DRV10983: RPM feedback is not accurate.

Speed read back from the period register is estimated value and can have errors. What is the value reported by the motor period register and the actual value? 

value after calculation is 370RPM but the actual RPM is 380 but this type of error is observed in some of the DRV IC. Most of the DRV IC shows error of only 2 or 3 RPM.

Karthik,

This device does not have an internal speed loop, so any slight variation in the load due to fan blade angle/weight/size etc can cause the speed variations. If you are looking to minimize the speed error, we recommend using an external MCU to implement an external speed loop. 

Refer to this reference design for more details on implementing speed loop: https://www.ti.com/tool/TIDA-01496

Regards,

Vishnu

I am using external MCU to execute the speed loop stabilization but the issue is  the drv speed readback itself sending less RPM then like if the fan is at 380 it  feedback RPM is around 370  so my speed loop is based on RPM so based on Feedback RPM it makes it to 380 and stabilizes but the actual RPM what we observe was 390 or more which is not accepted. This is happening in only some of the DRV IC? Why is that so?

Will get back to you shortly. 

Karthik,

Can you check the frequency of FG pin and compare with the motor period register on the devices that report incorrect speed? How many pole pairs does your fan have? 

Regards,

Vishnu