AN938 Datasheet, PDF (23/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

AN938
3.9
Typical application
PWM gain
Equation 15
V_-----C---C--
V_ct
=
-^V----^C----^C-----^â‹---⁶---
V_CC – 1
≈
6
where V_ct is the peak-to-peak sawtooth oscillator.
Pole and zero values are as follows:
Equation 16
Equation 17
F_o
=
-------------------¹-------------------
2 ⋅ πR_esr ⋅ C_out
=
------------------------------¹-------------------------------
2 ⋅ π ⋅ 0.065 ⋅ 300 ⋅ 10^–6
=
8.162KHz
Equation 18
Equation 19
Equation 20
F_p
=
-------------------¹-------------------
2 ⋅ π ⋅ L ⋅ C_out
=
------------------------------------¹-------------------------------------
2 ⋅ π ⋅ 68 ⋅ 10^–6 ⋅ 300 ⋅ 10^–6
=
1.087kHz
F_ocomp
=
2^-----⋅^---π^-----⋅^---1R-----_c^----⋅^---C^----_c^-
=
-------------------------------¹--------------------------------
2 ⋅ π ⋅ 15 ⋅ 10³ ⋅ 22 ⋅ 10^–9
=
492Hz
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 ⋅ 150 ⋅ 10^–12
=
70KHz
The compensation is found by choosing F_ocomp close to the frequency of the double pole
introduced by the LC filter.
Using a compensation network with R1 = 15 kΩ, C6 = 22 nF and C5 = 150 pF, the gain and
phase bode plots are shown in Figure 32-34. The cutoff frequency F_c is 22 KHz and the
phase margin is 52°C.
Doc ID 5410 Rev 10
23/33