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BQ24770: bq24770: How to disable charging?

Gerrit Telkamp
Prodigy 190 points
Part Number: BQ24770

We have found an unexpected behavoiur of the bq24770 battery charger in our 4S application. We have a max. charge voltage of 16.8V and a max. charge current of 1.6A.

We need to disable charging by software if the temperature becomes too high. So when the battery voltage reaches the programmed max. MaxChargeVoltage(), we inhibit charging by setting Bit[0] of the ChargeOption0() register. Now, the internal charge regulator is disabled - but the problem is that BATFET is still kept on. Therefore, a maximum current flows into the battery (up to 3A in our case), over-charging the battery in an uncontrollable way. The learn mode is disabled in our application. What might be the reason for that? Is there an other way to disable charging?

We've also found another unexpected behaviour, but its less critical. We supply the system by an external voltage supply (24V). Charging is disabled (this works if the battery voltage is low enough). Now we remove the external power supply - the bq24770 switches over to battery. Next, we connect the external power supply again. The bq24770 will not switch over to external power supply again, so the system is still supplied by the battery. This changes when we enable charging by clearing the charge inhibit bit of ChargeOption0() and writing a valid charge current to register ChargeCurrent(). Why does the system power supply depends on the charging state?

  • 0
    TI__Mastermind 20570 points
    Can please you show your schematic?
    For the second issue, can you show scope capture on VBAT, VSYS, VBUS, VILIM?
  • 0 in reply to Jing Zou
    Prodigy 190 points

    Plase find the schematic below.

    More or less the evaluation's board schematic, but with the following modifications:

    • additional PFET (SIS413DN, T18008) allows to disconnect the battery completely during power off
    • optional overvoltage protection allows to disable input voltage at VSYS >17.5V
    • /BATPRES is generated by internal comparator of bq24770, configured as default
    • ILIM is expernally programmed to 170mV, but disabled in configuration by software

    bq24770_schematic.pdf

  • 0 in reply to Gerrit Telkamp
    TI__Mastermind 20570 points
    Can you show /BATDRV voltage? and VILIM voltage?
  • 0 in reply to Jing Zou
    Prodigy 190 points

    The bq24770 does not have a VBUS pin. Do you mean VIN?

    There is also some confusion in the "bq2477x EVM User’s Guide" (SLUUAO3C), Note 3.3, noting something about VBUS and PMID. Maybe a relict of an older document version?

    Also in the datasheet SLUSC03C, Chapter 8.6.8 the headline is confusing "Setting the Minimum Charge Voltage". This should be "Setting the Minimum System Voltage" I think.

    I will measure VILIM and BASTDRV and come back again.

  • 0 in reply to Gerrit Telkamp
    Prodigy 190 points
    I think we found the reason for the second problem (bq24770 does not automatically switch over to external power supply).
    In the moment when the external power supply is unplugged and plugged again, the bq24770 resets its registers MaxChargeCurrent() and MaxChargeVolate() to their reset values. In our case MaxChargeVoltage() is set 13.504V, because CELL Pin is high (3s/4s application).
    But when the battery voltage is higher, the system will still be supplied by the battery, until we set the register MaxChargeVoltage() to 16.8V.
    This has to be done by software.
    It is an additional step, and this is required in 4s applications only. For 1s, 2s and 3s applications the default value of MaxChargeVoltage() semms to be correct.