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DRV8412: Can the output pull down resistor dissipate excessive power when a inductive load discharges frequently with /reset being low?

Raymund Hofmann
Prodigy 20 points
Part Number: DRV8412

Lets assume power supply is 48v and a inductive load that takes 100e-6s to decay to zero current when /reset input goes low repeating every 250e-6s.

This means a power of 2.3W dissipated in one half bridge pulldown with a duty cycle of 40%, so in effect 1.8W for the device when the two bridges do this.

Is this asumption correct?

  • 0
    TI__Mastermind 23431 points

    Hey Raymund,

    1.84W seems correct to me, but I am not sure about what you're initially asking.  Do you mean for the sense resistor?

    Best,

    Jacob

  • 0 in reply to Jacob Thompson
    Prodigy 20 points

    No, I mean the half bridge "weak pulldown":

    DRV8412, DRV8432
    SLES242G –DECEMBER 2009–REVISED DECEMBER 2014

    RPD
    Internal pulldown resistor at the output of each half-
    bridge
    Connected when RESET_AB or RESET_CD is
    active to provide bootstrap capacitor charge
    1
    kΩ

    7.3.3
    Device Reset
    Two reset pins are provided for independent control of half-bridges A/B and C/D. When RESET_AB is asserted
    low, all four power-stage FETs in half-bridges A and B are forced into a high-impedance (Hi-Z) state. Likewise,
    asserting RESET_CD low forces all four power-stage FETs in half-bridges C and D into a high- impedance state.
    To accommodate bootstrap charging prior to switching start, asserting the reset inputs low enables weak
    pulldown of the half-bridge outputs.

  • 0 in reply to Raymund Hofmann
    TI__Mastermind 23431 points

    Ahh okay,

    If the device "Recommended External Components" do not mention needing a separate resistor in this location, then the device internally can handle the current specified in the datasheet for the normal operating conditions.  I don't think you need to add any extra resistance or worry about it.

    Best,

    Jacob

  • 0 in reply to Jacob Thompson
    Prodigy 20 points

    My question is not about needing an extra resistor, the intent is to avoid the excessive power dissipation I assume to be in this internal pull down resistors in the case I described.

    Nevermind, I assume they are just linear resistors, meaning worst case power dissipation of around 2W in my example and will make the circuit so it actively discharges the inductors by controlling the switches before setting /RESET low.