L5985_08 Datasheet, PDF (20/37 Pages) STMicroelectronics – 2 A step-down switching regulator

L5985
Figure 11. Open loop gain: module bode diagram
Application information
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 k and 5 k.
2. Choose a gain (R₄/R₁) in order to have the required bandwidth (BW), that means:
Equation 18
R₄
=
B_-----W_----
f_LC
⋅
-¹--
K
⋅
R₁
where K is the feed forward constant and 1/K is equals to 9.
3. Calculate C₄ by placing the zero at 50 % of the output filter double pole frequency (f_LC):
Equation 19
C₄
=
-------------¹--------------
π ⋅ R₄ ⋅ f_LC
4. Calculate C₅ by placing the second pole at four times the system bandwidth (BW):
Equation 20
C₅ = ₂----_π-----_â‹---_R-----₄----_â‹---_C----^C₄----_â‹⁴---₄-----_â‹---_B----_W--------–----₁--
5. Set also the first pole at four times the system bandwidth and also the second zero at
the output filter double pole:
Equation 21
R₃
=
-----------^R----¹----------- ,
4_-----⋅_---B_-----W_{---- – 1}
f_LC
C₃
=
-------------------¹---------------------
2π ⋅ R₃ ⋅ 4 ⋅ BW
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