TPS54335A: unstable start up at EN = floating

Yusuke Mukuno

Expert 7395 points

Part Number: TPS54335A

Hi,

My customer have an issue of unstable startup at EN=floating (1000pF cap).

Issue rate : sometimes

If customer applied 3.3Vdc to the EN pin from external, it didn't occur.

Waveform

Pink, Vin:24V

Green, Vout: 5V

Blue, EN

Could you please let me know the root cause of this behavior?

Thanks

Muk

over 6 years ago

0 Shawn Nie over 6 years ago

TI__Genius 10070 points

Hi,

Please remove the 1000pF capacitor.

Shawn

0 Jason Wang (BSR-MV) over 6 years ago

TI__Genius 9550 points

hi Muk,

Quckly feedback for you.

From your description and waveform, the root cause is:

VIN''s ramp up slew rate is slow.

When device's SS time is end, VIN is still lower than 5V (or near 5V, so Vout is also lower than 5V), then device enter HICCUP function (refer datasheet page 15).

How to resove it?

Pull EN high after VIN is hihger ~7V

increase the VIN ramp up slew rate, suggest >2V/ms

0 Yusuke Mukuno over 6 years ago in reply to Jason Wang (BSR-MV)

TI__Expert 7395 points

Thanks!!

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TPS54335A: unstable start up at EN = floating