Question about maximum input ratings on BQ20Z95 SAFE, MRSTn and PRESn pins...

Neil Ragsdale

The datasheet for the BQ20Z95 says the SAFE, PRESn and MRSTn pins can't go more than 0.3V above the REG25 pin levels.

A few comments and questions:

1) The BQ20Z95 reference design has the OUT (pin 1) from a BQ29412DCT fed to a voltage divider and then a diode to the SAFE pin on the BQ20Z95.

It seems like the voltage fed to the SAFE pin would exceed the recommended absolute maximum voltage when 14V is applied to the VDD pin of the BQ29412DCT.

Is this true?

2) My design interfaces to a standard 3.3V processor. And the SAFE, MRSTn and PRESn lines need to tie to that micro.

May I assume that I must add some clamping-current limiting scheme if I am to interface SAFE and PRESn to a 3.3V micro?

over 14 years ago

0 Neil Ragsdale over 14 years ago

Prodigy 165 points

Note: Disregard Item 1 from the previous post. The datasheet lists SAFE as in input when referring to the Absolute Maximum ratings Table. See Page 4.

0 Neil Ragsdale over 14 years ago in reply to Neil Ragsdale

Prodigy 165 points

This is another addendum to my previous 2 e-mails related to absolute maximum ratings on certain pins. Let's just focus on PRESn...

1) The datasheet says: "System / Host present input. Pull up to TOUT". The reference schematic just shows resistors and a protection diode with no pullup of any sort.

Questions:

1) Does this PRESn input need a pullup?

2) And one end of that pullup may not exceed a voltage of approximately 2.5V + 0.3V?

0 Jackie Hui over 14 years ago in reply to Neil Ragsdale

TI__Expert 3865 points

Yes, you should limit the current to less than 10mA going to those pins. You can put a 1K resistor in series when connecting to a 3.3V micro.

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Question about maximum input ratings on BQ20Z95 SAFE, MRSTn and PRESn pins...