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DRV8801: VPROPI Output current

SheldonTT
Prodigy 225 points
Part Number: DRV8801

I am using the DRV8801 with a total of 50K load resistance on the VPROPI output pin, with the resistance the voltage is reading 1.31V ( this is not 5X the current) if i remove the resistance I am getting 2.0V ( this is the 5X coil current). It seems that the pin has an output impedance I am exceeding but I can not find anything in the datasheet. What is the max load that this pin can drive accurately?

Thank you,

Sheldon

(8801_VPROPI_OUT is connected to an ADC) 

  • 0
    TI__Guru** 114905 points

    Hi Sheldon,

    We will investigate and reply soon.

  • 0 in reply to Rick Duncan
    Prodigy 225 points

    Update: Increasing the output resistance to ground to 150K still does not solve the problem. Is there a resistance in series or a voltage divider circuit inside the part?

     

  • 0 in reply to Rick Duncan
    Prodigy 225 points

    Anything from your side? 

  • 0 in reply to SheldonTT
    TI__Guru** 114905 points
    Hi Sheldon,

    The VPROPI signal should work properly with the resistors shown.

    Is the signal divided and then read by an ADC? Is there some resistance inside the receiving device that is changing the divider ratio?

    Can you try changing R319 to 100 Ohm and R318 to 49.9k?
  • 0 in reply to Rick Duncan
    Prodigy 225 points
    Hello Rick,

    I swapped the resistors as you said. Voltages measured (using an oscilloscope) are as follows:
    vsense = 0.1V
    VPROPI = 0.348V ( should be 0.5V)
    8801_VpropI_out = 0.348V (should be 0.5V)

    I then put 50K resistor at R319 and removed R318
    vsense = 0.1V
    Vpropi = 0.500V
    8801_vpropi_out = 0.500V

    I do not believe my ADC is loading the circuit.

    I did notice the sample and hold portion of my circuit would load the circuit periodically so i added a 0.1uF cap in parrallel with R318, got the same results without the periodic loading.

    The total input resistance of of my ADC is 200 ohm.

    I am doing the divider circuit because my ADC max input voltage is 1.0V.
  • 0 in reply to SheldonTT
    TI__Guru** 114905 points
    Hi Sheldon,

    The output of VPROPI is not a standard X5 op amp. It is expected to drive a high impedance input and no more than approximately 110uA.

    Think of the VPROPI output as a current mirror driving an internal load. At the SENSE pin, the voltage is converted into current using a typical 4.6k resistor. This current then drives an internal load resistor of typically 23k (5X) to create the VPROPI voltage. Adding external resistance to ground creates a parallel resistance path and affects the gain of VPROPI.

    To avoid or minimize the effects on the output, there are several options:
    1) Add an external amplifer to reduce the voltage into your ADC.
    2) Use the voltage on the SENSE pin as the input to your ADC. The full scale voltage at the ADC would be reduced.
    3) Change the resistor divider values. Using 3M/2M (or 300k/200k depending on accuracy desired) should reduce the effect of the external load. This could create issues with system noise.
    4) Recognize this parallel resistance in the calculation and adjust according. For you, an external load of 15.33k to ground on VPROPI should work.
    The gain can be modified by the equation G = (23k||Rext)/4.6k.
  • 0 in reply to Rick Duncan
    Prodigy 225 points

    Thank you!

    That information would be very helpful in the datasheet.