TPS82130: Output ramps when disabled

Hi Brian,

It seems there is a diode from 1V8 to 3V3. Could you remove R191 and R192 and test only U58 to see if it works normally?

Thanks.

Regards,

Shipeng

Hi Shipeng.

After removing R191 and R192 I found that the part still does not behave normally.

Unfortunatly I can't post a scope shot yet, but referring to the first shot above with all 4 rails, the behavior is now the same except that the 3.3V rail (U58, red) does not increase up to 3.3V after the 1.8V rail (blue) rises. It stays at the initial ~1.5V level.

I guess this is what would be expected under the circumstances with the enable pin now floating.

Hi Brian,

I mean you can disable U56 and U57, only test U58 with control EN manually. 

Or you can try to remove U58 and see if 3V3 can still get 1.5V when U56 and U57 start up.

So interesting result.

The easiest way to disable U56 and U57 was to disconnect the input 5V by removing L27 and L29.

When I do this, U58 output (P3V3) still increases to 1.5V just as it did when U56 and U57 were running normally.

I confirmed the U58 enable pin is still low as expected.

I can't completly disconnect the P3V3 output from the rest of the board. Could something be going on with this part or is this voltage somehow being pushed onto the P3V3 output by a load elsewhere on the board?

Hi Brian,

You can try to remove L7 to disable U58 to see if the voltage is powered by something else.

Back to this today. With L27 and L29 removed, U58 is the only remaining active power supply on the board (aside from +5V).

Still though, after removing L7, the P3V3 rail still increases to 1.5V.

So there must be another power supply in your circuit. Please check it.

Thanks.