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LM63615-Q1: Request for schematic review support

Vishnu V P
Prodigy 30 points
Part Number: LM63615-Q1

We have created a power supply schematic with the buck regulator LM63615 with the following specification. Please find the attached schematic for your review.  Kindly review it and provide your comments if any. What is the preferred inductor value for this specification.?

V_BAT+ = 12V to 24V

I_in = 500mA

V_Out = 5V

I_Out = 10mA to 500mA

Fsw = 2100 kHz

5V_Buck_Supplier.pdf

  • 0
    TI__Mastermind 30955 points

    Hi Vishnu,

    Schematic looks fine here. For a minimal inductance, with inductor ripple ratio of 30%, you can use 1.4uH, so your value of 4.7uH is fine here. I would make sure to account for the DC derating of the capacitors.

    Please let me know if you have any other questions. 

    Regards,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 30 points

    Hi Richard,

             Thank you for your response. Will this design works always in CCM mode. As per our calculation the duty cycle is around 20%. Will this switch to DCM mode.? And when it is DCM, what happens to the spread spectrum.?

    Regards,

    Vishnu V P

  • 0 in reply to Vishnu V P
    TI__Mastermind 30955 points

    Hi Vishnu,

    Depends on the load, the part may enter DCM. Spread spectrum will disengage if duty cycle hits either the high/low limit, and then frequency will fold back. If you want to avoid this, you can enable FPWM mode to have constant frequency. 

    Regards,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 30 points

    Hi Richard,

     What is the low limit of duty cycle at which spread spectrum disengage. We can not find this info in datasheet.

    When regulator enters DCM mode, how it will impact the inductor ripple current percentage with given specification.?  

    Regards,

    Vishnu V P

  • 0 in reply to Vishnu V P
    TI__Mastermind 30955 points

    Hi Vishnu,

    Low limit of duty cycle is minimum on-time operation, so duty cycle is dependent on frequency, as minimum on-time = duty/Fsw. 

    When the regulator enters DCM, the inductor ripple current will increase. Please see figure 8-6 as compared with figure 8-9.

    Regards,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 30 points

    Hi Richard,

             Sorry to ask you again and again. This looks bit complicated. In our application we want to make sure the device should always been in CCM and spread spectrum is active. For an input voltage of 24V and a load current of 50 - 200mA with a switching frequency 2.1MHz how can we confirm the CCM and spread spectrum is active always.?

    According to formula the duty cycle calculated is 20% and the minimum on-time=9.5us. From datasheet the minimum switch on time is 50ns. Does this mean our application requirement will not be falling to DCM.??

    Regards,

    Vishnu V P

  • 0 in reply to Vishnu V P
    TI__Mastermind 30955 points

    Hi Vishnu,

    You can enable FPWM mode to solve your issues. You can do this by tying MODE to VCC.

    Regards,

    Richard 

  • 0 in reply to Richard Fu22
    Prodigy 30 points

    Hi Richard,

        Thanks for your feedback. Can I get the efficiency vs load current and efficiency vs input voltage curves for the FPWM mode.

    Regards,

    Vishnu V P

  • 0 in reply to Vishnu V P
    TI__Mastermind 30955 points

    Hi Vishnu,

    Unfortunately, we do not have any current data for efficiency curves in the FPWM mode. I can get you this data, but will need to order and EVM and it may take up to a week to arrive. You may also order an EVM and do the testing if it is easier for you that way. 

    Regards,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 30 points

    Hi Richard,

          Is it possible to control the MODE pin from a microcontroller pin.? We want to make the regulator to work on FPWM mode in normal operation of the product and AUTO mode during sleep mode of the product to ensure lesser sleep current at sleep mode. Is this possible to do by making the MODE pin high and low from the controller.?

    Regards,

    Vishnu V P

  • 0 in reply to Vishnu V P
    TI__Mastermind 30955 points

    Hi Vishnu,

    Yes, you can control the MODE pin from a microcontroller pin. The MODE changes can be made anytime on the fly after device is powered up. Thresholds are located in datasheet, but I've attached them below.

    Regards,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 30 points

    Hi Richard,

              Thank you for your response. There is a confusion with respect to the max voltage that can be applied to the SYNC/MODE pin, as per the recommended operating conditions the max voltage to SYNC/MODE pin is 5V and as per the image attached in your above response it is 1.8V. Which value we need to consider when controlling this pin from MCU. As per our controller the digital pins are 5V, so do we need to use a potential divider to control the pin or can we connect directly.? 

                 

    Regards,

    Vishnu V P

  • 0 in reply to Vishnu V P
    TI__Mastermind 19891 points

    Hello Vishnu, 

    If your MCU has 5V output, it seems to be OK to connect directly. 

    The absolute maximum voltage on sync/mode is 6V. 

    The maximum recommended voltage is 5V. 

    The logic high level threshold is as high as 1.8V, as indicated by the table. This means when you need pull the pin high, you need to provide more than 1.8V to the pin in order to ensure logic high input. The logic low level threshold is as low as 0.8V. This means you need to pull the pin below 0.8V in order to ensure logic low input. 

    Regards, 

    Denislav

  • 0 in reply to Denislav Petkov
    Prodigy 30 points

    Hi Denislav,

              Thank you for your response. What will be the state of SYNC/MODE pin during the power up before the controller make this pin HIGH or LOW.? Will this be in an indeterminate state.?  Do we need to have a pullup or pulldown to this pin to avoid any such indeterminate state during power up and controller bootup.? If this is required what is the recommended pullup or pulldown value required.?

    Regards,

    Vishnu V P