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BQ51050B: Behavior of *CHG

Robert Marsanyi
Prodigy 20 points
Part Number: BQ51050B

We use a standard bq51050B circuit for managing a 320 mA-hr LiPo in our product. After our LiPo is fully charged, we have observed the CHG pin pulsing low in response to pings from the charging transmitter, low for 90uS with a 500mS period. The datasheet doesn't seem to predict this behavior. Can you provide any further information about how CHG is defined?  Specifically, we're considering using this to detect that we're connected to an active transmitter and our battery is fully-charged.

  • 0
    TI__Guru* 75656 points
    Hello
    After fully charging the battery BQ51050B will send a charge complete to the TX to stop power transfer, TX will turn off. The TX will power up typically every 500ms the BQ51050B will respond with End Power Charge Complete (EPT01), the TX should then shut off.
    I checked this in the lab this morning and /CHG pin will remain Hi-Z then entire time, no low to indicate charge.

    After RX sends charge complete it enters a low power state and will only respond if the TX powers it up. The /CHG pin will be in a Hi-Z state.

    Can you send a scope capture of what you are seeing.
  • 0 in reply to Bill Johns
    Prodigy 20 points

    Here's a couple of scope traces from our test engineer.  He points out that the LOW signal isn't really dragged low, it looks more like it's floating.

    and a close-up of the LOW portion:

  • 0 in reply to Robert Marsanyi
    TI__Guru* 75656 points
    The /CHG pin is open drain device and will use a pull up resistor, check the voltage on the pull up resistor.
    Also check the voltage at RECT pin, this is voltage that will power internal circuits for device. But the bq51050B will not power up for about 40mS.
  • 0 in reply to Bill Johns
    Prodigy 10 points

    Hi Bill,

    Sorry for the late reply.

    I am the one doing the testing for Robert, so I'll be responding from now on. Here is what we see on the RECT pin when the battery is fully charged.

    I also have been looking through the datasheet and from my understanding of the Figure 20 and Figure 21 flowcharts, the only conditions that would cause the RX to immediately end communication are EN2/AD/TS/CTRL EPT conditions. Seeing that a full charge is not any of these conditions(we have TS/CTRL tied to a NTC) the next possible place to send a full charge EPT would be the first decision in Figure 21 which is after the output is enabled at the end of the Figure 20 flowchart. See highlighted sections below.

    Giving this, I tried to connect TS/CTRL to 3V and also GND, which did halt the wireless charging but the CHG line still does the same thing.