TPS25982: Not turning ON

David Wende

Intellectual 680 points

Part Number: TPS25982

Tool/software:

Hi,

We are using the attached circuit (part of a larger schematic) for some time quite successfully.

The output current draw is around 2.5 - 3 A for an input of around 20V.

Recently we added an additional load to 15V_IN.

This load, as STANDALONE consumes about 300mA for an input voltage of 20V.

It's power-on transient current is less than 1A.

However we are seeing that whenever this relatively small additional load is connected, the TPS25987 (U1) fails to reach the ON steady state, and the output (15V_IN) is essentially zero.

I have tried:

- increasing R(ilim)

- changing C(retry_delay)

- changing C(nretry)

without any success whatsoever.

Do you have any ideas on how to solve this problem?

Thanks

David2538.input.pdf

3 months ago

0 Arush Gupta 2 months ago

TI__Mastermind 39315 points

Hi David,

It looks like the continuous current draw is primary cause of power dissipation during startup and the startup itself is little slow with 10nF. Can you reduce the dvdt cap from 10nF to around 2nF.

Let me know if this works. We have design calculator using which you can also calculate the value.

Best regards,
Arush

0 David Wende 2 months ago in reply to Arush Gupta

Intellectual 680 points

Hi Arush

I reduced dvdt cap to 3.3nF by removing C5,C6 but the situation did not improve.

What can you suggest?

Thanks

David

0 Arush Gupta 2 months ago in reply to David Wende

TI__Mastermind 39315 points

Hi David,

Can you try with dvdt floating. This should be easier to evaluate at your end.

Best Regards,
Arush

0 David Wende 2 months ago in reply to Arush Gupta

Intellectual 680 points

Hi Arush

Removing ALL the dvdt caps does help.

Can I leave these unconnected for my production run?

i.e. is this a safe situation?

Thanks

David

+1 Arush Gupta 2 months ago in reply to David Wende

TI__Mastermind 39315 points

Hi David,

The dvdt open means fastest startup time. If you don't have big capacitive load then there is no issue. I calculated using the design calculator, you will have to chose dvdt cap of <= 1nF for your 20V/3A startup condition (assuming 100uF output cap)

Best Regards,
Arush

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TPS25982: Not turning ON