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MCT8316Z: MCT8316ZT Buck in Resistor mode

Koshi Ninomiya
Expert 5425 points
Part Number: MCT8316Z

Dear Team,

Could you let me understand how to caluculate the losses of RBK resistor unnder Buck in Resistor mode of MCT8316ZT?

Conditions are as follows,
 - VM=24V
 - VBK=5V by VSEL_BK=Hi-Z
 - No external load, only FB_BK current with ~15mA
 - RBK : 22ohm

My customer evaluates the MCT8316ZT on their board under Buck in Resistor mode. And the customer has a heat issue with the RBK resistor now.

About the SW_BK operation under Buck in Resistor mode, does the SW_BK output PWM pulse of VM-GND amplitude with On duty cycle of VBK/VM?

I look forward to your reply.
Thank you.

Best Regards,

Koshi Ninomiya

  • 0
    TI__Expert 5425 points

    Dear Team,

    One more question. 

    In the datasheet of the MCT8316Z, it is described as follows,
      If the buck regulator is unused, the buck pins SW_BK, GND_BK, and FB_BK cannot be left floating or connected to ground. The buck regulator components LBK/RBK and CBK must be connected in hardware.

    Please let me know the reason why the SW_BK pin cannnot be left floating.
    The customer is also considering not using the integrated mixed-mode buck regulator, and supplying external 5V to the FB_BK.

    Best Regards,

    Koshi Ninomiya

  • 0 in reply to Koshi Ninomiya
    TI__Mastermind 26416 points

    Hi Ninomiya-san,

    Thanks for your questions! I will aim to respond to you by the end of the week. Tuesday is a holiday for TI, so responses may be delayed.

    Regards,

    Anthony Lodi

  • +1 in reply to Koshi Ninomiya
    TI__Mastermind 26416 points

    Hi Ninomiya-san

    The SW_BK pin cannot be left floating because any time the driver is reset either during powerup or after an undervoltage fault or after exiting sleep mode the default setting is for the buck to be enabled until the buck is disabled over SPI. Even if the buck is not being used the buck will operate for some time after device reset until it is disabled over SPI. You can use small footprint components since power dissipation isn't much of a concern since you plan on disabling the buck soon after reset.

    Regards,

    Anthony Lodi