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TPS22997: CT Waveform question

Yishan Chen
Intellectual 1740 points
Part Number: TPS22997

Tool/software:

Hi Team,

Customer is measuring the voltage between CT and GND, they found the waveform of CT is:

They want to know why CT waveform is like this. Why CT will become same voltage potential with GND at the end.

They also measure the CT to Vout at the stable status, they found CT-Vout is -3.3V, I think it should be positive 3.3V, why is that happened?

Could you please help us understand the structure of Charge pump and how it connect with CT and Vout and GND?(The structure in Datasheet is not very clear).

Thank you,

Yishan Chen.

  • 0
    TI__Mastermind 32761 points

    Hello Yishan,

    CT pin charges the capacitor with a current source as proportional to VOUT.

    Once gate high (GH) is reached, it discharges the capacitor. That means Gate voltage has reached to its limit as per VOUT and FET is fully enhanced.

    CT pin voltage doesn't represent the GATE voltage. 

     Thanks 

    Amrit 

  • 0 in reply to Amrit Jit
    TI__Intellectual 1740 points

    Hi Amrit,

    What is the reference ground of CT capacitor, will it switch from Ground to Vout like bootcap?

    Could please give us more insight about how charge pump works in load switch? Why we measure the voltage between CT and Vout is -3.3V?

    After the MOSFET is fully enhanced, If we consider the leak current of GS junction capacitor, will we need to recharge the junction capacitor when the voltage drop to a certain of level?

    Thank for you support.

    Yishan Chen

  • 0 in reply to Yishan Chen
    TI__Mastermind 32761 points

    Hello Chen, 

    TPS22997 has integrated switch with internal charge pump.

    CT pin gives you programmable slew rate. I don't see the need to measure voltage with respect to VOUT.

    Yishan Chen said:
    After the MOSFET is fully enhanced, If we consider the leak current of GS junction capacitor, will we need to recharge the junction capacitor when the voltage drop to a certain of level?

    No need to recharge the junction capacitor.

    Thanks 

    Amrit