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DRV8662 doesn't operate with high (but legal) REXT values

Tim Gipson
Prodigy 170 points
Other Parts Discussed in Thread: DRV8662

The DRV8662 datasheet specifies an valid range for the REXT resistor of 6K to 35K. However, in our testing, with an REXT of 30K, the device comes up only intermittently, maybe 30% of the time. The other times no switching appears to occur, and VBOOST=VDD. The same part/board/configuration works as expected when using "normal" REXT values on the order of 6-10k.

Test conditions:

Vboost setting: 100V

Vin: 5.0V

Inductor: 4.7uF

Given the observed behavior, what is a safe actual maximum value for REXT in a production device?

Thanks!

  • 0
    TI__Expert 3280 points
    Hello Tim,
    It sounds like you are on the edge of a stability problem of the hysteretic control loop of the device. This could be due to some parasitics or other parameters in the inductor on your board. Since you do not need much current in your application, you can provide more margin in the system by increasing the REXT value as you described. Is there any reason you do not want to increase the REXT?
  • 0 in reply to Rodney Miranda
    Prodigy 170 points

    That's right; per the above, I would like to increase REXT to its maximum specified value of (per the datasheet) 35K. However, when approaching this value (e.g. 30K +/-1%), the device no longer operates reliably. It seems as though 35K is not actually a safe value to use in a production device. So, I would like to know the maximum "safe" value to use in a production product to minimize peak current over a 4.7uH inductor while guaranteeing the device operates 100% of the time, across all devices (part-to-part variance), over the full specified voltage and temperature range.

    Thanks!

  • 0 in reply to Tim Gipson
    TI__Expert 3280 points
    Hi Tim,
    As I mentioned, I think there are some parasitics that are not allowing it to function reliably up to the 35K. Therefore it is difficult to determine what would be "safe" across a variance boards/inductors/your application. Rather than diving into a board parasitics discussion, can you explain what you are trying to accomplish by minimizing the peak current?

    Also, feel free to send me a message if you do cannot reveal more details on the forum.
  • 0 in reply to Rodney Miranda
    Prodigy 170 points
    Rodney,
    You guessed it, the application is a bit more complicated than I've let on in the public forum (I tried contacting an engineer via TI's support site, but got an auto-reply bouncing me to here). Do you have an email address or some method I could use to make that contact? I couldn't find an obvious way to send a private message or equivalent through this forum, but if there's a way to do that, that works too.
    Thanks!