Missing resistor on BQ ILIM pin. DA0P5JMB6D0

[liquidzorch]

Hello. I can't find the schematics anywhere for this motherboard, does anyone have one, and can give me the resistor value for the BQ728H missing in pic below? I already read the dadsheet, and this resistor value is calculated, so I can't just go with whats in the "typical" diagram.

On a second note, could this resistor cause it to only boot with the battery and not the charger? It boots fine with a charged battery, but will not turn on with the charger (nor charge). The 19v does not get past the first mosfet, it recieves pulses to turn on, but something is preventing it. No shorts found.

Here is an image. Thanks in advance!

https://i.ibb.co/1d4SXbT/PXL-20231218-154330801.jpg

[mon2]

The ILIM will define the max current for charging of the battery. It will not be related to the observed fault.

ILIM = pin #10 and contains a resistor based voltage divider. See attached for a partial schematic from another motherboard that can be used as a guide. Remove all power and measure the resistance of the resistor that is still onboard for pin #10.

Does this resistor go to ground or +3v3 ? What is the value of that resistor?

Based on that value, you can review the value of the opposite resistor to support the peak current to be applied for the charging leg of this component.

WRT to the fault - review the DCin mosfets that are also nearby this charger IC. Often one of them will be leaky. Review the sticky above by piernov on charger circuits.

[liquidzorch]

Thanks, the resistor that is still attached is going to gnd, it's a 10k if I'm not mistaken as I was checking it yesterday. If it's a voltage divider circuit to determine the max amperage, then I could calculate it, but I don't know the max current this laptop should deliver to the battery. I will check on the sticky you mention. Thanks agian.

[liquidzorch]

Ok. So I finally found the fault, and now ac in works, but I have a problem. If the battery is connected it works, but the light goes orange and does not charge the battery. The batfet mosfet will choose for the system to use battery instead of ac. If the battery is disconnected, the acfet turns on and the system will use ac power. If I disconnect the battery while ac is in, the thing will just turn off, and never seitch from battery to AC. So it's either one or the other. While the image below is from the datasheet, not the laptop. It seems the laptop is pretty close to the same. The fault it had was Q3 was shorted. I swapped with another n channel, 30v 24a mosfet. What should I be looking at if the chip won't turn on ac if the battery is connected?

[mon2]

Check if the battery mosfet is leaky. Remove all power. Meter in resistance mode.

Measure the resistance across the mosfet:

source & drain
source & gate
gate & drain

Post each measurement. Also double check the DCin mosfets with the same resistance mode method. If you have a leaky mosfet here, same issue as you are reporting will surface.

[liquidzorch]

Battery mosfet:
S-D: 3.6Mohm
S-G:2.5Mohm
G-D: OL, reversed leads: above 1 Mohm

The two dc mosfets are also all in the megaohm range

[liquidzorch]

It does seem to be something to do with the missing ILIM resistor. Reading the datasheet, it says to calculate a 1.6v voltage on ILIM to disable external regulation and stick with the programmed limit. That's what I did. I also tried 1v on ILIM, but still the same thing. If I remove the resistor from 3.3v to ILIM, I now get AC power on ssytem rail, instead of battery, but no charging. So, without a voltage on ILIM I can use the battery, use the ac, hot plug and hotunplug the battery and it swaps between the two, just no charging (even though the led is orange whe both are connected).

[mon2]

Review the resistance between SRN and SRP pins at the charger ic pins. Just to confirm the full path resistance is in line for the charger circuit.

what is the voltage of ACDET pin when both are connected? This voltage should be > 2.7v but confirm my memory against the same datasheet. Respectively , ACOK should be a stable logic high = ~3v3 under a normal case.

[liquidzorch]

esistance between SRN and SRP pins is about 15ohms. I can confirm ACOK is stable at 3.3v in all conditions when ac plug is connected, as I already checked that, thinking the same. ACDET pin is 2.01v (which is below what data sheet says it should be but this is a BQ728H and the datasheet I am looking at is for a BQ24725, maybe a bit different? as with this 2.01v it does turn on.

And an update. I replaced the q3 mosfet again, and got it to charge for a quick 2 seconds, when plugged ac in, the orange led flashed twice, then stable, and when it went stable, the charging stopped. And now, it no longer blinks on ac plug, back to the exact same scenario.

[mon2]

The resistor based voltage divider on the ACDET pin is too low. The combination of the high and low side resistors must yield a value of 2v7 or higher. Review any of the BQ data sheets to obtain common values for each resistor. You can also use an online resistor calculator for this divider circuit. Focus on this and post your findings. The mosfets may be faulty but fix the ACDET problem first.

[liquidzorch]

Thank you for all your help. I have found the charging problem. Phase was not connected form the mosfet to the BQ, it runs below Q3 mosfet, so I guess when the mosfet burned, it damaged the phase line. I made a temporary jumper and it is now charging and swapping between battery and AC. The only thing left is to adjust the ILIM voltage divider, I am guessing that I should leave it at 1.6v, to disable the voltage divider option and make it check it's programming (if I understand the datasheet correctly). Right now I have ir at 1v, which I have not done the calculations as to see what charging amperage that equates to, but I have no knowledge of what it shoud be, I'm guessing smc should take care of the amperage.

[mon2]

Very nice find and fix. Shadow the datasheet for the charge current and test the charge time. I would review if the battery pack is heating more than usual or not. Personally leave the charge current there be lower than the max. The battery charge mosfets + inductor will also be impacted by the charge current. This was a good case study for the forum. Well done. I think in the future, we should post all voltage levels of each pin on the charger IC to smoke out (not literally) a faulty signal line like this case.