Hi Alessandro,
please refer to the Figure 8. in the MC33771C full datasheet. The MC33771C can switch to Idle mode only from the INIT mode if the communication is not active. If the MC33771C is not initialized within 60s, then it switches to Idle mode. Please refer to the sections 9.3.2 and 9.3.3. From Normal mode you can send it to Sleep mode by software command. Please refer to section 9.3.4.
With Best Regards,
Jozef