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bq51013B - Considerations for sizing C_RECT

Jonathan Bender
Expert 7895 points
Other Parts Discussed in Thread: BQ51013B, BQ51013BEVM-764

Hello,

What is the correct way to size the capacitor on the RECT pin for bq51013B?

In the datasheet it states, "Depending on the power levels, the value may be 4.7 μF to 22 μF."

In the BQ51013BEVM-764 User's Guide, it recommends C_RECT >= 10 uF, along with high frequency bypass caps.

Is there a table of the power levels and corresponding recommended C_RECT values? Since the end application is size-constrained, the lowest possible capacitance while not compromising the IC / system performance is preferred.

Thanks,

Jonathan

  • 0
    TI__Expert 7730 points

    Jonathan,
    There is not a table of power level and corresponding RECT cap. The main purpose of the cap is to filter out the noise and also sustain the load step from low to high. So you can start with the lowest 4.7uF and then observe the transient response of VRECT when you are doing the load step from zero to the maximum load of your system. If the transient is bad, then increase the cap value.

    Thanks,
    Wenjia

  • 0 in reply to Wenjia Liu
    TI__Expert 7895 points

    Hi Wenjia,


    Thanks for the reply.

    What would V_RECT look like if C_RECT was insufficiently sized? On a load step, would it dip below V_OUT before recovering, or some other type of observable waveform abnormality (on V_RECT or V_OUT)?

    Below is from the EVM User's Guide. I'm using this as a reference for what V_RECT / V_OUT should look like with a load step.

    Thanks,

    Jonathan

  • 0 in reply to Jonathan Bender
    TI__Expert 7730 points

    Jonthan,

    Below is an example when you have the load step from 500mA to 1A and VRECT has a big transient. You may want to add caps to both VRECT and VOUT when you see something like blow.

    Thanks,

    Wenjia

  • 0 in reply to Wenjia Liu
    TI__Expert 7895 points

    Hi Wenjia,


    Perfect, thanks for the clear illustration! We'll start with C_RECT = 4.7 uF, and if V_OUT and/or the output current sag with the max load step, will increase C_RECT to reach the optimal value.


    Jonathan