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DRV8312: DRV8312, DRV8332 PWM frequency

Part Number: DRV8312
Other Parts Discussed in Thread: DRV8332,

Hi 

According to the datasheet of DRV8312 and DRV8332, it is mentioned that PWM frequency can accept up to 500kHz.

I want to ask about this spec.

  -Is there a any constraint about PWM minimum on duty time? 

  -According to the datasheet, it is mentioned that dead time is  5ns.

   Does this mean that deadtime is okay if the input PWM is up to 500kHz?

Best regards

Naoki

  • Hello Naoki,

    On the surface, the device was designed and tested to handle 500kHz PWM, so there's no concerns. With that being said, yes, the dead time (5.5ns) and the Minimum PWM pulse duration, low side, for charging the Bootstrap capacitor (50ns). With a budget of 2000ns for 500kHz, you can work the percentages but 50ns means max of 97.5% duty cycle and <1% impact on the minimum duty cycle for the dead time. You'll have to be the judge if it is worth dropping down the frequency so you can make the percentage larger. As a result, I highly recommend trying it in a real system and having a plan to tweak it if you're not reaching a specification you planned on (e.g. lower PWM frequency, to increase duty cycle and potential max speed).  

    Edit:

    One thing I forgot, the device is not intelligent when it comes to the duty cycle. It will actually try to output anything that is applied to the input. You can see this by figure 5. My recommendations try to take into account the duty cycles needed to actually make the device work without any problem controlling the motor.

    Best,

    -Cole

  • Hi 

    Thank you. I can clearly understood this device spec.

    You mentioned that minimum pulse duration is 50ns due to Bootstrap capacitor spec.

    If the PWM Duration of the input is less than 50ns, does it mean any concern about device damage?

    Or the device can set deadtime automatically?

    Best regards

    Naoki

  • Hello Naoki,

    No, the less than 50ns PWM pulse width on the !PWM_RESETx will not damage the device, it will cause a failure to turn on the external high side power FET (maybe cause a boot strap cap UVLO).

    The bootstrap capacitor spec is related to generating the voltage required to turn on the high side FET and is fundamental to understand how a bootstrap circuit operates. If you take a look at the "Bootstrap Circuitry Selection for Half-Bridge Configurations" app note, the first section explains how an external low side power FET must be on to charge C_BOOT. Just be sure to skip the sections about selecting a different diode because that advice is not applicable for the DRV8312 and DRV8332: https://www.ti.com/lit/an/slua887/slua887.pdf 

    Essentially the spec accounts for the turn on time for our logic to react, turn on the internal diode, and start charging the capacitor (100nC typical).  

    Best,

    -Cole