• State TI Thinks Resolved
  • Locked Locked
  • Replies 8 replies
  • Answers 1 answer
  • Subscribers 31 subscribers
  • Views 1064 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.

Original question:

DRV120: PWM Frequency setting not working

DRV120: PWM Frequency setting not working

gotmilk
Prodigy 120 points
Part Number: DRV120

HI  the Relay  being driven is generating hum.

the problem is similar to this thread. https://e2e.ti.com/support/applications/motor_drivers/f/38/t/641857?tisearch=e2e-sitesearch&keymatch=drv120 

The data sheets states a value for OSC is 160K. Can a smaller value  be used?  60 to 100K?

thanks

  • 0
    TI__Expert 6330 points
    Hi,

    I have notified our expert of your issue. Please expect a response by Monday May, 21st.

    Regards,
    Aaron
  • 0
    TI__Intellectual 1965 points
    Hi gotmilk,

    The thread you linked demonstrates an issue where the duty cycle of the PWM waveform is not constant. Is this the issue you are having?

    I do not recommend setting the f_PWM frequency higher than 25kHz. There is likely another way to solve the issue you are having. Can you post your schematic including the solenoid inductance and resistance?

    Best,
    Grant
  • 0 in reply to Grant Falkenburg
    Prodigy 120 points

    Hi the resistant is 300ohms, the measured L= 87mh @10kHz..

    thanks

     

  • 0 in reply to gotmilk
    TI__Intellectual 1965 points

    Hi gotmilk,

    The stability of the DRV120's feedback loop additionally depends on R_PEAK, R_HOLD, and V_SUPPLY. Can you provide the values you are using for these parameters?

    Best,

    Grant

  • 0 in reply to Grant Falkenburg
    Prodigy 120 points

    R_Peak 82K
    Hold 130K
    OSC shorted 20K, or 165K 25K, 81K(best case), 200K
    V supply 24v
     

    I replaced  R_Peak, R_Hold and R_OSC with a pots. 

    with OSC fixed  at 165K,  adjusting I_Hold

    I_hold = 136K, the PWM is ~1/2 the predicted value, looks to be constant.

    I_hold,= 160K, the PWM is  ~ the predicted value  and looks to be constant.

    The results suggest that the PWM behavior is depended on the set current, In either case the device is controlling the current, 

    The issue is the hum generated if R_OSC is set  above 100K, the hum generated is annoying.

    The sweet spot :  R_OSC = 70K to 85K  and R_Hold in the range of 165K, unfortunately  with R_OSC set below 130K,  switching the relay is intermittent. Can you confirm if this is true?

    thanks

  • 0 in reply to gotmilk
    Prodigy 120 points

    HI Grant  can you share your thoughts

    thanks  

  • 0 in reply to gotmilk
    TI__Intellectual 1965 points

    Hi gotmilk,

    It is known that when F_PWM is set above 30 kHz, the peak current is skipped and the DRV120 goes directly to the hold current. This is why I do not recommend setting F_PWM above 25 kHz.

    For your application there is a possible workaround. Can you try setting R_OSC to 80 kOhm, R_HOLD to 165 kOhm, and installing a 0.1 uF capacitor in parallel with R_HOLD? This effectively bypasses R_HOLD for a short period of time when the DRV120 is first powered, thus pulling the output low and closing the solenoid. This works with your application because the resistance of your solenoid is so high (300 Ohms) that if the output is pulled low, the resistance limits the current through the solenoid.

    There is one caveat to this method: I cannot guarantee that the EN pin will function properly.

    Please give this a try and let me know if it solves your issue.

    Best,

    Grant

  • 0 in reply to Grant Falkenburg
    TI__Intellectual 1965 points
    Hi gotmilk,

    Let me make a small correction: Instead of installing a 0.1 uF capacitor in parallel with R_HOLD, please install the series combination of 82kOhm and 0.1uF in parallel with R_HOLD. When the DRV120 is first powered, the 82kOhm resistor will be in parallel with R_HOLD=160kOhm, giving a net resistance of 54kOhm, thus setting the current to 100mA. After the 0.1uF capacitor charges, the 82kOhm resistor will not affect the current setting, giving you the 41mA needed to keep your solenoid closed. The 0.1uF may not allow the solenoid enough time to close; if this is the case, you can use a 1uF capacitor.

    Please let me know if this solves your issue.

    Best,
    Grant