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TPS56121: +5V unreliable startup

Part Number: TPS56121
Other Parts Discussed in Thread: TPS56221

We added a resistor and FET 2N7002 N-CH 60V to the design to allow controlled startup and shutdown.

When the FET switch is enabled, the +5V supply does not start.

Checked part values, temperature and current and there seems to be no issue.

The unit is operated at 300Khz but there is a large ripple current 3AMPS  at 70kHz that may be the reason.

 

Attached a Pico scope capture file of the power enable (red), 12V supply(blue), 5V output (Yellow)

There is not a large droop on the +12V line to indicate a large current.

The enable line looks very clean.

The +5V starts to ramp but stops. The supply will try to restart every 50mS

Is there another internal fault condition that can occur causing this issue?