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BQ24251 Input Current Limited to 800mA

Richard Prosser
Prodigy 30 points
Other Parts Discussed in Thread: BQ24251

Hi,

We are using the bq24251 in a new product but would like to decrease the battery recharge time. At present we have set the ILIM value to about 1.5A and the ISET limit to about 1A but cannot obotian an input current greater than about 800mA. I can get up to 900mA  - 1A or so by increasing the input voltage to about 5.6V, but at the 5V we are using I only get the 800mA. I have changed the system load and can reduce the charge and/or input current using software settings but still hit the 800mA input limit.

I have also shorted out the ILIM resistor, again without improvement.

I have looked at and tried alternative inductors in case I am hitting a saturation problem, although I don't think this should effect the input limit.

The charger is being reported as a SDP type, but, again, I've tried different types without improvement.

Any other ideas as to what the problem could be?

Thanks,

Richard Prosser

  • 0
    TI__Guru**** 166916 points

    Are you using the ILIM pin or I2C to set the input curren limit?

    If you are certain that you system load is activation the DPPM loop, which prioritizes system load over charge current, can you confirm that:

    1.  The input voltage is drooping to the VINDPM limit and therefore activating the VINDPM loop?

    2.  The IC temperature isn't too hot and therefore activating the thermal loop?

  • 0 in reply to Jeff F
    Prodigy 30 points
    1. I thought I2C was used to set the input current lmit. However, register 2 reads as 0xEC, corresponding to exernal ILIM. This is set using a 196ohm resistor for 1.37A. Shorting the ILIM resistor has no effect. I'll try playing with this register setting though. (will need to get my test app modified).
    2. The input voltage is drooping, but even when I correct for this the input current still limits. I have set the Vdpm to 4.2V using resistors also, still no improvement.
    3. The chip doesn't appear to be getting hot at all. Spraying cold spray on the chip doesn't improve things, and the current limit stays fairly stable, it doesn't decay as things warm up.
  • 0 in reply to Richard Prosser
    Prodigy 30 points

    I have discovered I have been fighting 3 or 4  different issues on this.

    1. The input voltage droop, already discussed, and countered by increasing the input voltage (or dropping the Vdpm limit).

    2. A firmware bug that was preventing me setting the USB host type without resetting all registers in the chip (R2.7 was being written every time)

    3. Significant voltage drop between the charger chip and the battery. This is mostly due to cable/connector losses, but a fuse is  adding 55mOhms and protection FETs and 5mOhm shunt also contribute.  In total I get about 300mOhm of series resistance. At a 1A charge current this results in a 300mV drop so my 4.2V charger voltage setting results in 3.9V at the battery, reducing the time the charger is in constant current mode.

    4. The actual voltage recovery on the batteries is reasonably quick (especially to 3.9V) further reducing the time that the maximum charge current is being applied.


    We are now looking at using the voltage measurement from the protection chip to set the charger output voltage (with safety measures).  This will enable the full 4.2V to be applied at the battery terminals and appears to decrease the charge time by almost half. I've written this up as it might be of assistance to others.