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BQ76940: REGSRC Source Follower Ckt

Rohan Patwardhan1
Intellectual 340 points
Part Number: BQ76940

Hi TI Team,

What can be the min max value range for Rsrc resistor as per following diagram. 

What must be the range of VC5X for correct functioning of the circuit. Currently we are providing VC5X from 12V to 7.5V. What can be the repurcussions if this range is not right,

Regards,

Rohan

  • 0
    TI__Guru 54715 points

    Hi Rohan,

    You want to maintain a good voltage on the REGSRC to meet the minimum recommended operating voltage, but also to maintain the drive voltage for the FETs which comes from the REGSRC pin.  See https://www.ti.com/lit/pdf/slua749 section 9.  If you drop too much voltage on Rsrc because you have extra REGOUT current, you could cause your FETs to heat more than expected. 

    If you have 20 mA current and 100 ohm you have 2V drop.  If the voltage drop is a concern for your design consider minimizing or eliminating Rsrc and moving the blocking diode to the drain path as in the right side of figure 14 of that apnote.

  • 0 in reply to WM5295
    Intellectual 340 points

    Hi Sir,

    Our current value is 100R. We get a voltage of ~8V on REGSRC which seems to be within limits. The REGOUT load it is about 2mA. There is no heating on the source follower configuration FET. Do you foresee 2mA current on REGOUT to have an issue?

    In your statement, you can may heat FETs, you mean Charge, Discharge FETs or the source follower FET. In our application, we are not controlling Charge-Discharge FETs with BQ IC.

    Regards,

    Rohan

  • 0 in reply to Rohan Patwardhan1
    TI__Guru 54715 points

    Hi Rohan,

    8V seems fine for REGSRC if you are not driving charge and discharge FETs. 2 mA regulator current in your 100 ohm is only 200 mV drop.

    The concern on FET heating mentioned was for the power FETs for the pack.  May FETs for this application have RDSON ratings at 6V and 10V, the 6V value may be high enough that a designer does not want to operate the FETs near that voltage for an extended time.  Other designs may be fine with the value and condition.  Heating of the source follower may be a concern in some designs also.  If a design has the full 20 mA regulator load and a 30V drop on the transistor it is 0.6W, they may need a special transistor, or drop some power in a drain resistor or use a switching regulator to provide the load.

  • 0 in reply to WM5295
    Intellectual 340 points

    Understood. Thank you for your support!!