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BQ77905: Help Diagnosing Fault Behavior on BQ7790509 for Lithium Primary Battery Protection

James Moriarty
Prodigy 10 points
Part Number: BQ77905

Tool/software:

Hello,

For my application, I am using a Lithium Primary Battery pack configured as 4S (3.9V D Cells), giving an OCV of 15.6V and a max discharge rate of 1A. I have four such battery packs diode-ORed together to achieve 15.6V @ 4A max. The load characteristics are:

  • Idle: 16mA
  • Operation: 200mA (typical)
  • Inrush: 2A

I would like to add overvoltage (OV), undervoltage (UV), and temperature protection to the battery packs without using a microcontroller. The BQ7790509 caught my attention due to its built-in UV (2.5V) and OV (4.25V) thresholds.

I made a PCB similar to the BQ77905EVM-707 evaluation board for testing, and I made the following modifications:

  1. Changed R18–R21 to 3.9MΩ to reduce battery current draw (from the original 3.9kΩ).
  2. Changed R16 (sense resistor) to 0.05Ω to accommodate the low load current. (OCD2/IMAX = .1/2= .05)

Setup for Testing:

  • Power supply connected to J1 to simulate a 15.6V battery pack.
  • Load connected to J5.

Observed Behavior:

  1. On power-up, the system briefly draws 16mA before dropping to the microamp range. The load voltage drops to approximately 2V.
  2. If the load is disconnected and reconnected, the system resumes operation, drawing 16mA at idle, and the load receives power.

This leads me to suspect a fault condition (UV, OCD1, OCD2, or SCD) occurs during the initial connection, requiring load removal to recover.

Scope Measurements:

  • At power-on:
    • Both CHG and DSG pins go negative then rise to 12.5V.
    • After ~2 seconds, CHG is pulled low while DSG remains high (12.5V).
  • After removing and reconnecting the load:
    • Both CHG and DSG return high, and power is restored to the load.

Questions:

  1. What might be causing the fault condition during power-on?
    • The datasheet states CHG and DSG should both be off (low) during UV or OCD fault conditions. However, I observe DSG remaining high (12.5V) even when CHG is pulled low.
  2. Could this behavior be related to my resistor modifications (e.g., the high 3.9MΩ values for R18–R21)?
  3. Are there specific steps to isolate which fault condition (UV, OCD1, OCD2, or SCD) is being triggered in this scenario?

Thank you for your assistance in resolving this issue!

Best regards,
Jay

  • 0
    TI__Genius 16275 points

    Hello James,

    The engineer is currently Out of Office for today. They will be back Monday to provide a response.

    Thank you for your patience.

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    TI__Genius 16275 points

    Hello James,

    How do you have your cells connected? Are you using the 3.9kOhms/3.9MOhms to simulate cells? Just from an initial glance, this would look to cause issues with cell measurements.
    If only the CHG pin is turned OFF, this sounds like it could be due to an OV fault instead of UV. For UV, only the DSG pin is turned OFF.

    Table 8-3. Fault Condition, State, and Recovery Methods and Section 8.3.2 Fault Operation in the BQ77905 Datasheet may assist in identifying which fault was triggered. Section 9.2.3 Application Curves also has images to show what some of the fault's work look like. As for some of the faults that have both the CHG and DSG fault turned off, the delay time for each fault could give an idea potentially to which fault it triggers if there is a difference in delay between them. 

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 10 points

    Hi Alexis, 
    Yes R18, R19, R20 & R21 are used to simulate the cells, why would this cause issues with cell measurements? In the BQ77905 EVM 200 ohm resistors are used to simulate cells. I had increased the resistance to reduce the current draw on the battery. 
    Thanks for the insight on the CHG and DSG delay time I will look into this. 

    Regards

    James 

  • 0 in reply to James Moriarty
    TI__Genius 16275 points

    Hello James,

    3.9kOhms aren’t a good representation of actual cell batteries and our part was made to use real cells for normal operation is all. The pins themselves also have an input resistance, so if the simulated cells are also quite large, that could potentially cause conflicts in how the voltage is read.

    Best Regards,
    Alexis