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BQ76952: Does not enter sleep mode with parallel path and discharge disabled

Lukas Simon
Prodigy 110 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQ25756

Tool/software:

Hello,

We use the bq76952 as a monitor in parallel path configuration and the bq25756 charger as shown in the sketch below:

We control the discharge and charge FETs with the DFETOFF and CFETOFF pins. When the discharge path is enabled(DFETOFF pin enabled) the device enters sleep mode if no load is connected to PACK+. This is as expected. 

But the device does not enter sleep mode if the discharge path is disabled. The WAKE bit is set then all the time. Then it would make even more sense for the monitor to go into sleep mode. How can we fix that problem? Or what can cause that? 

We do not have a blocking diode or blocking FET behind the charge FET and therefore the PACK pin is always high. Does this cause the behavior we see?

Here are some of our register settings:

What can cause that and how can we achieve sleep mode also with disabled discharge path? 

Thank you in advance

Lukas

  • +1
    TI__Mastermind 19015 points

    Hello Lukas,

    Section 5.2.3.2.2 PREDISCHARGE Mode in the BQ76952 Technical Reference Manual talks about this occurrence in the 3rd paragraph after the Note. It mentions that SLEEP mode can also be allowed if the host sends the DSG_PDSG_OFF() or ALL_FETS_OFF().

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 110 points

    Hello Alexis,

    Thank you very much for your answer. This is unfortunately very disadvantageous. I tried to disable the PDSG_EN bit on our test platform. If it is disabled the Monitor transitions to sleep if the discharge path is disabled as you mentioned. 

    Unfortunately we want the monitor in fully autonomous mode without commands from the host. We control the DFETOFF and CFETOFF pins by a hardware circuit. If the discharge path is disabled also the microcontroller circuit is disabled that only "reads" data from the monitor when discharge path is enabled.

    We need the predischarge functionality because we have high capacitive load on the bus.

    1: Why is there a connection with the enabled predischarge FET and the fact that the device does not go into sleep mode if the discharge FET is disabled?

    2: What do you recommend us to do in this case? Is there a workaround? We want sleep mode when discharge FET is disabled, we need predischarge functionality and we can't send a command because a deactivated discharge FET will disable also MCU to preserve power and only the monitor is on.

    Best regards and thanks in advance

    Lukas 

  • 0 in reply to Lukas Simon
    TI__Mastermind 19015 points

    Hello Lukas,

    It seems to be how the device was designed. A potential workaround might be to have a safety fault that disables DSG FET through configuring a thermistor pin, that was previously unused, as an OTF. 

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 110 points

    Thank you for your answer Alexis! Now i understand the cause. I will try to fix that with a workaround or accepting the higher current draw in normal mode because it is a very large battery.