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Original question:

BQ76942: Recommended PNP transistors for sluaa09a circuit

BQ76952: How to claculate the required transistor for the current loop circuit

Criton Kallkkaran
Prodigy 70 points
Part Number: BQ76952

Tool/software:

Hi Team,

I had a go through the document Multiple FETs with the BQ76952, BQ76942 Battery Monitors (Rev. A) to understand the control of multiple FET's using BQ76952. Unfortunately, there was no reference on the the paramters to be considered to choose the Q30 AND Q31. From my understanding obviously we have to consider the pack voltage. Could you please tell me what else should be considered in choosing Q31 and Q30. A thread related to similar topic suggests to choose  "MMBT3906 is a good starting point with 40V and 200mA ratings". Is the collector current related to the maximum load current? Could you please explain it in detail?

I look forward to hearing from you. 

Many thanks and Kind regards,
Criton

  • 0
    TI__Expert 5395 points

    Hello Criton,

    The collector current most likely would be considerably less than the maximum load current so I wouldn’t be basing it off of that. The MMBT3906 is a good starting choice for choosing a transistor as it should be handling up to what you need. You can consider BAT+ for Q30/Q31. To better understand what happens when the FET driver turns-off, Section 16.5 FET Driver Turn-Off in the BQ76952 Datasheet explains this

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 70 points

    Hi Alexis,

    Many thanks for your response.
    I went through the section 16.5 which describes the use of series gate resistance that is used to reduce the transients during the turn off of FET's. I could not find the significant details of the Transistors in this portion. 

    Alexis_H said:
    You can consider BAT+ for Q30/Q31

    Could you please elaborate on this as I did not understand this statement.

    Kind regards,
    Criton

  • 0 in reply to Criton Kallkkaran
    TI__Expert 5395 points

    Hello Criton,

    I apologize for the confusion from that statement. Instead of considering the pack voltage, I think considering the BAT+ voltage would be better when considering the transistor.

    The section I recommended, is more to help better understand what our part does when the FET driver turns-off. This could help with seeing the voltage and current flow the transistor might see when you add that into the circuit. 

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 70 points

    Hi Alexis,

    Many thanks for the clarification. Do you mean the total voltage of the cells in series (battery) by BAT+ voltage? and how would I relate/ choose the collector current for the particular transistor?

    Kind regards,
    Criton

  • +1 in reply to Criton Kallkkaran
    TI__Expert 5395 points

    Hello Criton,

    Yes, that is correct. Top of the battery stack to GND.

    As explained in the FET Driver Turn-Off section, the CHG pin discharges toward the BAT pin level when it’s off. That means the voltage at the collector for the transistor sees at most, is the difference between BAT+ and CP1 (Charge pump). 

    Best Regards,
    Alexis