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DRV120: DRV120: What is the minimun PWM OSC freq.

Chad Southwell
Prodigy 90 points
Part Number: DRV120

The fluid valves I'm using have a 24VDC solenoid, 150ohm, 110mH@10kz. I've slowed the PWM clock to ~5KHz (1Meg resistor) to get them to work. Faster PWM clock, say 20KHz does not work at all, the solennoid cycles off and on in a random way. At 10KHz the valve works and sings rather clearly. At 5KHz the singing is reasonable and the valve works, the output waveform looks good.

Rhold =100k

Rpeak =100k

Ckeep=1uf

Vin=24VDC

Rosc=1Meg

So what is minimum frequency the PWM section do tolerate.

this image just before the end of keep time...

  • 0
    TI__Intellectual 1965 points

    Hi Chad,

    Thank you for your question. The DRV120 should be able to work up to 25kHz. I have a few questions about your application:

    What frequency do you require for your application?

    What supply voltage are you using?

    Does the random behavior happen during the peak current or hold current?

    If you have a current probe, can you measure the voltage on the OUT pin and the current through the solenoid at the frequency of interest?

    With this information, I can start troubleshooting.

    Best,

    Grant

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

    Were you able to resolve your issue? If so, could you describe what steps you took to solve it? If not, I can still help given the answers to the above questions.

    Best,
    Grant
  • 0 in reply to Grant Falkenburg
    Prodigy 90 points

    Hi Grant,

    I'm a bit side tracked by other tasks. However as I asked in my first  posting, what is the minimum frequency the PWM can operate?

    I do plan on getting back to this part and understand the odd behavior with the two solenoids I have.

    I have no specific frequency in mind, certainly above audio would be nice. I've tried valves from Parker and Cole/Parmer but both behave differently even though they both are 24VDC, 140-160 0hms and ~120mH (@10kHz) coils. The Cole/Parmer being the worst as it was the one that would cause the DRV120 to cycle off/on randomly at OSC = 20kHz.

    I was reading one the comments on setting the frequency in this forum, that got me thinking about the inductance vs. frequency and di/dt. So I experimented with slowing the frequency way down so that during a single off cycle (5kHz) the current would fall to the low historesis window and then turn on the gate on the next cycle (see the out waveform in my first post) whilst in HOLD mode. The transition from PEAK to HOLD looks normal.

    Supply is 24VDC.
    Hard to determine if the random on/off behavior occurs during PEAK or HOLD. I'm assuming during hold.

    I do note that at 20 or 25KHz in the HOLD state the OUT waveform requires ~10 cycles to allow the coil current to fall  before turning on the gate of the pass MOSFET. But this does seem normal based on how the PWM circuit is designed.


    The valve solenoid I plan on  using:
    www.parker.com/.../Series 2 Data Sheet.pdf

    More later when I get back to the part.

  • 0 in reply to Chad Southwell
    TI__Intellectual 1965 points

    Hi Chad,

    The minimum PWM frequency is determined by the maximum R_OSC value of 2 MOhms as defined by Equation 4 in the datasheet. Thus the minimum PWM frequency is 2 kHz.

    I am happy to help more once you have more information. In the meantime, I will think about what could be causing your issue.

    Best,

    Grant