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BQ27531-G1: Is it possible to operate without ntc resistor?

Petr Mlynar
Intellectual 615 points
Part Number: BQ27531-G1
Other Parts Discussed in Thread: AM3358, BQ24190

Hi,

on our custom board we plan a following setup:

MPU: AM3358

PMIC: TPS65217D

CHARGER: BQ24190

FUEL GAUGE: BQ27531

BATTERY: 8000mAh without ntc resistor

Is it possible to operate setup as above mentioned if the battery doesn't have a ntc resistor. From the BQ27531 TRM I understood that battery insertion pin works only if NTC is present, otherwise the voltage will be 2.5V and FG will assume that battery is not inserted.

Any idea how to solve this?

  • 0
    TI__Expert 3030 points

    Hi Petr,

    If the battery doesn't have a thermistor but battery insertion detection is needed, you'll need to pull BI/TOUT to VCC by a 1.8M resistor, and the pack needs to have a pin pulled down to PACK- by 10K. When battery is inserted, that pin of the pack will pull BI/TOUT low, then gauge knows the battery is inserted.

    Best Regards,

    Abraham

  • 0 in reply to Abraham Rashid
    Intellectual 615 points

    Hi Abraham,

    I thought that its like that but let me have another question.

    We will design our board with ntc circuitry supported, but it might happen that we will use battery (different supplier) without the ntc wire. It effectively means that BI will always be high (2.5V).

    Is fuel gauge able to operate without battery insertion functionality? How does this limitation influence fuel guaging and charging with BQ24190? Will our board boot or does FG limit current (charger settings) without proper battery insertion?

    Thanks for clarification.

  • 0 in reply to Petr Mlynar
    TI__Expert 3030 points

    Hi Petr,

    Gauge will enter BAT INSERT CHECK Mode without battery insertion. For this application, you can configure Operation Configuration B[BIE] = 0 to disable battery insertion detection, and inform the gauge with command BAT_INSERT: 0x000D when the battery is inserted, thus gauge will rely on the host to detect battery.

    Best Regards,

    Abraham

  • 0 in reply to Abraham Rashid
    Intellectual 615 points

    Hi Abraham,

    thanks for the answer. I understood that we can have a workaround via SW running on the host but what about the case when the host is turned off.

    If I insert a battery for the first time and press power button, will the host boot or will FG limit the input current of the charger and therefore prevent boot? I cannot find it now but I thought I read in datasheet that the current will be limited to 100mA only. This would of course prevent the boot and we would be in deadlock situation.

    If the host can boot and checks the charger registers through the FG (we use combo FG controlling charger) to find out that we run on battery and SW will then set the BI flag in FG, we should be able to operate normally right?

    Thanks.

  • 0 in reply to Petr Mlynar
    TI__Expert 3030 points

    Hi Petr,

    Could you upload a block diagram of your system including the gauge, charger, MCU and battery connections? If the charger can power the MCU up without charging the battery, MCU will boot. If the gauge controls the power from charger to MCU, then MCU won't be able to boot.

    Best Regards,

    Abraham

  • 0 in reply to Abraham Rashid
    Intellectual 615 points

    Hi Abraham,

    its exactly same schematics as it is in the bq27531 datasheet, Figure 12 in chapter 9.2

    Then we connect the SYS voltage pin into the TPS65217D PMIC as a battery input. Because we use the beaglecore module you can see the connection between PMIC and MPU here:

    Thanks for clarification.

  • 0 in reply to Petr Mlynar
    TI__Expert 3030 points

    Hi Petr,

    Default current limit of BQ24190 could be set to 500mA with OTG = 1, refer to bq24190 datasheet table-8. BQ27531 enters halt state without battery insertion so charger will use default configuration if gauge doesn't interfere.

    Best Regards,

    Abraham  

  • 0 in reply to Abraham Rashid
    Intellectual 615 points

    Hi Abraham,

    isn't the input current limit applied only in case where Vbus is connected? In my case I have no charger connected (Vbus is 0) and only battery without ntc is inserted.

    I am pretty sure that charger will let the current flow into the system (SYS pin) but I do not know how the fuel gauge will react if it find out that battery is not inserted (no ntc). Then FG can drive the charger to turn off battery switch or reduce the otuput current, preventing the system to boot and set battery insertion via SW.

  • 0 in reply to Petr Mlynar
    TI__Expert 3030 points

    Hi Petr,

    The switch of the charger is turned off when powered up by default, so it won't start discharging the battery without instructions from gauge. You may try to set REG01[5:4]=10 to enable discharge when reset. An other approach is connecting BI pin to P-, gauge can be informed when battery is inserted. 

  • 0 in reply to Abraham Rashid
    Intellectual 615 points

    Hi Abraham,

    but how should I set REG01 if my MCU will not boot?

    Your proposed alternative will hardwire the absence of NTC resistor, right? Our idea was that user will be free to choose the battery with ntc or without as we have two suppliers.

    It seems to me that this is not feasible and we must stick to one solution only, right?

  • +1 in reply to Petr Mlynar
    TI__Expert 3030 points

    Hi Petr,

    My initials were to configure the charger separately prior to connecting the battery.

    Batteries both with NTC and without NTC cannot be supported with same hardware configuration. Two separate solutions can be used to support two kinds of batteries or you can ignore the NTC pin of the batteries with thermistor and use the no NTC solution for all batteries.

  • 0 in reply to Abraham Rashid
    Intellectual 615 points

    Hi Abraham,

    now its clear to me, thanks.