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TPS62140A: Discharging time by PG pin

Part Number: TPS62140A
Other Parts Discussed in Thread: TPS62148, TPS62125

Hi team

Thanks for the usual support.

One of our customer would like to control the discharging time with using PG discharg function in datasheet.

It seems internal discharging FET has around 150ohm impeadance thru my simulation.

Is it possible to be advised discharging FET RDSon range min max for calcurating the time at the customer?

Thanks

Best regards,

Shidara 

  • Hi Shidara-san,

    You can obtain the PG FET resistance from the PG voltage level on page 6 of the D/S. 150 Ohm is the maximum value from this PG voltage parameter. However, with this IC you must limit the discharge current to 10 mA or less. This requires a series resistor with PG--the resistor will typically be much bigger than the PG FET Rdson.

    However, I recommend the smaller, more accurate TPS62148 which has a built-in stronger output discharge.
  • Hi Chris-san
    Thanks for your prompt advice.
    I could understand that internal On resistance is 150 Ohm(max) and 35 Ohm(Typ) from VOL_PG specification.
    Yes. We understand that we have to keep discharge current less than 10mA.
    Since the min specification is not specified, Assuming 0 ohm.
    Is the idea that the designer must be less than 10 mA with only external resistors is correct?
    Or is it possible to present the minimum resistance value of the design guarantee value?
    Thanks
    Best regards,
    Shidara
  • Hi Shidara-san,

    Since it is not specified, you need to assume the worst case of 0 Ohm Rdson.

    This circuit is detailed in section 8.2.2.3 of the TPS62125 D/S. That device has a different PG FET, but the concept is the same.
  • Hi Chris-san
    Thank you very much.
    I could understand.
    Do you have any idea to accerarate the discharge speed with low cost
    because I could calcurate that it is difficult to meet the requred time within
    this limitation?
    Conditions are
    Vout=3.3V, Tdischarge<35mS, Cout =200uF
    If I put external 330 Ohm, Tconst=66mS at best case of internal 0 Ohm.
    Thanks
    best regards,
    Shidara
  • The TPS62148 mentioned before has a stronger discharge circuit. This should be used.

    As well, what is your required discharge voltage level? What does the 3.3V have to get down to? For any RC discharge circuit, theory says that it will never get to 0.000V as it will asymptotically approach the final value. How low does it have to be to be considered discharged?

    What the effective capacitance? I think you have 200uF typical/nominal, but how much is there after DC bias? Can some of this be removed?
  • Hi Shidara-san,

    Were you able to find out more specifics about the requirements for discharge?
  • Hi Chris-san

    Thank you very much for the reply.

    I could simulate the same phenomenon with theory and inform it to the customer.

    The customer is thinking now then I'd like to close this talk because more information

    will be the customer's information. If I need to talk more, please allow me to send email directly

    to you.

    Thank you so much

    Best regards,

    Shidara