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L7980 Fiches technique(PDF) 25 Page - STMicroelectronics

No de pièce L7980
Description  2 A step-down switching regulator
PDF  44 Pages
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Fabricant  STMICROELECTRONICS [STMicroelectronics]
Site Internet  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

L7980 Fiches technique(HTML) 25 Page - STMicroelectronics

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L7980
Application informations
Doc ID 15181 Rev 4
25/44
Figure 14.
Open loop gain: module bode diagram
The guidelines for positioning the poles and the zeroes and for calculating the component
values can be summarized as follow:
1.
Choose a value for R1, usually between 1 kΩ and 5 kΩ, in order to have values of C4
and C5 not comparable with parasitic capacitance of the board.
2.
Choose a gain (R4/R1) in order to have the required bandwidth (BW), that means:
Equation 25
Where fESR is the ESR zero:
Equation 26
and Vs is the saw-tooth amplitude. The voltage feed forward keeps the ratio Vs/Vin constant.
3.
Calculate C4 by placing the zero one decade below the output filter double pole:
Equation 27
4.
Then calculate C3 in order to place the second pole at four times the system bandwidth
(BW):
R
4
f
ESR
f
LC
------------
⎝⎠
⎛⎞
2
BW
f
ESR
------------
V
S
V
IN
--------- R
1
⋅⋅
⋅
=
f
ESR
1
2
π ESR C
OUT
⋅⋅
--------------------------------------------
=
C
4
10
2
π R
4
f
LC
⋅⋅
-------------------------------
=



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