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TPS82140: Switching Frequency

Li Zhang94
Prodigy 10 points
Part Number: TPS82140
Other Parts Discussed in Thread: TPS62135

I'm using TPS82140 to convert a 12 V rail to 5.7 V, and the load will be drawing roughly 0.5 A from the 5.7 V output. I do get the right output voltage but the switching frequency I measured is about 500 kHz, which is far off the 2MHz nominal switching frequency. I need the switching frequency as high as possible because I'm using it to supply a 13 GHz VCO and I have spurious requirement below 1.5 MHz offset. The only thing I did different from Figure 5 of the datasheet is the R1 and R2 values. I've used R1 = 51 kOhm and R2 = 8.2 kOhm to set the output voltage. Is the R2 I used too small so that it messed up the switching frequency?

  • 0
    TI__Guru*** 142860 points
    This is a pretty light load, so you are likely in PFM mode. See equation 2 in the D/S. Based on that, you are drawing much less than 0.5A.

    If there is another way to re-architect the power in your system, such that more load is added to this 5.7V rail, this would increase the frequency. One way might be to run a 1.8V rail converter from this 5.7V instead of the main 12Vin.

    Or you can use the discrete TPS62135 which has the ability to force PWM mode and maintain a higher switching frequency.

    With 5.7V, do you have an LDO after it to clean it up? This LDO may not even be needed, but may also be more effective at providing some PSRR at 500 kHz compared to 2 MHz. Many LDOs do not provide any rejection at such high frequencies.