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BQ24040 and ISET Short to Ground

George Ioakimedes
Genius 4950 points
Other Parts Discussed in Thread: BQ24040

We've got a new design and have sent out for PCBs but in a cost cutting review I made a mistake and eliminated the ISET resistor and shorted it directly to ground. ISET2 is also shorted to ground. Here is my circuit:

The battery we will be connecting is only a 100mAh battery. What would happen if we try to use this circuit? I'm going to try and cut the trace to ground and add an external resistor but I thought I would check to see what would happen if we try to use the circuit like this.

Thanks,

George

  • 0
    TI__Mastermind 20570 points
    There is a short circuit detection on ISET pin. If a short is detected on ISET, Iout latches off.
  • 0 in reply to Jing Zou
    Genius 4950 points
    That was my understanding as well but I figured it wouldn't hurt to just double check. Thanks for responding!
  • 0 in reply to George Ioakimedes
    Genius 4950 points

    Hello again! I have my boards built and the technician was able to cut the trace from pin 2 to ground. She also added an external 10k resistor from pin 2 to ground. I verified that pin 2 is not shorted to ground by checking the resistance from pin 2 to ground and I measure 9.8k ohms.

    The problem is that I'm on vacation and I only brought limited tools with me to test and debug. I have some 100mAh batteries and a Li-ion battery charger (based on a MCP73831 set to 100mA charge, Adafruit #1304). I also have a USB power port analyzer (goes in series on the USB line and measures voltage and current draw).

    If I take a battery that measures 3.3V and use the Adafruit charger I can see that it charges the battery at 100mA. If I take the same battery and plug it into my board the charge LED goes on but I only see 2mA and the battery voltage does not increase.

    I have 2 boards built and I have done some testing with them powered by the battery and all chips seem to be functioning so I know I don't have any major issues on the board. Another odd fact is that one board starts to get warm rather quickly when I try to charge it yet I don't see any shorts. It is possible that maybe this board has a short under the BQ24040 and that short only matters when the chip is trying to charge?

    I'm confused why the USB port analyzer is not reporting any current draw if the charge LED is on. I've reviewed the schematic and I don't see any other issues but perhaps there is something that I've overlooked.

    I have a protection FET on the battery ground line but that shouldn't be an issue since the voltage regulator that's on the board works (as well as the other components) when I'm powered by the battery alone.

    What else can I check with just the limited tools that I have with me?

    Thanks,

    George

  • 0 in reply to George Ioakimedes
    TI__Mastermind 20570 points
    10k resistor between ISET and GND will program the fast charge current to around 50mA.

    If the IC gets hot when the current is only 2mA, then there is definitely a short somewhere. Can you use a multimeter to measure the continuity of each pin on bq24040?

    Please also try to remove the protection circuit and connect the battery directly to the OUT pin.

    Measure if it is 5V at IN pin of the IC.

    Make sure TS voltage is within the normal temperature range.

    These are what I can think of based on the information you have provided. I would also suggest to replace the IC on the board that has the hot issue.
  • 0 in reply to Jing Zou
    Genius 4950 points

    Yes, I agree that I think the warm board has some short under the chip but it's a little odd that the USB port analyzer doesn't show a high current. For now I'll forget about this board and X-Ray it when I get back. Since the chip has only a 0.4mm pitch I don't think I can safely get 2 multimeter probe tips on the pins next to each other.

    ON the other board I measure 500mV on the TS pin. PG seems to be 0V. I would assume that if PG was active I would see some voltage on it without a pull-up resistor attached. Since the charging LED is on I think that also tells me that input power (5.1V) is seen as OK.

    If I keep the USB cable connected to the board and disconnect the battery the charge LED turns OFF. If then reinsert the battery the charge LED comes back ON so the battery detection circuit is working.

    I kept the board charging for more than 30 minutes but the current reading on the USB analyzer never goes above 2mA. So it seems like everything is working except the charger really isn't charging and is in some kind of standby mode. If was actually disabled I would think that the charge LED would be OFF and it's not. We've both calculated that the charging current should be 50mA and the battery voltage is less than 4V so I'm really stumped as to what the problem is.
     
    I don't have the ability to remove the protection FET but the switching regulator on this board is working so I don't think that could be a potential problem.

  • 0 in reply to George Ioakimedes
    Genius 4950 points
    What would happen if ISET is shorted to Vin? When the technician added the 10k external resistor from ISET to GND I think there is a short under this resistor between pins 1 and 2. If I measure pin 2 I see 5.1V which is Vin. What would the BQ24040 do if ISET is Vin?
  • 0 in reply to George Ioakimedes
    TI__Mastermind 20570 points
    At input source plug in, there might be a large ring voltage on connection due to the parasitic inductance. This ring voltage might get higher than the maximum voltage rating on ISET. If ISET is shorted to IN, then ISET might get damaged.
  • 0 in reply to Jing Zou
    Genius 4950 points
    What would happen if ISET did not get damaged but is at Vin=5.1V
  • 0 in reply to George Ioakimedes
    TI__Mastermind 20570 points

    If ISET is not damaged, then there should be no charge to the output.

  • 0 in reply to Jing Zou
    Genius 4950 points
    OK, thanks for letting me know that. I'll have to check the boards next week but that would at least explain why they don't seem to be charging but all of the charger functions appear to be working. Kind of ironic that I started with ISET shorted to ground and now to Vin!