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LM5164-Q1: Switching Frequency

Rohan Patwardhan1
Intellectual 340 points
Part Number: LM5164-Q1
Other Parts Discussed in Thread: LM5164

Hi TI Team,

LM5164 frequency shall be set based on Ron value. For our design, we have set the value to 178k. However, we have observed, the switching frequency reduce during light load operation. Is this expected behaviour?

Our intention was to change the Ron value to a higher value when light load operation would be needed. However, since the switching frequency reduces, we believe efficiency shall not dip as expected. Is there any advantage in using two different resistor configuration?

Thankyou,

Rohan 

  • 0
    TI__Mastermind 25590 points

    Rohan,

    Yes, diode emulation prevents negative inductor current. So if the output current is less than half of the peak-to-peak inductor ripple, the inductor minimum current would normally go negative if operating in continuous conduction, but diode emulation cuts off the inductor current from going negative. This increases the average inductor current which increases the output current which results in a lower switching frequency to bring that output current back down to the necessary value for the given load.

    Two resistors can help maintain a more consistent frequency as you mention, depending on operating conditions.

    -Sam

  • 0 in reply to Samuel Jaffe
    Intellectual 340 points

    Hi Sam,

    So can I assume that when in DEM, the switching frequency depends on load current and in a way is independent of Ron value. Can I also infer that the efficiency shall not vary on using 2 different Ron configurations?

    Thanks,

    Rohan

  • 0 in reply to Rohan Patwardhan1
    TI__Mastermind 25590 points

    Rohan,

    Yes, you are correct, in DEM, frequency is mostly controlled by load.

    Efficiency will be lower for a higher switching frequency (more switching losses) but small adjustments should not affect the efficiency more than component tolerances.

    -Sam

  • 0 in reply to Samuel Jaffe
    Intellectual 340 points

    Hi Sam,

    Thank you for your support.

    Regards, 
    Rohan