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DRV8432 Parallel Full Bridge operation - PWM_A/PWM_B relationship

Anup Rajput
Expert 1920 points
Other Parts Discussed in Thread: DRV8301, DRV8302, DRV8711

Hi,

We are designing a DC motor drive (brushed, PWM controlled, 0.5hp, 19.5A peak). We have evaluated the DRV8432 as a perfect candidate for something a little less powerful than this particular motor (since DRV8432 has 14A max current), using the very useful DRV8432EVM.

We are using it in Parallel Full Bridge (PFB) mode. We see that even at a PWM frequency of 50% (generated and confirmed as correct, using an MSP430), the IC tends to reach it's over temperature warning (OTW) in about 7 minutes of constant operation (despite just 10A drawn, and 35 degrees ambient temperature), on the DRV8432EVM. Also, after about 15 minutes of this constant operation, the FAULT signal comes up.

1. While we try to debug this problem, is it acceptable to use power MOSFETs after the DRV8432 output pins in order to reduce the current load on the DRV? Of course, we would take care of the MOSFET specifications as per our load requirements. In this case, we could make do with a lower wattage DRV instead of the DRV8432.

2. What is the relationship between the PWM_A and PWM_B w.r.t. OUT_A and OUT_C in PFB mode? A diagram depicting the timing/signal relationship would be great. Any details on the braking, coasting, direction, etc would be most beneficial, as the same are missing from the device datasheet. We are OK with referring to another device's datasheet in case you confirm that the functionality/behaviour would be the same (except for the different current capacity, etc).

3. Are there any application notes using the integrated DRVs as pre-drivers?

4. How about using current and RPM sensing with these DRVs? We understand the principles, but any additional pointers would be helpful!

Thanks,

Anup

  • 0
    TI__Mastermind 21320 points

    Hi Anup,

    It sounds like you are looking for a DC motor pre-driver. While we do not have a dedicated single DC pre-driver we do have several other options.

    DRV8301 - BLDC Pre-driver with SPI comm and onboard buck converter. Can also be used to drive single DC motor.

    DRV8302 - BLDC Pre-driver with onboard buck converter. Can also be used to drive single DC motor.

    DRV8711 - Stepper/Dual DC Motor Pre-driver

    If you still wish to use the DRV8432 we can try and answer your questions. But no it cannot be used as a pre-driver for an NMOS power halfbridge since it cannot properly bias the gate voltage.

  • 0 in reply to Nicholas Oborny
    Expert 1920 points

    Hi Nicholas,

    Thanks a lot for your detailed response.

    We are proceeding on two tracks now. We will consider the pre-drivers that you have recommended and build up a board with independent FETs.

    But, we will still use DRV8432 on the other track, in parallel! We went through a promising app note on parallelizing the 8801 IC for driving a larger load, while still being able to use the internal features like OCP, UVO, etc. Could you please answer our other questions on the DRV8432 then? (#2 and #4).

    Thanks & Regards,

    Anup

  • 0
    TI__Mastermind 21320 points

    Anup.

    2. In Parallel Full Bridge mode it should be a one to one relationsship from input to output. If PWM_A goes high OUT_A and OUT_B will high. If PWM_B goes low OUT_C and OUT_D will go low.

    With this setup there will be 4 states.

    PWM_A = 0 + PWM_B = 0 = Brake

    PWM_A = 1 + PWM_B = 0 = Forward 

    PWM_A = 0 + PWM_B = 1 = Reverse 

    PWM_A = 1 + PWM_B = 1 = Brake 

    You can Hi-Z the outputs through the use of the RESET pins in order to enable a coast mode.

     

    4. Current sensing can be performed with a low side or high side sense resistor on each dual bridge. You can see an example of this in the datasheet (high side sensing). You would feed each end of the sense resistor to a differential amplifier the could then feed to an ADC. You can find a more detailed example of this on the DRV8301 EVM.

    There are several ways to go about RPM sensing including opto or hall sensors. Also, you can use the average current and voltage level and with a torque constant attempt to determine an RPM.

  • 0 in reply to Nicholas Oborny
    Expert 1920 points

    Nicholas,

    Thanks a lot for helping me understand the modes. I did see this set of stated for other DRVs, but needed confirmation that the DRV8432 doesn't have something different.

    I will mark this answer as verified, and will start a fresh thread for my other questions.