We are using an ADT7470 to control 4x fans in one of our products. We are observing an issue with operating the fans in "low-frequency" mode (bit 6 of 0x40 = 1) whereby every second the PWM output is held HI for approximately 35ms (see attached trace). This is independent of automatic or manual control mode, or of the PWM duty cycle, and causes audible variation in the running fan speed.
More confusingly, if the tachometer is disabled using 0x74, the output is held HI for 750ms instead (attached). And if fast tach readout is enabled (bit 5 of 0x40) then this occurs every 250ms instead of every 1s.
I therefore conclude that this is related to the tachometer readout in low-frequency mode, as this is the only thing in the datasheet that occurs once per second.
Is there a workaround for this issue?