rt9911 Richtek Technology Corporation, rt9911 Datasheet - Page 29

• Manufacturer Part Number: rt9911
• Description: 6 Channel Dc/dc Converters
• Manufacturer: Richtek Technology Corporation

The results we will get for CH4 asyn-boost in CCM
compensation are as follows:
The major steps for getting above results :
1.
2. Select suitable inductor to ensure I
3. Find RHPZ(Right Hand Plan Zero) location.
4. Set f
For example :
5.
6.
7. Find
8.
DS9911-04 August 2007
L, inductance
C
on ceramic output capacitor.
R
(ceramic output capacitor)
R2, the voltage divider resistor in between FB and ground.
C
R
C
connect to ground
C
ignored if C
L >
RHPZ(Boost
R
Find
R2 =
CCM,
where
Get
which is duty to V
D
which is the conjugate double pole from LC filter.
C
OUT
ESR
F
C
P
C
LOAD
C
, feedforward capacitor in parallel with R1.
, connect in between COMP pin and ground. (Can be
=
, compensation resistor on COMP pin.
, compensation capacitor in series with R
=
2
duty
R
, output capacitance. This compensation is based
, ESR (Equivalent Series Resistance) of C
C
the
R1
2
V
I x
C
f
(cross over frequency) sufficiently below RHPZ.
=
cdp
π
M
IN
=
OUT(MIN.)
x
I
load
f x
cycle
OUT(MAX)
=
x
GM
=
V
V
D
P
f
cdp
V
f
1
LP
OUT
f
2
OUT
V
C
C
< 10pF)
OUT
)
x
π
pole
V
IN
x
=
=
x
x V
(1
=
- 1
x
FB
G
-
R
RHPZ
R
f x
- 1
(LC)
-
,
V
doc
LOAD
,
D
C
R
P
OUT
D)
f :
OSC
D
FB
6
V
LOAD
to
LP
V
=
,
OUT
2
IN
where
transfer function.
cancel
=
duty
(1
o
,
2
2
r
=
-
.
π
π
lower.
D)
I
L
OUT(MAX.)
cycle
x
2
V
G
one
(M
OUT
,
doc
where
-
=
1)
of
=
2
- 1
x
(1
x
.
the
OUT(MIN.)
M
R
V
V
-
V
D)
LOAD
IN
OUT
1 -
double
IN
2
,
x
works in
C
pole.
OUT
C
and
OUT
,
9. Find C
10.Evaluate C
CH5 Asyn-Inverter Controller with External MOSFET
CH5 is an asyn-inverter controller driving external logic
level P type MOSFET, which employs voltage mode
control to regulate the output voltage. Compensation
depends on designing the loading range working in
discontinuous or continuous inductor current mode. (DCM
or CCM).
Asyn-Inverter in DCM :
We call it DCM because inductor current falls to zero on
each switch cycle. The benefit of designing in DCM is the
simple loop compensation, which has no RHPZ (Right
Hand Plan Zero) and conjugate double pole in the frequency
domain to worry about, but has a single load pole instead.
However, the output ripple and efficiency are worse than
in CCM (Continuous Inductor Current). If the loading is
around tens of mA, it is not bad to design in DCM with
less impact on the output ripple and efficiency, but gain
more easy to stabilize the control loop.
The fixed parameters for CH5 asyn-inverter in DCM
compensation are as follows:
Transconductance (from FB to COMP), GM = 200us
Internal voltage ramp to decide duty cycle, V
Feedback voltage, V
Reference voltage, V
double pole.
(ceramic output capacitor).
C
C
COMP
F
P
=
=
C
2
OUT
f
π
by placing its zero on f
×
C
f
x
cdp
P
1
P
R
. C
R
ESR
×
R
C
C
C
R1
P
C
GM
is for canceling the zero from C
.
+
-
.
20k
C
FB
FB
REF
P
Figure 10
0V
can
= FB = 0V
= 1V
V
OUT
be
R1
R2
4.7uF
ignore
cdp
C
F
V
REF
to cancel another
if
RT9911
R
C
www.richtek.com
= 1V
C
ESR
OUT
P
<
10pF.
P
= 1V
I
OUT
OUT
29