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DRV8701: Operating DRV8701 and H-Bridge on Separate Supplies

Holger G.
Prodigy 30 points
Part Number: DRV8701

Hey there,

i would like to use the DRV8701 for a commercial project. I have a question regarding the datasheet chapter '7.4.1 Operating DRV8701 and H-Bridge on Separate Supplies - Figure 33. VM Operating Range Based on Motor Supply Voltage' (Page 27).

In my application I may have have the situation that the battery supply (vbat) drops below 10V but the DRV8701 supply (VM) will remain at 24V. ( I know that i have to connect the IDRIVE Pin to AVDD to disable the high side OCP for situations when VM is >> VBAT).

But this situation described above (VBAT < 10V, VM = 24V) would still be outside the "allowable area" in your figures. My question is: what will happen in that specific case? Will it damage the DRV8701 or will it simply disable the high side?

In my application it won't be necessary that the high side FETs work in this situation. Its only a turn off situation and only the low side FETs should work to keep the connected coil shortet to ground to avoid self induction.

Thanks a lot in advance.

  • 0
    TI__Expert 4435 points

    Hello Holger,
    Vbat <10V falls under Case 3 for the most part. Can you please confirm what is the lowest voltage you expect Vbat to drop?

  • 0 in reply to Luis Riveros
    Prodigy 30 points
    Hello Luis,
    thanks a lot for your fast response. Well, the problem - as I understand it - is not only that Vbat drops below 10V, but that VM remains at 24V at that moment. In figgure 33 on page 27 a Vbat < 10V(14V) is ONLY allowed if VM falls below 14V as well. So if VM would fall below 14V as well (even if it still would be higher than Vbat) it would not be a problem. But I understood from the both figgures that thats not only a case 3 szenario but its additionally outside the allowed area for VM vs. Vbat in that case or any other case. Its just outside the 'allowed area' but I dont know what 'not allowed' means ... no High Side FETs oder danger to the device ?

    The szenario I am talking/thinking about is the power off szenario. In that case our coil (not a motor!) must be grounded on both sides to prevent the coil from self induction if the magnetic field is still active (otherwise it would become an ignition coil -> DANGER). We do that simply by keeping ENABLE low. So while the voltage in the whole system drops when I switch it off, VM would fall below 5.9V and the undervoltage lockout would open the coil connection to ground on both sides of the coil. thats a critical situation. therefore i was thinking about a backup capacitor + diode (to Vbat) that backups VM until the magnetic field in the coil collapses and the low side FETs can open without danger . But in that szenario Vbat could become nearly 0V and VM could still remain 24V (from the backup capacitor).

    So my question is ... what happens outside the "allowed area" in figgure 33 for a case 3 szenario.

    Will it damage the DRV8701 or will it simply stop the high side FETs from working ? (I only need the low side FETs to protect the system and keep the coil grounded to prevent it from self induction). Sorry for my bad english. Please let me know if I have to explain in better words.
  • 0 in reply to Holger G.
    TI__Expert 4435 points

    Hi Holger,

    If VM remains at 24V then this would be a case 3 scenario. I can see how the figure creates confusion stating "allowable" and showing VM only up to 14 V when in reality VM can go up to 45V as per the Table 9, case 3. I have noted this and we will update the datasheet to better explain this part, thank you for your input.

    For the question when VM falls below 5.9V, and low side FETs driving, yes you can add a backup capacitor or just place a bigger bulk capacitor for VM + the diode. It would not damage the DRV8701 device. I hope this helps, thank you.

  • 0 in reply to Luis Riveros
    Prodigy 30 points
    Thank you for your help Luis. I very much look forward to use this driver IC in our project.