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# TPS62130A: Formula for deriving the switching frequency from the Vin, Vout, and FSW pin setting (high/low)

Part Number: TPS62130A

Hi,

As far as the data sheet of the TPS62130A is concerned, the switching frequency is described as follows.

1) When FSWpin=High
Switching frequency is set to 1.25MHz

2) When FSWpin=Low
Switching frequency is set to 2.5MHz

However, when I set the input voltage = 12V and the output voltage = 1V in Webench and calculated, the result was different.
FSWpin High: Webench calculation result 0.796MHz
FSWpin Low: Webench calculation result 1.33MHz

Please let me know if there is a formula that can derive the switching frequency from the input voltage, output voltage, and FSW pin setting (high/low).

Thanks,

Astro

• Hi Astro,

The switching frequency for this device will depend on the load current, Vin, and Vout.

An estimate in Power save mode (DCM) is:

To calculate switching frequency in FPWM mode, first calculate the on-time (2.5MHz FSW equations shown):

Note :  1.25MHz FSW is selected then the 400ns will be 800ns in the above equation.

Then validate that  t_on > 80ns for the minimum on-time constraint. From this value of t_on calculate t_off = (1-D)/D *t_on. Then FSW = 1/(t_on+t_off).

Let me know if that helps.

Thanks,

Joseph

• Hi, Joseph

We calculated the switching frequency for Vin = 12V, Vout = 1.0V, FSW = High (1.25MHz).

When ton is calculated by the formula, ton is limited to 80ns because ton = (1V/12V)*800ns = 66.7ns.

and toff = (1-D)/D *ton = (1(1/12))/(1/12)*80ns = 880ns.
And FSW = 1/ (ton+toff) = 1/(80ns + 880ns) = 1.04MHz.

Similarly, when Vin=12V, Vout=1.0V, FSW=Low (2.5MHz), the switching frequency is 1.04MHz.

I calculated the switching frequency with Webench,
It was different from the above calculation result and frequency.

Webench calculation result
FSWpin(high)・・・switching frequency =0.79526MHz
FSWpin(low) ・・・switching frequency=1.33MHz

Please let me know if you know the reason why Webench's switching frequency and calculation results are different.

Thanks,

Astro

• Hi Astro,

What is the output current for the WEBENCH setup? Switching frequency also varies with load current which isn't captured in the calculation above.

Thanks,

Joseph

• Hi, Joseph

The load current (Iout) set in Webench is 3A.

Thanks,

Astro

• Hi Astro,

One reason for difference between WEBENCH and the equations is that WEBENCH uses actual measured bench data in calculating frequency where the equations only capture the ideal behavior. Since this device uses constant on-time control, we have seen some differences in calculated vs. measured frequency due to the on-times being slightly different.

The WEBENCH frequency is likely to be more accurate since it came from measured data.

Let me know if you have any other questions.

Thanks,

Joseph

• Hi, Joseph

I understand that switching frequency of WEBENCH are more accurate since they are from actual measured data.
By the way, is my calculation correct? The switching frequency was 1.04MHz regardless of whether FSW was low or high.

Thanks,

Astro

• Hi Astro,

You calculated them correctly using the ideal equations. The reason they are the same is due to the minimum on-time constraint. With the same duty cycle and on-time we get the same frequency, 1.04MHz in your case.

Thanks,

Joseph

• In addition you can check out this Application note on DCS-Control devices and variations in switching frequency.

Thanks,

Joseph