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DRV8847: Strange behaviour of DRV8847 in parallel bridge mode

Part Number: DRV8847

I am using DRV8847 with MODE=1 and IN3=1. At PWM frequencies below 2kHz, the device operates as expected. Above this frequency the outputs that should be L have a signal that is in opposite phase to the H outputs (ref Table 5 in datasheet) - e.g. if IN1=0V and IN2 has the input PWM signal, OUT2 and OUT4 produce the expected PWM but OUT1 and OUT3 produce the antiphase signal up to approx 30% PWM and then the OUT1/3 pulse width decreases as the input pulse width increases. The result is that the connected motor does not start until the input signal has reached approx 30% pulse width.

Has anyone observed this effect and know what is causing it?

-- Andy