[Power supply column] No. 6: Power supply that repeatedly turns on and off at startup!

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Basic Power SupplyAnalog Devices

This is Sasaki in charge of power supply dock technology.

In this column, I would like to introduce some examples of problems related to power supplies that I encountered multiple times during various customer support sessions.

One day, I received the following advice from a customer.

Be careful when turning on the power!

When I start DC/DC, I'm having trouble with a staircase-like rise that looks like it's repeatedly turning on and off. . . (Figure 1)

Article header library 127857 pic01 1 — Figure 1 : Power rising waveform

Looking at the data sheet, it should be a clean monotonically increasing rise. . .

hmm...
It certainly has a stepped rise.
By the way, is the load current within the specification range?
Is there a current flowing that will affect the overcurrent protection?

Does it turn on and off repeatedly when overcurrent flows? ?

yes.
Rush current flows when the power supply is started, and overcurrent protection is activated, which may cause it to turn on and off repeatedly.
Also, have you made the soft start extremely fast?

a! The startup power supply sequence for the microcontroller and FPGA on the board is strict, so
Start with soft start in μsec order,
A large capacitor is inserted between the power supply and the microcontroller to increase the responsiveness of the power supply. . .

If that's the case, I think you can avoid this by setting the soft start function setting time to the order of milliseconds or optimizing the capacitor capacity.