Quad-Comparator Circuit Provides Power-Down Sequencing At Low Cost

Focus:

Many applications with multiple supply rails now require both power-up and power-down sequencing. Typically, the power-down sequencing needs to be in reverse order of the power-up sequencing. If only power-up sequencing were needed, this could be implemented at no added cost simply by tying the power good signal of one voltage regulator to the enable pin of the next regulator. However, such an approach does not allow for a controlled power down. A microcontroller could be used for this purpose, but it would require programming. A simpler, low-cost, no-programming alternative is presented here using a low-cost voltage reference and a low-cost multi-output comparator to sequence multiple power-converter outputs. This circuit requires that each power converter (a switching or linear regulator) has either an enable or a soft-start pin, and that the shutdown sequence is in reverse order of the power-up sequence. Circuit operation is explained and test results are given for a prototype of the sequencer used to control the outputs of three TPS54620 synchronous stepdown regulators. For cases where shutdown sequence is not in reverse order of power-up, alternative solutions using a microcontroller or dedicated sequencer are briefly described.

What you’ll learn:

• How to implement a low-cost circuit for power-up and power-down sequencing of linear or switching regulators with enable or soft start pins
• How to design a low-cost power-down sequencer for point-of-load power converters that requires no programming

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Author & Publication:

Josh Mandelcorn, Texas Instruments, Dallas, Texas, How2Power Today, Jul 27 2011

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