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DRV2604L: Output Stage Behavior

hideyuki sakai
Mastermind 6250 points
Part Number: DRV2604L
Other Parts Discussed in Thread: DRV2605L

Dear, Sir. 

My customer is considering to apply DRV2604L for their platform as LRA haptics. 

At this time, I would like to makes sure several regarding output stage behavior as followings. 

Please give your advice. as 

1. I suppose ; 

   High-side transistor would be work as current limitation during Over-drive. 

   Low-side transistor would be work chopper(PWM) during Sustain. 

   I wonder my understanding is correct? 

2. High-side transistor. 

  If it was work as current limitation, which is correct? 

  - It would be work like lower duty or pulse-skip similar as DCDC converter. 

  - It would be work like linear current limit similar as LDO. 

Best Regards, 

H. Sakai

  • 0
    TI__Intellectual 1910 points

    Hey Hideyuki, 

    I'm not sure what you mean by over drive and sustain, but perhaps an explanation of the DRV2605L output will help:

    The DRV2605L is a class-D amplifier, which means the outputs are driven by transistors in an H-bridge configuration that are switching at about 20.5 kHz. These transistors are either fully on or off, and the result is a square-wave output signal with a duty cycle that is proportional to the amplitude of the signal. The output switches from 0V to ~VDD. The LRA acts a low-pass filter, so it does not see the 20 kHz PWM but instead more of a sine wave. The higher the duty cycle, the higher the amplitude on the output as seen by the LRA. The picture below shows the current path with a speaker instead of an LRA but the principle is the same. Does that help?

  • 0 in reply to Micah B Brouwer
    Mastermind 6250 points

    Dear, Micah-san. 

    Thank you so much for your detail explanation. 

    I understood DRV2604L is a kind of class-D amplifier. So the output behavior is

    PWM(chopper). 

    There would be 2 stage current control for LRA driving, one is over drive and another

    is sustain. 

    The other side, there would be 2 way of PWM for such 2 stage current control, I suppose. 

    1. High-side transistor = PWM & Low-side transistor = ON

    2. High-side = ON & Low-side transistor = PWM

    My previous question is ; 

    Is there the relation between 2 stage current control & 2 way of PWM? 

    Best Regards, 

    H. Sakai

  • 0 in reply to hideyuki sakai
    TI__Guru** 100835 points
    Hi Sakai-san,

    Let me try to explain further how the driver works.
    As Micah explained above, the output stage of DRV2604L is a Class-D. So the output signal is a PWM which amplitude goes from 0 to ~VDD.
    Overdrive is the action performed by the DRV2604L to enable an accelerated startup. You can see the Overdrive as an opposite of Braking. Overdrive/Braking use a higher level voltage than the normal drive, but this is related to the PWM Duty Cycle.
    So the PWM frequency is fixed at 20.5kHz (typical) and its amplitude is fixed to be switching between 0 and ~VDD. The PWM Duty Cycle will determine the amplitude and frequency of the signal driven into the actuator, there is no current control as the output transistors are only turning On and Off. This is like any Class-D amplifier.
    Please note that most graphs from data sheet state "[OUT+] − [OUT−] (Filtered)". The output of DRV2604L without filter would be like shown in data sheet Figure 9.
    The "output with feedback" plot from the graph you posted above is a representation of the signal driven into the actuator, and not the actual signal coming out of the DRV2604L outputs. This is why the figure is titled "Waveform Simplification".
    I hope this explanation makes it more clear to understand the way DRV2604L works.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators
  • 0 in reply to Ivan Salazar
    TI__Guru** 100835 points
    Sakai-san,

    I'd like to add one additional comment.
    The difference between the DRV2604L and conventional Class-D modulation is that:
    For ERM, only OUT+ is switching during overdrive and normal drive and OUT- is a return path to GND. During braking only OUT- is switching and OUT+ is a return path to GND. This is because ERM are driven with DC voltages which polarity determines the spin direction.
    For LRA, only OUT+ is switching during the positive cycle of the signal and OUT- is a return path to GND, and only OUT- is switching during the negative cycle of the signal and OUT+ is a return path to GND. This differs with a conventional Class-D modulation in the way that OUT+ and OUT- are not driving complementary duty cycle PWM signals but they drive complementary full PWM signals (not only duty cycle).

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators