BQ24079T: SYSOFF function

Lasse Korff

Part Number: BQ24079T
Other Parts Discussed in Thread: BQ24079

Hi TI!

This question is slightly related to some of the other questions in this BQ2407x group.

1)
In the datasheet it says "Do not leave SYSOFF unconnected to ensure proper operation." Is it correct to leave SYSOFF unconnected if one intentionally wants the device to be turned OFF?

2)
In the application which we are working on we are thinking of using the SYSOFF to turn OFF the entire system in order to minimize current consumption from the battery during OFF mode.
It could be circuit much like this:

So a push button or power to turn ON the BQ24079 and the uMic to keep the BQ24079 alive if power is not present. Would this be an intentional use of the SYSOFF pin?

over 5 years ago

0 Raheem over 5 years ago

TI__Mastermind 21590 points

Hello Lasse,
I'll suggest pulling up SYSOFF to VBAT through a big resistor to reduce current consumption in addition to your circuit diagram so SYSOFF is not floating.

0 Lasse Korff over 5 years ago in reply to Raheem

Prodigy 115 points

OK. So the internal pull-up resistor will not be enough? How large a resistor will you suggest?

0 Raheem over 5 years ago in reply to Lasse Korff

TI__Mastermind 21590 points

Hello Lasse,
A 200k resistor should work.

0 Lasse Korff over 5 years ago in reply to Raheem

Prodigy 115 points

Hi Raheem!

Thank you for the answer.
But I guess the 200k resistor will add to the OFF current consumption?

Do you know what is the input impedance of the SYSOFF pin? I read in another post that it was below 1Mohm - is this correct?

0 Raheem over 5 years ago in reply to Lasse Korff

TI__Mastermind 21590 points

Lasse,
That is correct. Since you'll pull SYSOFF to VBAT, that will add to current consumption. However, SYSOFF pin has a high input impedance.

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BQ24079T: SYSOFF function