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TPS40170: ILIM

Kaitlin McCarthy
Prodigy 10 points
Part Number: TPS40170
Other Parts Discussed in Thread: LM25145, LM5146, LM5145

TPS40170 Inquiry

1. What is the relationship between Temperature Temperature coefficient of ILIM current (Tc =1400ppm) and ILIM performance over temperature?

In section 8.2.2.14 ILIM Resistor (R9, C17) of the datasheet, it is recommended to allow for 30% over the minimum current limit for transient recovery and 20% rise in RDS(on)Q2 for self-heating of the MOSFET. In the recommendation text, it states 20%. In the equation is 25%.

2. Is the recommended rise in RDS(on)Q2 for self-heating of the MOSFET 20% or 25%?

3. Why are these factors 30% and 20/25%?

  • 0
    TI__Guru 74052 points

    Hi Kaithlin,

    MOSFET Rdson current sensing isn't super accurate over temperature as each MOSFET may have a slightly different tempco (TC), depending on the ratio of copper and silicon in the FET. In general, silicon has a lower TC than copper, so the net TC of the Rdson is somewhere in the range of 3000-4000ppm/degC.

    The controller ILIM current TC is different as the controller is not perfectly thermally coupled to the FET.

    Note also that some margin is required for load transients as the inductor current will over during load-on transients. I would go with at least a 30% margin for that.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 10 points

    Thank you for this information. That answered those questions I had. 

    I have another question about TPS40170. In section 7.3.4 of the datasheet, the OCP and SCP calculations are explained. I would like to measure the voltage that is compared to the ilim pin voltage for the OCP.

    The lowside FET voltage at the switch-node (SW pin) is explained to be the product of inductor current and low-side FET turn-on resistance RDS(on).

    How do you suggest we accurately measure the lowside FET voltage at the switch-node (SW pin) when the LDRV is ON after a blanking time?

  • 0 in reply to Kaitlin McCarthy
    TI__Guru 74052 points

    Hi Kaitlin,

    It's better to measure that at lower Vin - the probe then doesn't have to deal with a large voltage swing on SW. The voltage drop is the MOSFET Rdson times the instantaneous inductor current (avg value being equivalent to Iout).

    Btw, if this is a new design, I recommend using the LM25145 (42V), LM5145 (75V), LM5146 (100V) controller family. These parts have the same control loop architecture as the TPS part but provide higher performance at lower cost.

    Regards,

    Tim