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BQ24091 CHG- pin is not going high

In the circuit above, 3 of 5 prototype boards fully charge the battery (determined by 0.00 v drop across the PTC) and yet the CHG- pin never goes high (i.e. is always low). nCURRENT_ISET is set low and CHRGR_EN is FLOATING  ISET max is 300 mA.  The resistance of the PTC is 0.4R.  There is minimal noise on the pins.  Do you have any ideas what could be causing this problem? The battery is a Tenergy14500 800 mAH.





  • I want to clarify my inquiry.  In the circuit above, ALL of the boards charged the battery but 3 did NOT turn off the CHG- pin (i.e. go high or FLOAT).

  • Hello Robert,

    The CHG pin is actually open drain.  I don't see what it is connected to in your schematic.  If it was connected to the cathode side of an LED, it would turn the LED on.  Check the LED orientation if you are using one.  They only way to get a high/low indication is through a pull up.


  • Thank you.  I wish it was that easy.  The CHG pin is pulled up with an internal weak pull-up at the microprocessor.  These pull-up may not be adequate, however for both this and the TS pin. What do you recommend?

    I am still playing with the circuit to ascertain what may be happening here. Thank you for your support and quick response.  It is nice to know TI is there to help.  Bob

  • I think we have found the problem and it is somewhat related to your comment on pull-ups.  We are using a microprocessor that allows you to pull-up a port (8 pins).  What I didn't realize is that when we use a compiler directive to float the TS pin, it actually put the pin at the microprocessor into an input mode and which then attached it to the internal pull-up.  Thus, we had a divider that put 2.2 V on the TS pin and put it into TTDM at the charger.  In the end, the problem was that a compiler output FLOAT wasn't really a float. 

    The solution is to either leave it in TTDM or add 2 external pull-ups and make sure that the microprocessor does not enable the pull-ups on the I/O port. I mention it in case others may make the same erroneous assumption.