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TPS746-Q1: Rdson of PMOS transistor when operating in dropout

Jessica Riley
Prodigy 200 points
Part Number: TPS746-Q1

The datasheet notes that the device into-to-output resistance is the Rdson of the PMOS device when operating in the linear region.  What is that resistance?  I would like to calculate the power losses while operating in this mode, but I do not see that information in the Electrical Characteristics section.

Thank you,

Jessica Riley

  • +1
    TI__Mastermind 32780 points

    Hi Jessica,

    The power loss of the LDO is simply:

    Pd = (Vin - Vout) * (Iout + Ignd)

    See figure 6-8 in the datasheet for Ignd vs Iout.  In most cases Iout >> Ignd and this can be well approximated by (Vin - Vout) * Iout.  If you are concerned with power loss of the LDO it is better to use this analysis than try to evaluate it with Rds(on).

    If you are still interested in Rds(on) you can use the datasheet to approximate it for your use case.  You can use the dropout voltage curves (figures 6-4 through 6-6) to approximate Rds(on).  Vdropout = Rds(on) * Iout.  As with all MOSFET's, Rds(on) is a function of Vgs and temperature.  For low Vin and Vout, Rds(on) can be higher than with high Vin and Vout. 

    Thanks,

    Stephen