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Problem with BQ29700 battery protection IC [SOLVED]
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Unixon:
I have a design powered from a single 18650 battery and I've been asked to integrate a battery protection circuit in the design itself so that a user could make use of unprotected 18650 cells which are easier to put in the holder because these cells are a bit shorter.

Without a battery protection, the power circuit looked like this:
[(+) bat. terminal]---[P-FET switch]---[DCDC+Load]---[(-) bat. terminal]

After inserting BQ29700 circuit the new power circuit looks like this:
[(+) bat. terminal]---[FUSE]---<BAT+=PACK+>---[P-FET switch]---[DCDC+Load]---<GND=PACK->---[BQ29700 Charge N-FET]---[BQ29700 Discharge N-FET]---<BAT->---[(-) bat. terminal]
Power to battery protection is constantly delivered from the node BAT+=PACK+ as soon as a battery is inserted.
Charging circuit based on BQ24090 is also connected to the same node.

The problem is BQ29700 keeps discharge N-FET off (DOUT is always low).
Shorting BAT- to GND=PACK- or connecting charger across PACK+/PACK- as prescribed in the datasheet does not get BQ29700 out of this half-disabled state.
Is there any way to enable protection IC in this setting?
Daixiwen:
Do you have a schematic? Is everything connected exactly as shown in the datasheet? Is there any load connected to the pack terminals (i.e. is the P Fet on)? What is the cell voltage? Is there any inrush current that would trigger the overcurrent or the short circuit protection?
Unixon:

--- Quote from: Daixiwen on July 31, 2019, 09:25:28 am ---Do you have a schematic?

--- End quote ---
Yes, but I'm not sure if I'm allowed to publish it. Well, OK, I guess this would be difficult to solve this problem without an actual schematic.


--- Quote from: Daixiwen on July 31, 2019, 09:25:28 am ---Is everything connected exactly as shown in the datasheet?

--- End quote ---
I've checked that and I think it is, but maybe I'm missing something over and over...


--- Quote from: Daixiwen on July 31, 2019, 09:25:28 am ---Is there any load connected to the pack terminals (i.e. is the P Fet on)?

--- End quote ---
Only DC-DC boosters and a power LED, the entire circuit draws about 50mA.


--- Quote from: Daixiwen on July 31, 2019, 09:25:28 am ---What is the cell voltage?

--- End quote ---
I've tried 3.8V-4.2V, no difference.


--- Quote from: Daixiwen on July 31, 2019, 09:25:28 am ---Is there any inrush current that would trigger the overcurrent or the short circuit protection?

--- End quote ---
Only capacitance in DC-DC circuits, but this must not trigger protection again after shorting grounds because all caps are charged.
Daixiwen:
You could have just shown the part of the schematic with the BQ ;)
I'm sorry but I don't see any problem there. You may have to start looking for less obvious problems, like components mounted in the wrong direction, or pin-out/footprint errors (The pinout of the BQ29700 is correct, but maybe the MOSFets?). It's probably a stupid question but did you check that the cell was inserted with the correct polarity? And by the way if this is a user serviceable cell, maybe you should check that your circuit can survive a cell inserted with the wrong polarity.
Did you check that you don't actually have a short circuit on your PCB? Between VBATT and ground?
Unfortunately I can't think of anything else. Maybe start probing around with a scope the pins around the BQ to see if anything is happening right when you connect the cell and/or charger.
Unixon:

--- Quote from: Daixiwen on July 31, 2019, 11:15:13 am ---You may have to start looking for less obvious problems, like components mounted in the wrong direction, or pin-out/footprint errors (The pinout of the BQ29700 is correct, but maybe the MOSFets?).

--- End quote ---
No, this is not the case. I will double check alternative MOSFETs (I've built 3 devices for different current thresholds, hence different Rds.on).


--- Quote from: Daixiwen on July 31, 2019, 11:15:13 am ---It's probably a stupid question but did you check that the cell was inserted with the correct polarity?
And by the way if this is a user serviceable cell, maybe you should check that your circuit can survive a cell inserted with the wrong polarity.

--- End quote ---
I'm testing with either a smaller LiPo battery, directly soldered with color wires to correct pads of 18650 socket, or just a lab power supply, not an actual 18650 cell.
However, I think I need to put an additional P-FET for polarity protection of BQ29700 (this will be 3rd polarity protection circuit onboard).
Alternatively, I could power BQ29700 from a node after polarity protection P-FET for the load, but this could bring some other problems if BQ29700 is powered off along with the whole device.


--- Quote from: Daixiwen on July 31, 2019, 11:15:13 am ---Did you check that you don't actually have a short circuit on your PCB? Between VBATT and ground?

--- End quote ---
Yes, I did. If I short BAT- to GND or connect PSU after the protection circuit the device powers on without issues, all voltages are OK.


--- Quote from: Daixiwen on July 31, 2019, 11:15:13 am ---Unfortunately I can't think of anything else. Maybe start probing around with a scope the pins around the BQ to see if anything is happening right when you connect the cell and/or charger.

--- End quote ---
I tried that, but couldn't catch anything significant yet.

My current decision is to put this protection circuit on a separate board with all requisite jumpers and test points and test against different conditions and state transitions.
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