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BQ24040: ISET2 Pulldown Value

Fred P
Expert 2235 points
Part Number: BQ24040

I try to set the ISET2 pin low so to have a charging current limit set according to ISET pin (1.8k -> 300mA).

Using a pulldown resistor of 10k on ISET2, the input current varies constantly between 100~170 mA (reading from my power supply).

The voltage on ISET2 pin is stable at 110mV.  This is well under the 0.4V for a logic LOW.

Shorting ISET2 pin to GND works: charging current is regulated at 300mA as it should.

Floating ISET2 pins works correctly as well: current reglated at ~90mA.

Why is the 10k pulldown not working correctly?  How low should I go?

Regards,

Frederic

  • 0
    Expert 2235 points

    Additional information:

    If I plug an electronic load in constant voltage mode (3.7V) instead of a battery, the charging current is regulated properly to 300mA with the 10k pulldown on ISET2.

  • 0 in reply to Fred P
    TI__Guru* 75906 points

    Hello

    We have looked at this in the lab and do not see this behavior.

    Can you share schematic?

    During your test do you have a current meter in the output, this has caused problem before. 

  • 0 in reply to Bill Johns
    Expert 2235 points

    Here you go:

    There is NO current meter anywhere.  I know this can cause problems. 

    USBC_VBUS is fed with 5V external power supply.  Voltage is stable.  Current reading on the power supply is erratic (and is known to work properly).

    The 3 signals are tied to an MCU.  MCU powered or not does not change behavior.

    If MCU drives ISET2 pin, it works as expected.  Leaving it to "float" with an 11k (not a 10k actually) pulldown gives an erratic charge current.

    Short-circuit ISET2 to GND works as expected.

    The behavior is the same on the two boards I have tested.

    Regards,

  • 0 in reply to Fred P
    TI__Guru* 75906 points

    No problem with the schematic.

    The 10k pull down should work, this is a typical value.  Have you tested with a lower value, 1k? 

  • 0 in reply to Bill Johns
    Expert 2235 points

    Just did.  It works.  

    I need to drive the pin high from time to time.  1k would still be ok but feels like the 60's...

    Anyhow, it does not make sense to use a 1k unless the logic level listed in the datasheet is wrong. 

    Pin current appears high to me as I measure 110mV for a 11k pulldown.  Specification is 2-9uA for ISET2=0.4V.

    With 1066k, I get 12.4mV.  Again, current is quite high.  

    Is the datasheet wrong about ISET2 pin or my parts are weird/damaged?

  • 0 in reply to Fred P
    TI__Guru* 75906 points

    I have looked at this in the lab with a range of resistors from 10k to 100k and cannot duplicate the problem you are seeing.

    Unit will enter 100mA mode until above 80k and 0.4V .  With a 10k pull down and 110mV you should be OK.

    Also checked the characterization data on device and it matches the datasheet, not a data sheet error.

    Do you have other parts you can try, different date code?

  • 0 in reply to Bill Johns
    Expert 2235 points

    Tested on 2 boards.  I can have 4 others untouched board with same marking on IC.  I'll measure and let you know.  This might take a day or two.