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TPS53355: Datasheet Overcurrent Threshold Calculation Confirmation Question

Darren (FAE)
Expert 5483 points
Part Number: TPS53355


There is a section on page 24 discussion the overcurrent detection threshold.

There is a statement:

Use an RDS(on) value of 1.5 mΩ for an overcurrent level of approximately 30 A. Use an RDS(on) value of 1.7 mΩ for overcurrent level of approximately 10 A.

Does just 0.2mΩ of resistance change, really result in a 3x overcurrent threshold change?

The equation (10) seems to indicate just a 3% change for this resistance change...

Could you confirm please?

Regards,
Darren

  • +1
    TI__Expert 5310 points

    Hi Darren,

    The RDS(on) value is an input into equation 10, which can be used to calculated the resistance needed to set the overcurrent threshold on the TRIP pin. The TPS53355 uses thermally compensated RDS(on) values for the low-side MOSFET, so the RDS(on) of the low side FET varies slightly as a function of current. If the customer wants to set a current limit between 10-20A, please use 1.7 mohm when calculating the resistance on the TRIP pin using equation 10. If the current limit setting is between 20A-30A, please use 1.5mohm as the RDS(on) value for equation 10.

    Looking at equation 10, the 0.2 mohm change in RDS(on) used in the calculation will not result in a 3x difference in the OCP threshold. The note regarding using different RDS(on) values for a desired current limit setting has to deal with the thermally compensated RDS(on) sensing the TPS53355 employs

    Regards,

    Alec Biesterfeld

  • 0
    TI__Genius 15187 points

    Hi Darren,

    The RDS(on) value changes as a function of current. 
    1.7mΩ should be used for RDS(on) in Equation 10 if the desired current is between 10-20A, and 1.5mΩ should be used for RDS(on) in Equation 10 if the current is between 20-30A.

    Best,
    Ryan

  • 0 in reply to Alec Biesterfeld
    TI__Expert 5483 points

    Hi Alec, Ryan,

    Thanks for the explanation. I understand now. Slight smile

    Quick follow-up.

    The Datasheet lists HPA01111DQPR as an orderable part number. Was this just a previous naming convention that was updated? There is no "HPA01111DQPR" stock floating around?

  • 0 in reply to Darren (FAE)
    TI__Expert 5310 points

    Hi Darren,

    I believe this was a previous naming convention. Looking at the orderable part numbers on ti.com, it appears that the TPS53355DQPR is the up to date orderable part number.

    Regards,

    Alec Biesterfeld