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Old 11-22-2021, 03:31 AM   #1
reformatt
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Default Asus X555DG and BQ735

Had a curly one on a BQ735 battery controller and an Asus X555DG motherboard and thought I'd share the fix.

Board not powering on, no 19V past the first input FET PQ8801 (ACFET in the datasheet). All FET's around the BQ735 all measured ok and no obvious signs of corrosion. Board boots fine if this first FET is bypassed, however no battery charging in this event.

ACDET, ACOK, VCC etc all ok on the BQ735. PQ8801's input gate is being driven off ACDRV, with source for that being CMSRC. Note that CMSRC is on the source of the FET, and ACDRV drives the gate. So initially for this circuit to work, the FET must be initially turned on by ACDRV to derive CMSRC. I initially changed out the first two FETS PQ8801 and PQ8802 but no difference. Then I noted that ACDRV was pulsing a number of times then shutting off. I noted in the datasheet the following:

"The gate drive voltage on ACFET and RBFET is VCMSRC + 6 V. If the ACFET/RBFET have been turned on for 20 ms, and the voltage across gate and source is still less than 5.9 V, ACFET and RBFET will be turned off. After 1.3-s delay, it resumes turning on ACFET and RBFET. If such a failure is detected seven times within 90 seconds, ACFET/RBFET will be latched off and an adapter removal and system shut down is required to force ACDET < 0.6 V to reset the IC. After IC reset from latch off, ACFET/RBFET can be turned on again. After 90 seconds, the failure counter will be reset to zero to prevent latch off. With ACFET/RBFET off, charge is disabled."

This was the exact behaviour. Note the last sentence where charge is disabled. So jumping the first FET will also disable battery charging. I noted that everything started working at one point after restoring the original ACFET/RBFET pair, only to fail again a bit later.

I didn't have the schematic, although the design is similar to the Asus UX31 with a BQ725 and N-Channel input FET's. Everything buzzed out for the FET's and current sense to the IC. However, the resistor to ground on the gate of PQ8801 (PR8801) measured only 600k. This is 1M in the datasheet and the other Asus schematic. So replaced it and that has fixed it.

According to the datasheet:

"To limit the adapter inrush current when ACFET is turned on to power system from adapter, the Cgs and Cgd external capacitor of ACFET must be carefully selected. The larger the Cgs and Cgd capacitance, the slower turnon of ACFET will be and less inrush current of adapter. However, if Cgs or Cgd is too large, the ACDRVCMSRC voltage may still go low after the 20-ms turnon time window is expired. To make sure ACFET will not be turned on when adapter is hot plugged in, the Cgs value should be 20 times or higher than Cgd."

This is the value of datasheets being published by the manufacturers. The resistor value change alters how fast the cap will charge between drain to gate, messing with the comparator timing within the BQ735.
Attached Images
File Type: jpg Asus X555DG FET input.jpg (51.9 KB, 21 views)

Last edited by reformatt; 11-22-2021 at 04:13 AM..
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Old 11-22-2021, 06:51 AM   #2
mcplslg123
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Default Re: Asus X555DG and BQ735

Yes thats a known fact that charging Ic makes 7 attempt to turn on the ACFET/RBFET before latching off until reset.
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