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TPS54160 Switching frequency

Morag le Roux
Prodigy 20 points
Other Parts Discussed in Thread: TPS54160

Hi 

I am using a 3 x TPS54160 devices to get a sequenced 3V3, 1V8 and 1V2 from a 24V input.  According to the datasheet placing a 270k resistor on the RT/CLK input should result in a switching frequency of about 444kHz on the 1V2 PSU.  However, it appears that, the switching frequency defaults to the minimum of 300kHz and in some instances the device starts skipping.

This 1V2 PSU is used to power the core of a DSP and this skipping (or Ecomode) seems to result in the DSP core not starting up properly.  My calculations based on the data in the datasheet suggest that the 444KHz is well below the curve of the skip and shift frequency.

Any ideas as to why this frequency would not be 444KHz as expected and how to get rid of the skipping?  

Thanks

Morag 

  • 0
    TI__Guru**** 191445 points

    The 444 kHz frequency and 24 V to  1.2 V conversion wil have a nominal on time of 113 nsec.  You may be running into minimum on time issues that can result in pulse skipping.  Eco-mode is a low current skip mode that is triggered when COMP drops to about 500 mV.   I would have to see waveforms and know more about your load current to make any further suggestions.  Make sure to include PH, inductor current, and COMP voltage.  Any additional waveforms that you deam useful as well.

  • 0 in reply to JohnTucker
    Prodigy 20 points

    Thanks, John.  I understand the possibility of skipping because of the low duty cycle but looking at the waveforms the device seems to be switching at 300kHz.  The conversion at this frequency should result in an on time of 167ns which is above the minimum of 130ns specified albeit still quite close (also obviously dependent on inductor current etc).

    I did notice that the skipping seems to occur under low load condition (30mA) at the output as opposed to the 300mA when the DSP starts up so it must be the COMP voltage that is causing the issue.  I will have to check this.  It almost seems like a catch22 because the low load caused by the DSP not starting up is causing the skipping on the PSU which in turn seems to affect the DSP startup.  To check this I put a resistive load across the output drawing about 100mA and the skipping stopped and the DSP started up successfully albeit that the switcher was still switching at 300kHz.

    I will get some waveforms and have a look at the COMP pin before getting back to you.

    Many thanks, once again.

    Morag