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bq24257 no PTM or VBATSHRT

Brant Martin
Prodigy 50 points

I'm trying to use this charger in my product, but I keep getting prototypes returned because the lithium cell protection circuitry has kicked in, and the charger won't assert VBATSHRT to reset it, or enter PTM with no cell connected.

Actually, I can get it into PTM if I remove a good cell while it's charging. When I do that, the charger keeps the device powered, and for a minute or two after that, I can unplug it and it will still go into PTM when plugged back in. After a couple minutes unplugged, it no longer goes into PTM or asserts VBATSHRT. Here's my circuit:

I've also added a 10k resistor from CE to ground, though I never see any voltage on that pin.

  • 0
    TI__Guru**** 166626 points

    /CE should not be left floating and needs a pull down resistor or be tied to GND.  If there is no thermistor connected to TS, R28 needs to 10k or R27 needs to be removed in order to disable the TS function.  Keep in mind that until you enter host mode (i.e. send your first write command), the D+/D- circuitry is in control of input current limit and if the battery voltage is above the VBATGD threshold of ~ 3.6V (i.e. a good battery) and D+/D- has resolved to a an SDP, the IC will not allow charging.  If SDP or CDP and weak battery, then ILIM =100mA for 45 minutes and then charging stops.  Also, the part does not automatically enter PTM.  The user has the write a 1 to the FORCE_PTM bit and it will not enter if the AnyBoot function thinks there is no battery (i.e., the square wave on BAT).

  • 0 in reply to Jeff F
    Prodigy 50 points

    The datasheet says that the Ts disable threshold is 70-73% VLDO, which is what that resistor divider is providing. A 10k resistor at the top didn't change anything.

    Are you saying that it has to do D+/D- detection before any charging will begin?

  • 0 in reply to Brant Martin
    TI__Guru**** 166626 points
    Actually, V(TS) > 70-73% V(VLDO) disables the TS function so you can simply connect TS to LDO to disable it.

    D+/D- controls the input current limit until you enter host mode. If D+/D- selected SDP then you have 100mA input current limit until you send the first WRITE command via I2C.