DRV10987: Different resulting speeds with 100% speed command

Part Number:DRV10987

Hi,

I am using the DRV10987 with a fan with one pole pair, Kt = 31,25 mV/Hz, Rph = 0,85 Ohm, Imax = 1.75A, RPM max = 40000.

My settings are:

0x90 0x103D
0x91 0x2A00
0x92 0x1888
0x93 0x30CB
0x94 0x3EAF
0x95 0x7853
0x96 0x6A

I have the problem, that the fan spins under identical mechanical conditions sometimes with 39500 RPM, but sometimes only with around 35000 RPM. Speed is set to 100%, and also buffered speed is 100%. Additionally, with the given settings, I frequently get an over current lock error, so I had to disable the lock bit to avoid stopping the motor.

Same problem with my own hardware and the DRV10987EVM. 

Any ideas?

Thanks!

Klaus