• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 30 subscribers
  • Views 1340 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DRV8838 Vm pin characteristics

Matthew Chapman
Prodigy 40 points

Good day,

Included here for convenience is my schematic and the functional block diagram from the DRV883x data sheet.  Battery voltage (V_BAT) is 7.2v.  I am driving a solenoid R-L-C circuit.  The inductor and resistor values are stated in the solenoid's data sheet as 90mH and 6 ohm respectively.  It is also stated to require a 4700uF capacitor in series with the solenoid.  I assume because the reactive values cancel out.  Expected current is to peak at 1.2A.  The result is that the circuit is under damped at approximately 0.69.    

Looking at the function block diagram it appears that the output is not isolated from the input.  Does this not mean that the current that flows through the output pins must come from and return to my battery source?  

I ask this because it states in the DRV883x data sheet that the operating current I_vm = 0.1mA max at V_m = 5v; Vcc = 3v with No PWM.  

http://www.ti.com/lit/ds/symlink/drv8838.pdf

Also, if I may ask here.  Would it not be beneficial for me to calculate a value of capacitance that is closer to critical damp?  At 0.69 damp I expect the transient waveform to dip below reference causing a reverse current.  

  • 0
    TI__Guru** 114905 points

    Hi Matthew,

    Matthew Chapman said:
    Included here for convenience is my schematic and the functional block diagram from the DRV883x data sheet.  Battery voltage (V_BAT) is 7.2v.  I am driving a solenoid R-L-C circuit.  The inductor and resistor values are stated in the solenoid's data sheet as 90mH and 6 ohm respectively.  It is also stated to require a 4700uF capacitor in series with the solenoid.  I assume because the reactive values cancel out.  Expected current is to peak at 1.2A.  The result is that the circuit is under damped at approximately 0.69.  

    Would you please confirm the location of the 4700uF capacitor? In series does not appear to be correct. Is it possible it is intended to provide the current pulse and should be placed in parallel with the 4.7uF cap shown on your schematic?

    Matthew Chapman said:

    Looking at the function block diagram it appears that the output is not isolated from the input.  Does this not mean that the current that flows through the output pins must come from and return to my battery source?  

    I ask this because it states in the DRV883x data sheet that the operating current I_vm = 0.1mA max at V_m = 5v; Vcc = 3v with No PWM.  

    The current flows from the battery to VM to the solenoid and back to GND as you stated. The current stated in the datasheet is without a load on the outputs. It provides a value of what amount of power is used by the DRV883x. Customers can use this value to determine the overall power; sleep, awake but idle, and awake driving.

     

  • 0 in reply to Rick Duncan
    Prodigy 40 points
    You are right. The solenoid requires a 20-100ms pulse and the capacitor would reach steady state in less time.