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Original question:

DRV8343-Q1: Typo in the datasheet?

DRV8343-Q1: Decouping capacitors for VM pin

John Pan
Intellectual 990 points
Part Number: DRV8343-Q1
Other Parts Discussed in Thread: DRV8343S-Q1EVM, DRV8353

Hi Cole,

I have a few questions on decouping capacitors for VM pin in DRV8343-Q1 data sheet.

In Figure 63 and Table 36, three decouping capacitors are recommended with C_VM1 = 0.1 uF, C_VM2 = 4.7 uF, and C_VM3 > 10 uF. However, DRV8343S-Q1EVM schematics doesn't have a C_VM3. I believe it is missing a 330 uF CAP next to TP6. Please confirm. If it is, does that mean that C_VM3 could be the same large capacitor for power filtering with L1?

Also in Layout section (page 80), C_VM2 is not mentioned. It seems that only a 0.1 uF bypass capacitor C_VM1 and a bulk capacitor that is > 10 uF are needed. My question is whether C_VM2 is really needed? Could the bulk capacitor (>10 uF) be the same power filtering capacitor or there must be a dedicated bulk capacitor for the VM pin in addition to another bulk capacitor for power filtering?

Thanks,

John

  • 0
    TI__Mastermind 18900 points

    Hey John,

    I'm sorry I can't give a more helpful answer but decoupling caps on VM and how many you need have been a hot topic for debate. So much so we added this section in some of our other datasheets (not the DRV8343Q1, this one comes from the DRV8353).

    So, you'll see the purpose is to provide local decoupling or current to the part. We at least recommend a bulk cap in the >10uF range and a smaller bypass in the ~100nF range. I would agree these are the minimum you need for a good rule of thumb and I would think this was an oversight on the EVM.

    But that's where the recommendations stop, no equation or guidance for how much capacitance for how much ripple, current, or whatever. As such, I suggest you put a footprint for C_VM2, test your system, and populate it if you think there's too much supply pumping or ripple on the supply as a result of testing. You might be able to change out the 10uF with the 4.7uF and be fine, it all comes down to testing.

    To be thorough, L1 and the rest of caps for that circuit are actually PI filtering that are used for batteries coming directly to the EVM in automotive applications so we won't count the caps in that circuit for the ones dedicated to VM. We can think of that are the left hand side of the figure above as "power supply output capacitance" instead of "local capacitance dedicated to the VM pin of the DRVx". This includes the 300uF electrolytic caps that are meant to provide charge to the power stage so they won't count towards VM decoupling caps either. 

    Let me know if this makes any sense.

    Best,

    -Cole

  • 0 in reply to Cole Macias
    Intellectual 990 points

    Hi Cole,

    This is very helpful.

    Thanks,

    John