L5981_08 datasheet(20/37 Pages) STMICROELECTRONICS

20 / 37 page

Application information

L5981

20/37

Equation 18

where K is the feed forward constant and 1 / K is equals to 9.

Calculate C4 by placing the zero at 50 % of the output filter double pole frequency (fLC):

Equation 19

Calculate C5 by placing the second pole at four times the system bandwidth (BW):

Equation 20

Set also the first pole at four times the system bandwidth and also the second zero at

the output filter double pole:

Equation 21

The suggested maximum system bandwidth is equals to the switching frequency divided by

3.5 (FSW / 3.5), anyway lower than 100 kHz if the FSW is set higher than 500 kHz.

For example with VOUT = 3.3 V, VIN = 12 V, IO = 1 A, L =33 μH, COUT = 22 μF, ESR < 1 mΩ,

the type III compensation network is:

In Figure 12 is shown the module and phase of the open loop gain. The bandwidth is about

56 kHz and the phase margin is 53 °.

----------

---- R

⋅⋅

π R

⋅⋅

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

π R

4BW

⋅

–

⋅⋅⋅

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

4BW

⋅

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

–

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

π R

4BW

⋅⋅ ⋅

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

4.99k

1.1k

110

5.6k

4.7nF

10nF

100pF

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