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DRV8840: DRV8840 datasheet data clarification

DU00000001
Prodigy 150 points
Part Number: DRV8840
Other Parts Discussed in Thread: LM317

Hello,

the datasheet (SLVSAB7D, p. 11) gives a pulldown resistance of "approximately 100 kΩ" for the I[4..0] inputs.

I've connected 2 of these inputs in parallel, pulling them via 1 kΩ to my supply of 5.1 V.

Can you imagine my surprise when measuring the voltage on the DRV8840 side of the resistor: 4.3 V ???

A bit of calculation gives a pulldown resistance of about 10 kΩ, which is by no way "approximately 100 kΩ" !

Am I correct that the datasheet needs some "rework" ?

Regards

     ...

  • 0
    TI__Guru 56650 points

    Du,

    The I[0,4] pulldown resistor should be 100kohm.

    1. What is the I[x] pin voltage if you only connect one I[x] pin from your pullup source?

    2. When you measure 4.3V on DRV8840 side, what is the voltage on the other side of 1kohm resistor?

  • 0 in reply to Wang5577
    Prodigy 150 points

    We have 2 PCBs built completely independently (one in Europe, one in the US).

    It's just the same layout and - by chance - the same PCB manufacturer (but from different orders).

    Got my DRV8840s from Mouser.

    All parts have been sourced independently as well. Both boards show the same phenomenon.

    Voltage on the other side is 5.1xx V (5.117 V on my board). And the supply is pristine and  "next-to-perfect" - emloying an LM317  :)

    Cutting one of the traces could be difficult - I'll have to see to that. Exchanging the resistor might be easier...

  • 0 in reply to DU00000001
    TI__Guru 56650 points

    Du,

    Yes. Please change 1kohm series resistor to 10kohm and measure the voltage across the that 10kohm resistor.

  • 0 in reply to Wang5577
    Prodigy 150 points

    OK - with the 10 kOhm resistor the voltage further dropped to 4.002 V. Supply still 5.119 V

    (Remember: With the 1 kOhm I saw about 4.3 V. And I have 2 inputs (I0 and I4) in parallel following the resistor)

    What kind of inputs are these? Obviously something non-linear.


     Regards

    U. Dreher

  • 0 in reply to DU00000001
    TI__Guru 56650 points

    Dreher,

    Do you test on an EVM or test it on your own board?

    1kohm with 0.8V voltage drop, that is 800uA sinking current. It is too much for DRV8840 logic input pin.

     With 10kohm, the voltage drops more and the sinking current drop to 100uA. To me, it seems another voltage source is attached to this network.  

  • 0 in reply to Wang5577
    Prodigy 150 points

    Hi,

    own PCB. But beyond the I[4:0] inputs  and GND (28) there's nothing in the vicinity.

    Have an eye on the board - it's R38 (right of the DRV8840). No vias and no nets in the vicinity other than described.

    Seems I can't upload, so have a look here:

    - leftmost of the 3 images.

  • 0 in reply to DU00000001
    Prodigy 150 points

    Edit of the previous post currently doesn't work.

    Direct link to the image:  cdn3.f-cdn.com/.../BypassValveController-1_(blurred).jpg

  • 0 in reply to DU00000001
    TI__Guru 56650 points

    U. Dreher,

    DRV8840 has V3P3OUT 3.3V voltage source output. Can you switch the pull-up source from 5.1V to 3.3V to see any change?

    V3P3OUT

  • 0 in reply to Wang5577
    Prodigy 150 points

    Referring to V3P3OUT will be "beyond tricky" considering the resistor size (0603). Certainly not today (already 11 pm here).

    Could it be you have the same suspicion as I: either a zener diode (with an "awful" characteristic line) or some protection diode(s) trying to limit the input to (probably) 3V3?

  • 0 in reply to DU00000001
    TI__Guru 56650 points

    I mean changing Ix pullup source from LM317 output to DRV8840's V3P3OUT. We don't need changing resistor size.

    If nothing is between pullup source and DRV8840's Ix pin (No vias and no nets in the vicinity other than describedCould be the internal), and the test result shows "another voltage source is attached to this network", we have to check if a limitation inside DRV8840. Let us see your test result tomorrow.

  • 0 in reply to Wang5577
    Prodigy 150 points
    It's not about changing the resistor size - it's about either soldering the resistor upright, soldering a wire to the upper end, attaching the wire somehow to V3P3OUT. On the 0603, the solder flows at both ends at about the same time :( Interrupting the trace might become just as "funny": soldering a wire to a trace is not funny. I'll see what I can do...
  • 0 in reply to Wang5577
    Prodigy 150 points

    Just ruined an otherwise good board to implement your "idea": traces coming off etc.  :(

    Anyway - I got the circuit "glued" together: roundabout 2.7 V on the I0/I4 side with 3.29 V at the V3P3OUT pin.

    According to this, the datasheet has some "holes" wrt the I[0:4]  :(

    (By some luck, I have another 1k pullup at the nSLEEP input: this one has the nominal 100 k pulldown. No protection circuit draining additional current.)

    Would be nice if you could answer on the "private" channel: it's not necessary to discuss all this in ful public.


    Regards

  • 0 in reply to DU00000001
    TI__Guru 56650 points

    1. For I[4:0] inputs, it could have an ESD diode between the pin and the V3P3OUT which causes a high pullup source current if the pullup source voltage is much higher than 3.3V. That is why I asked changing pullup source to 3.3V to verify it.

    2. Which Ix pin do you pullup? I0 and I4 have other functions which could be related other internal circuit. At IC sleep mode or VM is low, the pullup source current could be higher.

    Let us do a private conversion from now for this issue.

    Regards,

    Wang Li

  • 0 in reply to Wang5577
    Prodigy 150 points

    Currently, my issues are not yet resolved.

    So I suggest to set this thread on hold ...

  • 0 in reply to DU00000001
    TI__Guru 56650 points

    I [4:0] pin setting, pulldown resistor and quiescent current were answered in this post or private message.

    For current regulation issue in private message, it is a debugging work on each pin voltage measurement, schematic and layer review... which is not related to this I/O pin resistance thread. Let me close this one. You can open a new post to discuss the current regulation debugging.

  • 0 in reply to Wang5577
    Prodigy 150 points

    OK - while it did not yet resolve all my issues, let me summarize:

    1. The pulldown resistor specification is correct.
    2. Control inputs I[4:0] are capable of input voltages as specifed.
    3. Control inputs I[4:0] have a protection diode to V3P3OUT.
    4. Due to the protection diodes, the input current specification is violated for input voltages significantly larger than V3P3OUT (but still within the specified range).