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DRV8256: PWM Drive chip heating but no Temperature fault

Cristian Espinoza
Prodigy 20 points
Part Number: DRV8256

HI TI,

My question is regarding the chips Thermal Fault output. When driving the chip with PWM, in any direction, the chip is getting hot and not latching the thermal fault. The output driving current seems to degrade as the chip temperature increases. When the PWM duty cycle decreased the chip quickly cools down.  How should the response of the fault output be at the different error states?

Thanks,

Cristian Espinoza

  • 0
    TI__Mastermind 19860 points

    Hi Christian,

    The fault responses are described in section 7.3.7 Protection Circuits of the datasheet. 

    Are you taking thermal images of the chip to confirm how hot the device is getting?

    The H-Bridges and charge pump should disable with a latched recovery for OTSD. Are you still seeing outputs when the temperature threshold is exceeded?

    Best,

    David

  • 0 in reply to David Medis
    Prodigy 20 points

    HI David,

    We are using an Optical Thermometer (OMEGA OS540). The top surface measurement gets to ~50C when driving a 2ohm Load. Input voltage is at 12V. The output efficiency seems to drop when there is a rise at the chip. It doesn't seem to reach the threshold limit to latch the over temp state.

    Thanks

    Cristian Espinoza

  • 0 in reply to Cristian Espinoza
    TI__Mastermind 19860 points

    Hi Cristian,

    This operation seems to be as expected. With a 12V motor voltage and a 2Ohm load the motor current should be ~6A. A higher input PWM duty cycle will cause more switching losses and cause the increase in temperature. 

     

    See the following app note to learn more about Calculating Power Dissipation for a H-Bridge or Half Bridge Driver. 

    Best,

    David

  • 0 in reply to David Medis
    Prodigy 20 points

    Hi David,

    The chip is increasing in temperature but in our application we were unable to force it into the over temperature state. Do we have a test case to understand the OTSD? I am assuming this is not adjustable and is set by the internal design of the chip.

    Thanks,

    Cristian Espinoza

  • 0 in reply to Cristian Espinoza
    TI__Mastermind 19860 points

    Hi Cristian,

    The Overtemperature Thermal limit is an internal threshold and can't be adjusted. If you are trying to test the temperature limitations by tripping the thermal limit it is recommended to run the device with high RMS current over some duration to heat up the device. 

    Please see the following application report on Understanding Motor Driver Current Ratings for more details on current and temperature relationships. 

    "The θJA number for the chosen package gives an estimate of how much temperature rise is expected if the device was mounted on a standard JEDEC PCB. Other data is used to provide an estimate of the thermal resistance on the PCB."

    The thermal information table in the datasheet will allow calculations to be made to estimate the die rise in temperature for certain power dissipation conditions. 

    A more detailed explanation of power dissipation calculations can be found here: Calculating Power Dissipation for a H-Bridge or Half Bridge Driver (Rev. A) 

    Best,

    David