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BQ76952: FET Status during Load Condition and without Load Condition

syed moosa
Prodigy 110 points
Part Number: BQ76952

Hi,

1.My [DSG_FET] is turning off during CUV only when load is connected. Without load even if CUV is triggered but the [DSG_FET] does not turn off why so? 

2.Without Load what will be the [DSG_FET] & [CHG_FET] status during CUV, COV, OCD, OCC Conditions?

3. With Load what will be the [DSG_FET] & [CHG_FET] status during CUV, COV, OCD, OCC Conditions?

  • 0
    TI__Guru 54715 points

    Hi Syed,

    See the technical reference manual of the BQ76952 for settings affecting the performance.  See Chapter 5 Primary and Secondary Protection Subsystems for general description, see settings for protections in 13.3.3.2 Settings:Protection:Enabled Protections A and FET actions for DSG in 13.3.3.8 Settings:Protection:DSG FET Protections A.  It seems you would have set these to get the DSG FET to turn off during CUV since this is not default.

    1. The DSG is driven to a voltage just below the LD pin voltage.  This will have the FET gate and source at about the same voltage and the FET will be off.  The gate and source may still be near the battery voltage unless the load discharges the PACK+ node.  Since the BQ76952 loads the PACK+ periodically to measure the LD and PACK pins, the PACK+ will gradually drop to 0V, but it will be slow.  If the load has a large capacitance the drop will be much slower.  If a charger is attached the charger may maintain a voltage on PACK+ and it will not drop, but that would potentially cause a CUV recovery.  With a load attached the load will discharge the PACK+ node, LD and DSG will follow it down to 0.

    2.  The default conditions:

    Settings:Protection:Enabled Protections A  0x88. Only SCD and COV are enabled by default.   If you want OCD, OCC and CUV the corresponding bits must be set, 0xFC for example.

    Settings:Protection:CHG FET Protections A 0x98.  SCD, OCC and COV are enabled to control the FET if also enabled as protections.

    Settings:Protection:DSG FET Protections A 0xE4.  SCD, OCD and CUV are enabled to control the FET if also enabled as protections.

    Assuming you set Enabled Protections A to 0xFC:

    Condition DSG CHG
    CUV 0 1
    COV 1 0
    OCD 0 1
    OCC 1 0

    When 1 the outputs are driven to the charge pump voltage, above the battery voltage.  When 0 the CHG is driven to the BAT pin voltage, the DSG is driven to just below the LD pin voltage. Without load DSG and LD will be at about the PACK+ node voltage which may sit near the battery+ voltage and drift down slowly.

    3. The status of the signals will be the same as 2, but with a load present PACK+ will pull to PACK- (0 V), so LD and DSG will go with it.

  • 0 in reply to WM5295
    Prodigy 110 points

    Hi thanks for your response. If both DSG & CHG FETS are off what will be the pack+ voltage without load ?

  • +1 in reply to syed moosa
    TI__Guru 54715 points

    Hi Syed,

    Without a load and both DSG and CHG FETs off PACK+ will normally drift slowly to PACK-.  Several of the turn off examples in the BQ76952 data sheet section 16.5 https://www.ti.com/document-viewer/BQ76952/datasheet/GUID-6B8A70B1-03C6-410A-B38E-CA75FF0C9D14#GUID-5ADEBB8A-EAD6-482F-9202-4CE0A42B97ED have the PACK+ connected to BAT+ to avoid the confusion of discharge of PACK+.  Figure 16-6 shows an example of the PACK+ falling slowly.  The scope probes provide some load.