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DRV110 external drive of Rhold pin?

JONATHAN GRAHAM98068
Prodigy 90 points
Other Parts Discussed in Thread: DRV110

We're trying to tune our circuit to have tighter control over the DRV110's hold current cor this solenoid:

http://acroassociates.com/wp-content/uploads/Model_958_Product_Sheet.pdf

Normally the hold current isn't that critical, but for this solenoid, the manufacturer has basically said that the pull-in current is not safe for continued application and a very specific reduced current is required.  I've had many conversations with them that go beyond the datasheet, so suffice to say that it would be in my best interest to control the current to +/-5%.

Can I directly drive the "Rhold" pin with a DAC output?  We are currently using http://cds.linear.com/docs/en/datasheet/2632fa.pdf (sorry, I didn't pick it), and we have an RMS current measurement of the free-wheeling solenoid current available to the microcontroler, so in theory I can develop a software closed loop control over the hold current.  ...but the block diagram shows a reference block connected to the Rhold pin, will I risk damaging the DRV110 if I drive it directly from a DAC?

Thanks!

  • 0
    TI__Genius 9975 points

    Hello  JONATHAN GRAHAM98068,

    I will ask the person responsible for support of this product to reply to your inquiry.

    Best regards,

    Jose

  • 0 in reply to Jose Duenas
    TI__Intellectual 1915 points

    Hi, I will check whether this is possible and let you know.

    Best regards, Alain.

  • 0 in reply to AlainC
    Prodigy 40 points

    Hi Alain,

    I am one of Jonathan's associates at a different location.  We have determined that the HOLD pin behaves as if it is a summing junction or feedback node.  The voltage at this node appears to be held at 1.00 volts (observed on several devices so far).

    We are able to develop a current that accurately reflects the PWM hold current which we desire.  When this current is applied to the HOLD pin, the approach works well.  The HOLD pin remains at 1.00 volts over a good range of control current.

    The question which we would like to present to the engineers is wheter or not we can treat this pin as a feedback node (i.e. part of an on-chip control circuit) and depend on it being held to 1.00 volts as we vary the current fed externally to it (within carefully selected ranges).

    Thanks,

    Dennis

  • 0 in reply to DennisF2014
    TI__Intellectual 1915 points

    Hi Dennis, sorry for my late reply: the design database has been archived at our side and I am asking to restore it. This will take some more days to get it available.

    As far as I can tell this node is not a feedback node but more likely a current source with some clamp. So it looks ok to drive it with an external DAC. I will give you confirmation once I have access to schematic.

    Best regards, Alain.  

  • 0 in reply to AlainC
    Prodigy 40 points

    No problem.


    Sounds good so far.  If there are specified limits for the voltage developed at this node (or the current that produces it) that would be helpful to know also.  The developed voltage has been very close to 1.00 volts across a small sample of devices, although all are from the same batch.


    Dennis

  • 0 in reply to DennisF2014
    TI__Intellectual 1915 points

    Hi Dennis, after checking design schematic actually I would not recommend driving this pin using an external voltage as internally it is a current source and voltage is tighlty controlled to 1V (as you have already noticed). Doing so may cause some instability. Instead you may use a current DAC or a resistor DAC to program this node. The current is equivalent to1V divided by the resistor needed to set the Ihold current.

    Best regards, Alain.  

  • 0 in reply to AlainC
    Prodigy 40 points

    We have a voltage DAC along with a series resistance (high enough to act as a current source)  which, together with a fixed resistance to GND from the HOLD pin, gives us an adjustment range we need, using the datasheet curve for Rhold as a guide.


    To re-state this, the equivalent range that we get is similar to the datasheet range (over Rhold values).  We avoid equivalent resistance that would trip the internal defaults of the chip (i.e. we do not go below 67k equivalent).

    Dennis

  • 0 in reply to DennisF2014
    TI__Intellectual 1915 points

    As long as there is a resistor is serial with the votage DAC that would work. The equivalent current to program is 1V/Rhold so in this case it will be (1V-Vdac)/Rdac. This should work fine as long as Rdac is not too small.

    Best regards, Alain.

  • 0 in reply to AlainC
    Prodigy 40 points

    Thanks Alain.  I believe we are all set.

    Dennis