Hi Cafain,
1. Is this a problem as how to use DRV8835?
No, the DRV8835 is able to handle VM voltage up to 11V
2. I can imagine that Ron of the built-in FET changes,
Is it possible to think that when the motor starts up, when the VM is low(For example VM=3.0V),
the current increases?
Ron change would be minimal. The start-up current will increase as the voltage increases.
3. In addition to changing the VM, what is the best way to reduce the motor startup current?\
To reduce motor start up current use PWM method and slowy increasing your duty cycle.
Also, depending on your end application, there might be other suitable DRV devices which have current regulation features such as the DRV8848.
Thank you.