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DRV8874: Does controller side needs PWM control when using PH/EN mode?

Hung Ching Hsu
Expert 3086 points
Part Number: DRV8874
Other Parts Discussed in Thread: , DRV8705-Q1

Hi Team 

My customer has question about PH/EN mode of DRV8874. 

Customer wants to rotate DC motor in forward direction using PH/EN mode.

First, he sets nSLEEP=EN=PH=1. ← Is this setting correct? 

Will PWM control still needed from controller side ? Or customer can fix Pin No1, 2 of DRV8874 to High state?

For my understanding, if customer wants to regulate motor speed, customer needs PWM control for EN pin and set PH pin to high state. Is it correct?

Thanks.

Regards,

Jo

  • 0
    TI__Guru 56650 points

    Hello, Jo::

    "he sets nSLEEP=EN=PH=1" correct.

    The controller side doesn't need to send the PWM control signal .The customer can fix Pin No1, 2 of DRV8874 to High state if they want the output current flowing direction always same. Datasheet "7.3.3.2 Current Regulation" section gives the detail.

    If the customer doesn't want to use DRV8874's current regulation, they can disable it or change to PWM mode to give the PWM signal. If you check the Table 3. PH/EN Control Mode logic true table, you can find PH/EN mode doesn't need PWM signal.

  • 0 in reply to Wang5577
    TI__Expert 3086 points

    Hi Wang


    Thanks forth reply. 

    My customer has following further questions. 

    Could you kindly help with this? Thanks.

    1. Is there difference between ILS1, ILS2 and ISEN1, ISEN2 in datasheet?

    2. Is it correct to calculate RIPROPI as following? 

      When 6A(initial) flows through motor and MCU's ADC is 3.3V rating. 

    　IPROPI=6A*455uA/A=2.73mA

    　RIPROPI < 3.3V/2.73mA≒1208Ω

    3. Regarding IMODE, when using at Quad-Level 1, I think VOUT will become BRAKE for toff duration and nFAULT signal will be low when current more than ITRIP is flowed. 

       Will noise have impact to ITRIP when motor is rotating? For example, ITRIP = 1A at first, then ITRIP became 1.2A for a short time when brush and commutator touched to each other. Will DRV8874 behaves like above or it will ignore the noise?

    4. Is there data for failure rate and failure mode? 


    Thanks.

    Regards,

    Jo

  • 0 in reply to Hung Ching Hsu
    TI__Guru 56650 points

    Hello, Jo,

    1. ILS1 is the current in low side FET1. ILS2 is the current in low side FET2. According to "Functional Block Diagram", ILS1 should be same as Isen1. ILS2 should be same as Isen2.

    2.The RIPROPI calculation is correct.

    3.when using at Quad-Level 1, the Current Chopping mode is the fixed off-time. Yes. VOUT will become BRAKE for toff duration and nFAULT signal will be low when current more than ITRIP is flowed. (check 7.3.3.2.1 Fixed Off-Time Current Chopping)

    Tdeg deglitch time is designed for ignoring the noise if the current spike is short than 0.6us.

    4. MTBF data can be found in quality page www.ti.com/.../report

  • 0 in reply to Wang5577
    TI__Expert 3086 points

    Hi Wang

    Thanks for the reply. 

    My customer has one question about ITRIP. 

    Can ITRIP be used to limit starting peak current? 

    For example, if motor's current is 10A when start to rotate(which exceeds DRV8874's 6A rating) and current when motor stably rotates is 1A. 

    If we set ITRIP=4A, will it help to suppress starting current to 4A(Both in Fixed Off-Time and Cycle-By-Cycle mode)?

    Or is ITRIP can only be used for current monitoring after motor starts rotate stably? 

    If  ITRIP can only be used for current monitoring after motor starts rotate stably, could you advise what customer can do to reduce current when motor start to rotate in order to use DRV8874?

    Regards,

    Jo

  • 0 in reply to Hung Ching Hsu
    TI__Guru 56650 points

    Jo,

    On the datasheet first page, "Using the external voltage reference pin, VREF, these devices can regulate the motor current during start-up and high-load events without interaction from a microcontroller."

    I would think  ITRIP can be used to limit starting peak current. But, we have to know the output current may over the 4A Itrip setting in the current sensing blanking time if the output inductance is not higher enough.

    "could you advise what customer can do to reduce current" Do you mean giving another customer name? I cannot. But, since ""Using the external voltage reference pin, VREF, these devices can regulate the motor current during start-up and high-load events without interaction from a microcontroller." on the datasheet first page, I would think a lot of people did it.

  • 0 in reply to Wang5577
    TI__Expert 3086 points

    Hi Wang

    Thanks for the reply. 

    "But, we have to know the output current may over the 4A Itrip setting in the current sensing blanking time if the output inductance is not higher enough."

    I think the output inductance you mean here is motor.

    How can we know the output inductance is not high enough? Is there any formula to calculate it?

    Thanks.

    Regards, 

    Jo

  • 0 in reply to Hung Ching Hsu
    TI__Guru 56650 points

    Jo,

    At start up, the motor doesn't spin yet. The output current follows the inductor equation: di/dt=Vin/L; dt=tBLK= 1.1us; assume Vin=24V; di=Itrip =4A; 

    Motor inductance, L, needs higher than  VIn*dt/di =24V*1.1us/4A = 6.6uH. For a motor, the inductance should be higher than 6.6uH.

  • 0 in reply to Wang5577
    TI__Expert 3086 points

    Hi Wang

    Thanks for the reply. I got two questions from customer. 

    It would be appreciated if you could help this.

    1. Do TI have data for DRV8874's failure mode? If yes, can we know the percentage for each failure mode?   

    Seems the page you provided before only shows MTBF and FIT.

    2. DRV8874's Peak output current MAX=6A. 

    Does it means customer can use DRV8874 to flow 6A all the time? (When motor rotates stably)

    Datasheet says customer should be careful of Power dissipation and thermal limits. Is there any other concern? 

    Thanks.

    Regards, 

    Jo

  • 0 in reply to Hung Ching Hsu
    TI__Guru 55605 points

    Jo,

    Thank you for your help supporting our products.

    For #1, we offer more details on our automotive grade version of the DRV8874, DRV8874-Q1.  If the customer needs those type of details, they need to consider the automotive grade device:

    https://www.ti.com/lit/fs/slvaev5/slvaev5.pdf

    For #2, there are calculations in the datasheet for temperature based on continuous current.  Section 8.2.1.2.2 has these calculations.  The device is rated at 6A peak, so 6A continuous will not be possible without a large heatsink mounted to the PCB.  

    If they need 6A continuous, I suggest they look at DRV8705-Q1 which use external FETs for driving larger currents without the need for a heatsink.

  • 0 in reply to Ryan Kehr
    TI__Expert 3086 points

    Hi Kehr

    Thanks for the reply.

    Customer uses DRV8874 for 1~2A continuous. 

    I'll let you know if there's any update.

    Regards, 
    Jo