turned on, an excessive current is drawn, and breakdown of the MOSFET is possible. In order to prevent such an occurrence, the pre-bias production function executes control so as not to turn on the low-side switch until the output voltage exceeds the pre-bias voltage.

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Basic studies for linear regulators and switching regulators as a DC-DC converter.