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LM5176: The buck-boost mode of the controller

Feng Xu
Prodigy 130 points
Part Number: LM5176

Hi

Recently i am designing a buck-boost DC-DC and LM5176 is used as the circuit controller.

Vout=20V      Fsw=600kHz.

The waveform of SW1 and SW2 have been monitored and it seems there is an error operation.

We know that when Vin is close to Vout the circuit will work in buck-boost mode,but for my DC-DC,the input voltage range of buck-boost mode is about 19V~30V.

There is hysteresis so when it wants to come back to pure buck mode,the input should be higher than 31V,which means that it works in buck-boost mode even when input is 30V,which leads to severe degression of conversion efficiency.

If it works in pure buck mode with Vin=30V the duty cycle of PWM is about D=20/30=66.67%,which means that the maximum duty cycle limit was not reached.

So what forces LM5176 make a judgement that Vin is close to Vout in such condition?

By the way,the buck-boost voltage range is 19~23V when switching frequency is 300kHz and 19~30V when switching frequency is 600kHz.

  • 0
    TI__Mastermind 21720 points

    Hi Feng,

    The transition region is more dependent on the the duty cycle and minimum off times of the switches. This means that the transition will vary with output voltage and switching frequency. This can be estimate by using the LM5176 quick-start calculator.

    -Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 130 points

    Hi Garrett,
    Yes, i know what you mean but...
    The off times of a buck converter is Toff=(1-D)*T=(1-Vout/Vin)/Fsw.

    When input voltage is reduced from a higher value to 30V, the converter which operates with 600kHz changes its mode into buck-boost mode.
    The minimum off time can be figured out : (1-20V/30V)/600kHz=556ns.

    When input voltage is reduced from a higher value to 23V, the converter which operates with 300kHz changes its mode into buck-boost mode.
    The minimum off time can be figured out : (1-20V/23V)/300kHz=435ns.

    It seems that the minimum off time of LM5176 is relative to switching frequency and not a fixed value.
    It actually leads to severe decrease of conversion efficiency because the converter works in buck-boost mode even when Vin is 10V larger than Vout.

    The Quick-start calculator's result show that the range of buck-boost is 16.5V~29.2V and my virtual range is 18V~30V.

    When in low load condition,the degression of efficiency becomes vevy striking !



    How can i do to improve the situation?

  • 0 in reply to Feng Xu
    TI__Mastermind 21720 points
    Feng,

    The only way to narrow the transition region is to lower the switching frequency.

    thanks,

    Garrett