AN938 datasheet(23/33 Pages) STMICROELECTRONICS

23 / 33 page

AN938

Typical application

Doc ID 5410 Rev 10

23/33

3.9

PWM gain

Equation 15

where Vct is the peak-to-peak sawtooth oscillator.

Pole and zero values are as follows:

Equation 16

Equation 17

Equation 18

Equation 19

Equation 20

The compensation is found by choosing Focomp close to the frequency of the double pole

introduced by the LC filter.

Using a compensation network with R1 = 15 k

Ω, C6 = 22 nF and C5 = 150 pF, the gain and

phase bode plots are shown in Figure 32-34. The cutoff frequency Fc is 22 KHz and the

phase margin is 52°C.

-----------

⋅

–

--------------------

≈

πR

esr

out

⋅

---------------------------------------

π 0.065 300 10

–

⋅⋅

⋅

-------------------------------------------------------------

8.162KHz

out

⋅

⋅⋅

---------------------------------------

68 10

–

300 10

–

⋅⋅

⋅

⋅⋅

--------------------------------------------------------------------------

1.087kHz

ocomp

π R

⋅⋅

⋅

---------------------------------

π 15 10

22 10

–

⋅⋅

⋅

----------------------------------------------------------------

492Hz

π R

⋅⋅

⋅

----------------------------------

π 1.2 10

22 10

–

⋅⋅

⋅

-----------------------------------------------------------------

6.029Hz

π R

⋅⋅

⋅

----------------------------------

π 15 10

150 10

–

⋅⋅

⋅

---------------------------------------------------------------------

70KHz

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