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DRV8837: how brake process work

Part Number: DRV8837

dear ti expert,

    i am using drv8837 to drive DC motor, i see that if both inputs to DRV8837 is high, it is brake . i wonder what is the purpose of brake ,just using to control the duty cycle of the driver's output(generally,from the word ,'brake', i think it is a process of stop something)? and in order to stop the motor ,only can use drv8837 in coast mode? 

by the way ,can i control drv8837 using an  external MCU's complementary pwm function?  thanks

  • Hi Jack,

    Brake mode occurs when the winding outputs are shorted. When the winding outputs are shorted, the current goes into slow decay (you can learn more about current regulation and decay modes here). In slow decay, the winding current does not change very much but it converts its mechanical energy to heat because of the winding short. This is why we call it "Brake".

    In coast mode, the output terminals are set to High-Z state (open) as shown in Table 1 of the datasheet. The current in this case will go into fast decay mode. In fast decay mode, the current is not maintained in the coils and instead flows back to the input source (input bulk capacitor). Effectively, the motor will continue to keep spinning and that is why it is called "Coast". If input cycles are continued to be sent to device, the motor will be accelerated during the ON time and will "Coast" during the OFF time and this will continue to happen until there is a balance between the energy added during the ON time and the energy dissipated during the OFF time. If input cycles are stopped being sent to device, them the motor should "Coast" to a stop but the time it takes to stop will depend heavily on friction. 

    [Q]: by the way ,can i control drv8837 using an  external MCU's complementary pwm function? 

    [A]: Yes, you can do that. Just make sure to follow the timing requirements on section 6.6 of the datasheet

    I hope that answers your question.

  •  ear Pablo,

        thanks for your reply. from drv8837 datasheet, in brake mode , both input1 and input2 should be high. but in complementary pwm mode ,i think that if input 1 is high, then input 2 should be low, vice versa.  if it is this case ,i don't think if is possible for complementary pwm to brake. i am confused ,will you kindly detial it? thanks.

    i got drv8837EVM from Ti, and measure its output as above , i found that it exists brake mode during motor spin because there is period in which both input is high ,according to datasheet ,it is brake ,also as you mentioned ,in brake mode ,mechanical energy converted to heat . so i wonder why not replace brake mode with coast mode ?

  • Hi Jack,

    In brake mode the motor winding terminals need to be shorted together. If they cannot be shorted, then brake mode will not work. When using complementary PWM, you might not be able to get both IN1 and IN2 to be high to get H-bridge in brake mode. The complementary PWM control method only gives fast decay or coast mode. Click here to look at how Coast mode works in complementary PWM mode vs conventional PWM mode. Hopefully this clears some of your confusion.

    General observation: For brushed motors, complementary mode is less efficient due to higher current ripple. If your application does require low ripple, then complementary PWM control method can be used.

    For more information, take a look at the following two E2E threads to learn more about Coast vs Brake mode and complementary PWM control method.