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BQ76952: Effects of using CP1 capacitor greater than 2.2uF

Embedded Developer
Intellectual 345 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQ76942

Tool/software:

In reference to TIDA- 010247 we are implementing a design with 6 Charge and 6 Discharge FETs of total gate capacitance 51.6nF  but after implementing the reference Multiple FETs with the BQ76952, BQ76942 Battery Monitors (Rev. A) (sluaa09a), the turn-on time of the DSG gate w.r.t PACK+, was not fast enough for our application (~250us).

 

On increasing CP1 capacitance to  4.4uF, which is double the maximum recommended value, we observed 25% improvement in the turn on time (~220us).

Current concerns:

  • What are the potential failure modes that we might encounter when using this solution long term?
  • Are there any other optimizations that can be done to improve turn-on time, except the ones mentioned in the Multiple FETs PDF?
  • 0
    TI__Intellectual 2080 points

    Hello,

    You are correct that we do not recommend going over the 2.2 uF. I will have to refer to my team and get back to you on the exact failures that may occur from increasing this capacitance or if it is just recommended for optimal device operation.

    I do not believe there is any method to improve turn-on time besides the ones mentioned the application note. Again, let me discuss with the team and get back to you on this as well.

    Regards,

    Rohin Nair

  • 0 in reply to Rohin Nair
    Intellectual 345 points

    Hi Rohan, have you discussed with your team?

  • 0 in reply to Embedded Developer
    TI__Intellectual 2080 points

    Hello,

    Apologies for the delayed response. The team member that will know more about this is currently out of office. I will get back to you later this week.

    Regards,

    Rohin Nair

  • 0 in reply to Embedded Developer
    TI__Intellectual 2080 points

    Hello,

    After speaking with my team, this is what I have gathered:

    Using this large cap is not an issue as long as you have tested it well. The main issue of having a large CP1 cap is that it takes longer for the charge pump voltage to rise to full value when it is first turned on (typically when you first power the device).

    You might look on a oscilloscope at the time it takes CP1 to rise to full voltage, and the time when you will turn on your FETs, and make sure it is up enough before the FETs turn on. If the voltage is still rising and you have turned on your FETs into a large dsg load, the FETs would have a strong Vgs, so it may overheat. Check this, and as long as this voltage is not rising then it may be safe to use.

    Regards,

    Rohin Nair