AN937 Datasheet, PDF (24/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

Typical application
AN937
F_p1
=
----------------¹-----------------
2 ⋅ π ⋅ R_o ⋅ C_c
=
--------------------------------¹---------------------------------
2 ⋅ π ⋅ 1.2 ⋅ 10⁶ ⋅ 22 ⋅ 10^–9
=
6.029Hz
F_p2
=
----------------¹-----------------
2 ⋅ π ⋅ R_c ⋅ C_o
=
--------------------------------¹---------------------------------
2 ⋅ π ⋅ 15 ⋅ 10³ ⋅ 82 ⋅ 10^–12
=
125KHz
F_oLN
=
----------------¹-----------------
2 ⋅ π ⋅ R₂ ⋅ C₉
=
---------------------------------¹---------------------------------
2 ⋅ π ⋅ 2.7 ⋅ 10³ ⋅ 4.7 ⋅ 10^–9
=
12KHz
F_pLN
=
-----------------------¹------------------------
2
⋅
π
⋅
-^R-----²----^â‹---^R-----³--
R₂ + R₃
⋅
C₉
=
----------------------------------¹------------------------------------
2 ⋅ π ⋅ 1.75 ⋅ 10³ ⋅ 4.7 ⋅ 10^–9
=
19KHz
Using a compensation network with R4= 15 kΩ, C4 = 22 nF and C5 = 82 pF, the gain and
Phase Bode plots are shown in Fig. 31-32. The cut-off frequency and the phase margin are:
V_OUT = 3.3V
V_OUT = 5V
V_OUT = 12V
V_OUT = 15V
V_OUT = 18V
V_OUT = 24V
Fc = 36KHz
Fc = 34KHz
Fc = 18KHz
Fc = 14KHz
Fc = 11KHz
Fc = 8.6KHz
Phase margin = 62°
Phase margin = 70°
Phase margin = 92°
Phase margin = 88°
Phase margin = 83°
Phase margin = 74°
Figure 29. Gain Bode plot open loop
90
79
68
57
46
35
24
13
2
9
20
0.1
1
Vout=3.3V
Vout=5V
Vout=12V
Vout=15V
Vout=18V
Vout=24V
0 dB
10
100
1 ^.10³
1 ^.10⁴
1 ^.10⁵
1 ^.10⁶
Frequency [Hz]
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Doc ID 5655 Rev 15