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TPS546B24A: SWITCHING FREQUENCY IMPLEMNETATION

Rajat Singh
Intellectual 280 points
Part Number: TPS546B24A

Hi Team,

VIN-12V

VOU-3.3V

IOUT- 5A

From the strapping PIN i am setting Switching frequencx 650 Khz but while measurement on board its showing 640 Khz.Please suggest what can be the problem?

Here is the attached waveform and schematic attachment.

Probe Point- SW PIN

  • 0
    TI__Expert 6926 points

    Hi Rajat,

    Seeing 640kHz is perfectly normal and within the tolerance of the oscillator. 650kHz switching frequency could range from 601kHz to 700kHz with the 650kHz setting, also shown in the datasheet. 

    Thank you,
    Tomoya

  • 0 in reply to Tomoya Ide
    Intellectual 280 points

    Hi Tom,

    I have checked above in datasheet.

    My question is when we are setting 650 from PIN strapping why internally it is changing?

    The device will work in CCM always regardless of output current so only varaible part is ouput voltage in my case that is also fixed to 3V3. 

    Many Thanks 

    Rajat

  • 0
    TI__Guru 50955 points

     

    The TPS546x24y family of devices uses an internal current source into capacitor generator to build the PWM clock with a nominal frequency selected by the pin-programming or PMBus setting.  In your case, this oscillator has been programmed to a nominal frequency of 650kHz, but has a +/- 10% tolerance, as referenced in the electrical specification table by extension of the 500kHz to 600kHz tolerance on the 550kHz nominal setting.

    640kHz is well within this expected part to part tolerance and is not an indication that anything is "wrong" with your design or the part.  Any randomly selected TPS546x24y device may produce an actual switching frequency between 585kHz and 715kHz when programmed to a nominal 650kHz.

    This fixed-frequency oscillator is not expected to vary with Input Voltage (for AVIN greater than 5V) output voltage, or under load, but may vary slightly with temperature.

    The switching frequency may also vary with AVIN voltage less than 5V as this may affect the bias voltage provided to the oscillator and shift its oscillation frequency, though the shift should still be within 10% of the nominal value.

    If you need tighter control of the switching frequency, the TPS546x24y family of devices can be synchronized to an external clock by providing an external square-wave signal to the SYNC pin and selecting "SYNC_IN" via PMBus or the ADDRSEL pin programming.