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TPS548A20: IOCP Threshold Calculation

Eric Fu51
Prodigy 90 points
Part Number: TPS548A20

I would like some clarity on the following items:

1) Tolerance on the I.TRIP: What is it? The datasheet only say 10uA typical. Variance on this value will affect the accuracy of the overcurrent protection threshold.

2) Low-side MOSFET on-resistance (R.DS(on)L): What is the actual max value? 4.94mohms (Pulled from Electrical Characteristics Table), 5.7mohms (Pulled from Current Sense and Overcurrent Protection), or neither? Do you have a plot of the Rdson (for the high- and low-side FETs) over temperature?

3) Discrepancy between the IOCP threshold in the Electrical Characteristics Table and Equation provided in the Current Sense and Overcurrent Protection: The equation and the characteristic tables do not align (see below for math).

From Datasheet:

Using Equation 3:

V.TRIP = R.TRIP * I.TRIP = (49kohms) * (10uA) = 490mV

Modifying Equation 4 to just represent the valley level of the inductor current:

I.OCL (w/ R.DS(on)L = 5.7mohms) = V.TRIP/(8*R.DS(on)L) = 490mV/(8*5.7mohms) = 10.74A 

I.OCL (w/ R.DS(on)L = 4.94mohms) = V.TRIP/(8*R.DS(on)L) = 490mV/(8*5.7mohms) = 12.39A

Both of these R.DS(on)L values do not match the 11.5A I.OCP threshold stated in the electrical characteristic table. For the table to align with the equation, the R.DS(on)L must equal to ~5.33mohms.

I.OCL (w/ R.DS(on)L = 5.7mohms) = V.TRIP/(8*R.DS(on)L) = 490mV/(8*5.33mohms) = 11.4916A 

Applying the same logic for the typical condition, the typical R.DS(on)L must equal to ~4.07mohms.

I.OCL (w/ R.DS(on)L = 4.07mohms) = V.TRIP/(8*R.DS(on)L) = 490mV/(8*4.07mohms) = 15.0491A

Any help will be appreciated!

 

  • 0
    TI__Genius 16720 points

    The electrical characterization table minimum and maximum values are set by using 3 or 6 sigma limits from testing >=30 devices over temperature.

    Equations matching on the min or max values in electrical characterization table most likely will not match.  For the current limit specification, the average value of the 30 measurements typical at 25C and the  max/min values are the +/-3 sigma. 

    The low side mosfet resistance over temperature in below table. 

    Rdsls (mOhm)        
    -40C 0 25 85 125
    3.76 4.09 4.33 4.97 5.55