• State TI Thinks Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 31 subscribers
  • Views 366 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.

DRV8251A: Output Capacitance

Dennis Chertkovsky
Prodigy 20 points
Part Number: DRV8251A

Tool/software:

Hello all,

I am considering designing a DRV8251A into a resonant application. For more accurate simulations, it would be good to know its output capacitance (high side COSS plus low side COSS for a single output). For my simulation, it would be sufficiently accurate to model the half bridge as a pair of idealized switches, idealized diodes, and capacitors. I already know the resistance for the switches (0.225 ohm) and forward voltage for the diodes (0.8 V), but I do not yet know the capacitance for the capacitors.

I attempted to estimate output capacitance by assuming that output rise/fall time is purely due to RC effects, but that yielded 444 nF, which feels far too large. I expected a result in the 100 pF - 1 nF range.

In theory I could measure this myself (impedance analyzer, bias tee, lots of voltage stepping, etc), but I first wanted to ask if anyone here has either measured this directly or been able to compute a convincing result from datasheet parameters.

Many thanks!
Dennis

  • 0
    TI__Mastermind 44855 points

    Hi,

    Thank you for your questions. Our expert will review your question, but US holiday on July 4th may cause a couple of days delay.

  • 0
    TI__Mastermind 23431 points

    Hey Dennis,

    I have reached out to our team to ask this and to see if 444nF seems reasonable or not.  I looked through the info I have on hand and am not finding output capacitance for any of our drivers, but we'll see what they say.  Give them a few days to respond to me.  

    Just curious, could you give more information on your load? Current, frequency, etc.   I wouldn't have thought that the 200kHz of this device would be enough for ultrasonic.  

    Best,

    Jacob

  • 0 in reply to Jacob Thompson
    Prodigy 20 points

    Hello Jacob,

    444 nF is obviously wrong.

    Looking at another FET with comparable parameters (50 V VDSS, 300 mOhm on-state resistance) the VN3205, I get 70-120 pF, which would make half bridge capacitance about 140-240 pF. I know this is a device from Microchip and likely made on a different process from TI parts, but I had trouble finding a TI part with the correct parameters.. I know effective output capacitance for resonant applications is a tricky concept (MOSFET drain-source capacitance varies heavily with applied voltage), but a rough guess would be good enough.

    Frequency is about 100 kHz, voltage is 30-48 V, load current is on the order of 200 mA RMS (highly variable, depending on how resonance/filtering is to be implemented). This is a an unusual application for a DC motor driver, but this was the cheapest H bridge on a chip we could find that met our electrical needs for a Class D driver and had built-in current sense. I am open to other suggestions, I haven't yet had a chance to look through the entirety of TI's DRV* series of products.

  • 0 in reply to Dennis Chertkovsky
    TI__Mastermind 23431 points

    Hey Dennis,

    Unfortunately we don't disclose this for our internal FETs.  Agreed with you that 444nF is very wrong, your thinking is likely correct with the 100pF - 1nF range.

    Best,

    Jacob