BQ76920: CHG voltage is very low

Scott Nelson

Part Number: BQ76920

I'm only seeing 1-2V on the CHG pin even though I have set the CHG_ON bit to 1 (I know the bit was written successfully because the CHG pin voltage is 0 before enabling CHG_ON). I am seeing ~12V at the DSG pin and there are no SYS_STAT bits being set other than CC_READY. I am using the exact schematic shown in section 7.3.1.3.1 of the datasheet. What could be the issue here?

over 6 years ago

0 Vish Nadarajah over 6 years ago

TI__Expert 6085 points

as to debug, try disconnecting the CHG FET and check voltage at the CHG pin. chances are the CHG FET is defective and drooping the CHG driver.

0 Scott Nelson over 6 years ago in reply to Vish Nadarajah

Intellectual 340 points

I pulled the CHG FET off the board and probed the CHG pin. I still saw ~2.5V for a little while (even after resetting and clearing SYS_STAT, etc.) and then it spuriously jumped up to ~12V. I suspect you were right that the FET was damaged but I think it must have been damaged by a drooping gate voltage (which caused too much power to be dissipated across the FET). Anything else to check?

0 Vish Nadarajah over 6 years ago in reply to Scott Nelson

TI__Expert 6085 points

even if you short the CHG PIN or DSG PIN directly to GND, you will draw about 2mA . This draw would not damage the internal circuitry of the pin. The drivers are sourced by REGSRC. Please check the integrity of the path.

0 Scott Nelson over 6 years ago in reply to Vish Nadarajah

Intellectual 340 points

So I replaced the FET yesterday and subsequently saw the 12V on the CHG pin (and on the gate of the FET). It sat idle for the night (no current through the FETs, no way for FET damage) and this morning I noticed there's only 1.8V on the CHG pin. I suppose I'll try leaving the pin disconnected for a while and seeing if the pin voltage continues to fluctuate.

This is the FET I'm using by the way: www.infineon.com/.../irfr7440pbf.pdf

0 Scott Nelson over 6 years ago in reply to Scott Nelson

Intellectual 340 points

Hmm, well that didn't work. I removed the Q3 FET (as seen in the schematic in section 7.3.1.3.1 of the datasheet) so that the CHG pin was floating. After doing that the BQ76920 is basically unresponsive. Setting registers does not seem to change anything and all the voltages and current are returned as 0. Nothing else changed except removing Q3. Is this expected?

0 Scott Nelson over 6 years ago in reply to Scott Nelson

Intellectual 340 points

I changed nothing (board was sitting untouched for a couple days) and now I'm seeing ~12V on both the CHG and DSG pins as well as proper voltage and current readings. What could explain this erratic behavior?

0 Vish Nadarajah over 6 years ago in reply to Scott Nelson

TI__Expert 6085 points

this type failure can only be traced back to the bqDevice having a cold solder joint; may on BAT pin that sources the REGSRC.

0 Scott Nelson over 6 years ago in reply to Vish Nadarajah

Intellectual 340 points

OK, thanks for your quick response. I hand soldered the BQ76920 so that is likely the issue. I'll go back and make sure all the joints are properly soldered. Thanks a lot!

Power management

Power management forum

BQ76920: CHG voltage is very low