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MCF8316A: Power Efficiency and need for BUCK components for proper operation

Chuck Moon
Prodigy 100 points
Part Number: MCF8316A

I am trying to estimate the power consumption of the processor, driver and associated current sense circuits, for a 3-phase Permanent Magnet Synchronous Motor (with L-L resistance of 2.6 ohms at PWM freq of 25Khz at room temperature).

It look like at 25degC with Cbk = 22 uF:

->in standby mode:

   * a Lbk inductor of 47 uH, provides the lowest processor current (Ivms = 10 ma)

   * a Rbk resistor value of 22 ohm, provides the highest processor current (Ivms = 27 ma)

->in operate mode with PWM at 25 kHz (no motor connected):

   * a Lbk inductor of 47 uH, provides the lowest processor current (Ivms = 14 ma)

   * a Rbk resistor value of 22 ohm, provides the highest processor current (Ivms = 30 ma)

With a motor connected as a function of L-L motor phase current, how much power does:

    * each of the motor phase current sense inputs consume?

    * each of the motor phase current half bridges (is P = Iphase^2*RdsON * 3)?

On page 19 of sllu338.pdf it says:

Only populate one of these options:

47 uH

22 uH

R1 resistor mode

Please verify that this means the BUCK either has 47 uH, 22 uH, or 22 ohms between SW-BK and VBK?

I'd like to reduce PCB space.  Is there any way not to use any BUCK components?

  • 0
    TI__Mastermind 26046 points

    Hi Chuck,

    You can use the MCF8316AEVM's integrated buck regulator with the 47uH inductor, 22uH inductor, or 22 ohm resistor as the buck filter to produce the buck output voltage. Each filtering component provides a range of possible buck output current:

    LBK = 47uH --> 0-200mA
    LBK = 22uH --> 0-50mA
    RBK = 22ohm --> 0-40mA

    Sometimes customers wish to minimize the BOM space on the buck. The buck components must  be populated whether you are using the buck or not; but generally if a very small buck output current is required (<40mA), customers can use a small 22ohm resistor and small voltage-rated 22uF capacitor (CBK) for the buck output filter. Different CBK values can be used for smaller BOM but will need to be tested for steady output voltage and small ripple as only 22uF has been validated and tested. 

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    Prodigy 100 points

    So other than powering the MCF8316A at a PWM frequency of 25 Khz, if I must populate the BUCK, what is the best value of Lbk and Cbk for maximum power efficiency?

  • 0 in reply to Chuck Moon
    TI__Mastermind 26046 points

    Hi Chuck,

    If you are not using the buck regulator, I recommend RBK = 22ohm and CBK = 22uF as stated above. 

    If you are using the buck and want the best power efficiency, I would recommend LBK = 47uH and CBK = 22uF, we have achieved 90% efficiency on the buck power with those components. 

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    Prodigy 100 points

    For Cbk=47 uH and Cbk=22uF, do you have any efficiency data as a function of current?

  • 0 in reply to Chuck Moon
    Prodigy 100 points

    Lbk = 47 uH, not Cbk!

  • 0 in reply to Chuck Moon
    TI__Mastermind 26046 points

    Hi Chuck,

    Let me reach out to the team to see if I can get this data. 

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    TI__Mastermind 26046 points

    Hi Chuck,

    I got more info on the buck regulator efficiency from the validation team. 

    Buck regulator efficiency is a function of multiple parameters: VM, buck load, and temperature.

    • Lower VM = better efficiency
    • Higher load current = better efficiency. At light load, buck losses will be limiting efficiency.
    • Lower temperature = better efficiency.

    With 47uH, the overall efficiency range is 57% to 95%. The highest efficiencies (74% to 95%) occur when VM <12V and the buck load current is near 200mA load, regardless of temp.

    With 22uH, the overall efficiency range is 40% to 90%. The highest efficiencies (60% to 90%) occur when VM <12V and the buck load current is near 50mA load, regardless of temp.

    Hope this helps!
    Aaron

  • 0 in reply to Aaron Barrera
    Prodigy 100 points

    Thanks for the efficiency data.

    It looks like Vm needs to be at a minimum of 6v to feed several LDO regulators which will an operating current of ~14ma (if LBK=47 uH) or 30 ma (if RBK = 22 ohm) at 25 degC.  If Vm is high (35 v), the device power dissipation is going to be Vm*Ioperating = 35*0.03 = 1.05 w!

    Is there any way to achieve better efficiency (like have Vm feed the BUCK which creates 6v that feeds the LDOs (so that the power dissipation doesn't increase with Vm)?

    Why is the BUCK is required to make the MCF8316A operate?  Can TI provide any power tree diagram for users to see how power branches in the device?

  • 0 in reply to Chuck Moon
    TI__Mastermind 26046 points

    Hi Chuck,

    You are correct, VM needs to be at least 6V to feed LDO and the buck regulators. Some of our points are captured at VM=6V, which showed higher efficiency than points above VM=12V. Higher VM will result in much higher power losses that needs to be mitigated through PCB configurations (thick copper, increased area, more layers, etc.) to improve thermal dissipation. 

    However, if buck efficiency is low (such as VM = 35V), this is actually not the main contributor of power losses in the device, you'll have more losses through the LDO dropout losses as well as switching and conduction losses through the 6 MOSFETs. 

    We have a thermal calculator for MCF8316A here you can use to estimate thermals of the device at different ambient temperatures, VM votlages, and PCB configurations: www.ti.com/.../slvrbi8

    The buck is designed into the 8316 family of devices and it is required to have the buck filtering components there or else you will receive a BUCK_UV fault, BK_FLT, and the nFAULT pin will go low. 

    Thanks,
    Aaron

  • 0 in reply to Aaron Barrera
    Prodigy 100 points

    And you don't know of achieve better efficiency...like have the output of the BUCK (which creates 6v from VM) directly feed the LDOs...(so that the power dissipation doesn't increase with Vm)?

  • 0 in reply to Chuck Moon
    TI__Mastermind 26046 points

    Hi Chuck,

    You can significantly lower the power dissipation of the AVDD LDO by setting the BUCK_PS_DIS bit to 0. This sources the LDO voltage from VBK rather than VM. 

    The equation for power dissipation will then be: 

     

    See section 7.3.3.4 of the MCF8316A datasheet. 

    Thanks,

    Aaron

  • 0 in reply to Aaron Barrera
    Prodigy 100 points

    Dear Aaron,

    Thanks for the insight into decoupling VM from the LDO.  It still looks to me like if I don't have any external load drawing current from AVDD, this doesn't help reduce my power?

    sincerely,

    Chuck

  • 0 in reply to Chuck Moon
    TI__Mastermind 26046 points

    Hi Chuck,

    Can you please share what specs you are trying to run MCF8316A to help estimate power dissipation and efficiency of the buck? Please share:

    - VM voltage
    - Buck voltage 
    - Buck load current required
    - Is AVDD required? If so, whether power sequencing needs to be enabled

    We don't exact calculation tools yet for buck power, so these specs above can help estimate power dissipation and efficiency based on the data we have internally. 

    Thanks,
    Aaron