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LM5175: LM5175 wrong switching frequency

Anton Schuler
Prodigy 50 points
Part Number: LM5175

Programming of RT/SYNC pin with resistor gives wrong frequency.

I used "LM5175 Buck-Boost Quickstart Tool -- EVM.xlsm" tool which result in RT resistor of 68k for 365kHz. This  assembled resistor produces frequency of 175kHz !?
We assembled new 33k  which generate 347kHz. What is the reason ?  Wrong Formula?    We use LM5175RHFT.

Regards Toni

  • 0
    TI__Mastermind 21720 points
    Hi Toni,

    Please make sure that you are not measuring the switching frequency in the transition region. This will appear to make the switching frequency to be 1/2 of the selected value. The resistor value calculation in the quickstart tool is correct.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 50 points
    Hi Garrett

    The measurements are not done in the transition region. Uout = 22,3V // Uin1 = 24V | Uin2 = 18V;

    Regards Toni
  • 0 in reply to Anton Schuler
    Prodigy 50 points
    Sorry Garrett

    Measurements are in the transition region. Found my EVM-Kit (SNVU439) which is programmed to Uout 22V. There I tested with different Uin and found your transiation behaviour. This is problematic in my design - SW switches down if voltages are higher then 26V or lower then 17V.
    If starting my EVM-Kit with Uin=18V the calculated swiching frequency could be measured. If rising voltage the frequency is lowering, if reducing voltage again to 18V the low frequency hold on. Is it possible to calculate this voltage borders and the hysterese of this values?

    Regards Toni
  • 0 in reply to Anton Schuler
    TI__Mastermind 21720 points
    Hi Toni,

    To understand how the LM5175 works both the buck switch node and the boost switch nodes should be observed. When VIN~VOUT the device will enter transition mode. During transition mode the converter will give one pulse of buck mode operation and then one pulse of boost mode operation. This alternating switching allows for buck-boost operation to be achieved. If you are only looking at one switch node the switching frequency will look like the is is half because there are alternating buck and boost pulses. If both switch nodes are observed it can be found that the switching frequency is matches the calculation of the datasheet.

    Thanks,

    Garrett