BQ2057 - Battery Charging While in Circuit

Gabriel Shaikh

Prodigy 35 points

Other Parts Discussed in Thread: BQ2057

I am using the BQ2057 in the following circuit:

For some reason, when the power supply (VSUPPLY) is not attached, +VBAT gets connected to VSUPPLY.

I have figured a couple of things out.

When Q4 is removed from the circuit, this problem does not occur. Of course, the batteries will not charge if Q4 is not in place so this does not solve the issue.
There is no current flowing through R18 or D18. Therefore, current must be flowing into the SNS pin of the BQ2057.
Removing R19 does not make any difference.
Clearly the connection between VSUPPLY and +VBAT is being made through the BQ2057.
Disconnection the VCC pin, the COMP pin, and the top of R19 from VSUPPLY and attaching them to the node between D18 and R18 did not solve the problem either. Replacing R19 with a 1k resistor also did not solve the problem. I have summarized some of my experimentation in the following tables.

VSUPPLY || VGS on Q4

--------------------------

0 || + 0.447 V

9.0 || - 0.796 V

VSUPPLY || R18 Current | R18 Voltage

0 || 0 mA | 0 V

9.0 || 0 mA | 0 V

VSUPPLY || R19 Current | R19 Voltage

-----------------------------------------

0 || 0 | 0

9.0 || 97.8 uA | 978 mV

Thanks in advance for the help!

over 12 years ago

0 Jeff F over 12 years ago

TI__Guru**** 166076 points

There is internal leakage through the IC SNS line. We recommend placing an isolation diode or FET on the Battery side of the pass element.

0 Michael Dierks over 9 years ago in reply to Jeff F

Prodigy 60 points

Hi Jeff F. Sorry to raise this from the dead, hope somebody is still watching. I am also using the BQ2057, and I am seeing back feed to VSUPPLY from VBAT when the supply voltage is removed. It is about 1V below the battery voltage, but it is messing up my charging LED configuration using the STAT pin, as I need VSUPPLY to be 0 VDC when the input is disconnected.

Question, as it has been awhile on the answer; is this still the preferred solution for eliminating the back feed voltage to VSUPPLY, inserting the isolation diode between the pass element and the battery? How does an isolation element between the pass element and the battery prevent the feedback from VBAT to SNS and VSUPPLY?

0 Jing Zou over 9 years ago in reply to Michael Dierks

TI__Mastermind 20570 points

We recommend to place the blocking diode, D1, inside the feedback loop to block the leakage path from BAT to SNS.

0 Michael Dierks over 9 years ago in reply to Jing Zou

Prodigy 60 points

Just a final followup if you are still with me.
The data sheet shows a blocking element before any sense resistors or other components. Is the suggestion to place the diode in the feedback loop to block SNS the more current solution?

Regards.

Michael

0 Jing Zou over 9 years ago in reply to Michael Dierks

TI__Mastermind 20570 points

Yes, we suggest you to place the diode inside the feedback loop to block leakage to SNS.

0 Michael Dierks over 9 years ago in reply to Jing Zou

Prodigy 60 points

Thanks Jing. I searched again on the BQ2057 tag and found several other suggestions along this line with support. I would click "verify answer", but I don't see the button on the page anywhere.

I consider this the correct answer.

Power management

Power management forum

BQ2057 - Battery Charging While in Circuit