rt9206 Richtek Technology Corporation, rt9206 Datasheet - Page 19

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

The design procedure as following :
(1). Selecting the zero crossover frequency f
(2). Place the zero of compensator is 70% fp that is
Design example
Design example of type 2 compensator: the schematic is
shown in Figure 4, where the parameters as following:
V
frequency=200kHz, L=15 H, C
parameters of RT9206 as following: gm=1.6ms, ramp
amplitude=1.9V,and reference voltage Vref=0.8V.
Step1. Determine the power stage poles and zeros. The
pole caused by the output inductor and output capacitor is
calculated as :
DS9206-11 March 2007
(3). Set a second pole to suppress the switching noise.
IN
=12V,
1/20 switching frequency. Then according equation (24)
set the resistor R
frequency.
resonance frequency of power stage. The compensator
capacitor Cc1 can be selected to set the zero. The
equation is shown in following :
f =
f =
Assume the pole is one half of switching frequency
f
P
Z
s
, which results in capacitor Cc2 as shows in following:
2
2
C
C2
L C
r
1
1
C
=
R
V
C1
OUT
C
O
O
R
=
=5V,
=
C1
C =
V
C1
2
Vr L f
C1
2
IN
to determine the zero crossover
1
fs -
15
gm r
0.7 R
22mΩ 940 F
C
L C
1
1
C1
I
OUT
C
O
1
940
C
=940 F, r
O
C1
=5A,
V
V
OUT
R
REF
= 1.34kHz
1
C1
C
= 7.7kHz
=22m , the
fs
(F)
( )
switching
C
is 1/10 ~
(F) (26)
(24)
(25)
Step2. Determine the zero crossover frequency and
Select desired zero-crossover frequency :
Step3. Determine desired location of poles and zeros for
Select:
Assume
Step4. Calculate the real parameters-resistor and
From equation (21), the R
Select R
Calculate C
Select C
Second capacitor C
(26)
Select C
C
C2
R
=
f
compensated type.
type2 compensator.
CZ
C 1
C =
capacitors for type2 compensator.
22m
C2
C1
C1
20kH z 15 H 1.9
C1
=
= 0.7 f = 0.7 1.34kHz = 938Hz
= 22nF
= 220pF
= 8.2k
C1
R
r
C
from equation (25)
0.7 R
C1
1
f
C
L C
f
CP
V
12V 1.6m s
f
P
L
S
IN
C2
Select f
=
O
C1
f
C
f
can be calculated using equation
2
Vr
S
gm
=
= 100kHz
f /10 ~ f /20
S
C1
8.2k
15
0.7 8.2k
is calculated as following :
C
V
V
= 20kHz
O U T
R E F
1
940
S
200kHz
0.8V
5V
RT9206
= 20.7nF
www.richtek.com
= 8.4k
194pF
19