TPS3808 power up and sense input sequence

Todd F

My customer is using a TPS3808G01, Powered from 3.3v and the voltage sense input is scaled from a 5V input via a 1/10 voltage divider. The customer is seeing a situation randomly where the reset output is holding the RESET output low after power up.

Question is: If the 5v (scaled by a 10k /1K divider) comes up first which will apply 0.5v to the internal comparator and then the 3.3v power to the device comes up next could the TPS3808G01 get into a situation were the RESET latches low and remains there?

They are seeing this behavior sometimes and they think the 5V is coming up sometimes before 3.3v.

Thanks.

over 14 years ago

0 Anthony F over 14 years ago

TI__Mastermind 35185 points

Hi Todd,

There has not been a case where startup sequence has led to any issue for the TPS3808G01. I put together the circuit described to test myself. I left the 5V monitored voltage turned on and varied Vcc with a 10 Hz 3.3 Vp-p square wave. RESET was pulled high each time after start up.

Is there any capacitance close to the SENSE pin or 5V input/can you post a schematic? Also can you have them capture some wave forms showing the SENSE pin, RESET, 3.3 V power and 5 V input when the error occurs?

Thanks,
Anthony

0 Todd F over 14 years ago in reply to Anthony F

Intellectual 590 points

Anthony,

There is a 10nF cap on the sense line as recommended in the datasheet (Datasheet: It is good analog design practice to put a 1nF to 10nF bypass capacitor on the SENSE input to reduce sensitivity to transients and layout parasitics.)

Capturing waveforms will be difficult as the fault does not occur all the time.

Todd

0 Chris Glaser over 14 years ago

TI__Guru**** 151130 points

Todd,

One test that you can do to see if the sequencing of the rails is the culprit is to tie the Vdd of the chip to 5V instead of 3.3V and see if the problem ever occurs. If so, then it is something else, other than the sequencing of the input rails.

0 George Hong over 14 years ago in reply to Chris Glaser

Prodigy 70 points

Todd/TI, has this issue been resolved?

0 Todd F over 14 years ago in reply to George Hong

Intellectual 590 points

Yes it has been resolved.

0 Chris Glaser over 14 years ago in reply to Todd F

TI__Guru**** 151130 points

Todd,

Thank you for letting us know that it has been resolved. May I ask you to reply with your solution? i.e. what did you do to resolve the issue?

Hopefully, others that have the same problem can learn from what you did.

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TPS3808 power up and sense input sequence