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DRV8889-Q1: Frequency of smart tune

Yoshiki Ochi
Genius 9685 points
Part Number: DRV8889-Q1


Hello team,

this is Ochi. Could you tell me operations with smart tune decay mode shown in Figure 14 and 15?

a) With smart tune dynamic decay mode, I understood toff is constant. Then, Is PWM frequency fixed once Itrip is decided (Itrip_A) and it reaches steady state? and, is a frequency with another Itrip (Itrip_B) different from the frequency with ItripA?

b) With smart tune ripple control decay mode, I have same questions as I wrote above.  Is PWM frequency fixed once Itrip is decided (Itrip_C) and it reaches steady state? Also, is a frequency with another Itrip (Itrip_D) different from the frequency with ItripC?

Best regards,

Ochi

  • 0
    TI__Guru 56650 points

    Ochi,

    a. In steady state inductor Von*Ton=Voff*Toff, If we assume the fast decay and slow decay timing is same in steady state, Ton and Toff should be stable and the frequency is fixed. If Itrip_B is different than Itrip_A, the fast decay and slow decay timing could be different which affects the average Voff in Toff period and causes the Ton difference. So, the final PWM frequency could be a little bit different.

    b. With ripple control decay mode, Von*Ton=Voff*Toff.   At Toff, IL(2* Rds_on + RL) -Vbemf=Ldi/dt   In this equation: di is the ripple amplitude, L is winding inductance; dt is Toff; IL is Itrip; RL is the winding resistance and Tds_on the switching FET's turn-on resistance.

    From this equation, Toff is changed with different Itrip level and motor spinning speed. If all the parameters in that equation are fixed, you can see the Toff is fixed and the frequency should be similar.

    In real circuit, the ripple amplitude could be very small. So, a noise could also affect the PWM frequency.