rt9206 Richtek Technology Corporation, rt9206 Datasheet - Page 18

• Manufacturer Part Number: rt9206
• Description: High Efficiency, Synchronous Buck With Dual Linear Controllers
• Manufacturer: Richtek Technology Corporation

RT9206
The effective series resistance (ESR) of capacitor and
capacitance introduces one zero into system, the zero is
given as :
In the voltage-mode Buck converter shown in Figure 5, the
loop gain of system is
The desired loop gain and phase margin is show in the
Bode plot of Figure 8.
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18
Figure 8. The Bode plot of desired loop gain and phase
T
L(S)
VIN/Vr
Phase
-180 °
TL(s)
-90 °
0°
Gain
G(s)
C
(S)
x
Vr
f
1
Z
fp
Power
x
stage
2
G
loop gain
Desired
P(S)
x
margin
rc x
1
x
H
fz
(S)
C
O
fc
C
(S)
x
margin
Phase
G
(S)
(Hz)
x
H
(S)
f
f
f
(22)
(23)
Where the f
frequency when the loop gain equals unity. Typically, f
chosen in range 1/10 ~ 1/20 of switching frequency. f
determines how fast the dynamic load response is. The
higher f
margin in the range of 45 ~ 60 is desirable.
So, the transfer function of compensator C(s) must be
designed to meet these requirements. In many applications,
use an electrolytic capacitor as the output capacitor, if the
zero (f
capacitor is a few kHz and smaller than 8 times f
type 2 (PI) can be used to get desired compensation. Figure
9 shows the typical type 2 trans-conductance error
amplifier and the Bode plot is also shown in Figure 10.
Figure 10. The Bode plot of type 2 trans-conductance
Figure 9. The typical type 2 trans-conductance error
Z
C
) caused by effective series resistance (ESR) of
gmRc1
with the faster dynamic response, and the phase
Ra
Rb
-90
C
V
OUT
is zero crossover frequency defined as the
Gain(dB)
Phase
V
REF
error amplifier
fcz
amplifier.
+
-
gm
Boost
DS9206-11 March 2007
Rc1
Cc1
Vc
fcp
Cc2
f
f
P
, the
C
be
C