IR3853M_15 Datasheet, PDF (22/35 Pages) International Rectifier – HIGHLY EFFICIENT INTEGRATED 4A, SYNCHRONOUS BUCK REGULATOR

Fig. 15. Type II compensation network
and its asymptotic gain plot
The transfer function (V_e/V_o) is given by:
V_{e = H(s) = −} Z_{f = −} 1+ sR₃C_{4 .....(19)}
V_o
Z_IN
sR₈C₄
The (s) indicates that the transfer function varies
as a function of frequency. This configuration
introduces a gain and zero, expressed by:
H(s) = ^R3 ...................................... (20)
R₈
F_z
=
2π
*
1
R₃ *
C₄
............................ (21)
First select the desired zero-crossover frequency
(F_o ):
F_o > F_ESR and F_o ≤ (1/5 ~ 1/10) * F_s
Use the following equation to calculate R3:
R₃
=
V_osc
*
F_o
V_in
*
*
F_ESR
F_L²_C
*
R₈
........................... (22)
PD-97516
IR3853MPbF
Where:
V_in = Maximum Input Voltage
V_osc = Oscillator Ramp Voltage
F_o = Crossover Frequency
F_ESR = Zero Frequency of the Output Capacitor
F_LC = Resonant Frequency of the Output Filter
R₈ = Feedback Resistor
To cancel one of the LC filter poles, place the
zero before the LC filter resonant frequency pole:
F_z = 75% F_LC
F_z = 0.75* _2π
1
..................................... (23)
L_o * C_o
Use equations (21), (22) and (23) to calculate
C4.
One more capacitor is sometimes added in
parallel with C4 and R3. This introduces one
more pole which is mainly used to suppress the
switching noise.
The additional pole is given by:
F_P
=
2π * R₃ *
1
C₄ * C_POLE
C₄ + C_POLE
...............................(24)
The pole sets to one half of the switching
frequency which results in the capacitor C_POLE:
C_POLE =
1
π*R₃*F_s
−¹
C₄
≅¹
π*R₃*F_s
....................(25)
For a general solution for unconditional stability
for any type of output capacitors, and a wide
range of ESR values, we should implement local
feedback with a Type-III compensation network.
The typically used compensation network for
voltage-mode controller is shown in figure 16.
Again, the transfer function is given by:
V_{e = H(s) = −} Z_f
V_o
Z_IN
By replacing Z_in and Z_f according to figure 16,
the transfer function can be expressed as:
H(s) =
− (1+ sR₃C₄ )[1+ sC₇ (R₈ + R₁₀ )]
sR₈ (C₄
+
C₃
)_⎢⎡1+
⎢⎣
sR_{3 ⎜⎜⎝⎛}
C₄
C₄
* C₃
+ C₃
⎟⎟⎠⎞⎥⎥⎦⎤⁽¹⁺
sR₁₀C₇
)
....... (26)
22
Rev 4.0