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DRV2700 HV piezo driver problem [SOLVED]

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Unixon:

--- Quote from: hv222 on August 17, 2019, 09:15:35 pm ---That explains negative values of voltage here  ;)

--- End quote ---
OK, here's correct shots of SW node at Vout set to 10V, 50V and 100V. At 100V actual Vout is degraded to 63V.

Unixon:

--- Quote from: hv222 on August 17, 2019, 08:50:13 pm ---Check REXT resistor. Is responsible for current limit. Maybe is damaged or have bad solder joint.

--- End quote ---
No, it's OK. I've changed that to several values in allowed range.
By default I put there 10k as in demo board, I've also tried 7.5k as a minimum value (max. current) and 30k as max. value (min. current).
With Rext=30k Vout becomes limited at 16-17V, with 7.5k vs. default 10k Vout is probably raises a bit, but I can't say that definitely.


--- Quote from: hv222 on August 17, 2019, 08:50:13 pm ---What is feedback voltage, when you set output to maximum value? If feedback voltage is close to 1.3V then check again feedback circuit and R38/C37. If it's below check around Q2. Maybe discharging circuit is on all time.

--- End quote ---
When Vout is below desired value, Vfb is also below Vref, it could be anywhere down to zero, 0.4-0.5V for example. But Q2 is closed at that time!

Unixon:
I think I found the problem. After all, it was the excessive loading by the H-bridge MOSFETS.
When H-bridge is enabled and low side is turned on, due to resistor ratios high side also gets turned on and I observe a short across safeguard 1k resistor.

Unixon:
I suppose this mod should fix the problem... But then I will have to re-spin the board, even at best pads selection this may not look very nice on existing boards.

Unixon:
OK, now this works fine. So it wasn't a DRV2700 problem, my schematics for H-bridge was insufficient.

Thanks everyone, problem solved.

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