nx2113 Microsemi Corporation, nx2113 Datasheet - Page 11

• Manufacturer Part Number: nx2113
• Description: 300khz & 600khz Synchronous Pwm Controller With Programmable Bus Uvlo
• Manufacturer: Microsemi Corporation

tion is based on ideal case, in reality, the droop or over-
shoot is typically more than the calculation. The equa-
tion gives a good start. For more margin, more capaci-
tors have to be chosen after the test. Typically, for high
frequency capacitor such as high quality POSCAP es-
pecially ceramic capacitor, 20% to 100% (for ceramic)
more capacitors have to be chosen since the ESR of
capacitors is so low that the PCB parasitic can affect
the results tremendously. More capacitors have to be
selected to compensate these parasitic parameters.
Compensator Design
power stage, the power system has 180
and therefore, is unstable by itself. In order to achieve
accurate output voltage and fast transient response,
compensator is employed to provide highest possible
bandwidth and enough phase margin. Ideally, the Bode
plot of the closed loop system has crossover frequency
between 1/10 and 1/5 of the switching frequency, phase
margin greater than 50
20dB/decade. Power stage output capacitors usually
decide the compensator type. If electrolytic capacitors
are chosen as output capacitors, type II compensator
can be used to compensate the system, because the
zero caused by output capacitor ESR is lower than cross-
over frequency. Otherwise type III compensator should
be chosen.
A. Type III compensator design
Sanyo oscap and poscap, the frequency of ESR zero
caused by output capacitors is higher than the cross-
over frequency. In this case, it is necessary to compen-
sate the system with type III compensator. The follow-
ing figures and equations show how to realize the type III
compensator by transconductance amplifier.
Rev. 2.0
11/18/05
It should be considered that the proposed equa-
Due to the double pole generated by LC filter of the
For low ESR output capacitors, typically such as
o
and the gain crossing 0dB with -
o
phase shift ,
the compensator. Their locations are shown in figure 10.
transconductance amplifier is given by:
compensator is
the compensator of transconductance amplifier must
satisfy this condition: R4>>2/gm. R1||R2||R3>>1/gm
is desirable.
For the voltage amplifier, the transfer function of
where F
The transfer function of type III compensator for
To achieve the same effect as voltage amplifier,
V
V
Zin
V
V
OUT
OUT
Figure 9 - Type III compensator using
C3
R3
e
e
F
F
F
F
Z1
Z2
P1
P2
1 g
Z1
Z
Z
Vout
2
2
,F
in
2
2
f
R2
R1
Z2
m
1 g
transconductance amplifier
,F
P1
Z
1
1
R
R
(R
R
m
in
and F
4
3
4
2
1
1
Vref
Fb
Z
C
C
Z / R
C2
NX2113/2113A
C
C
R ) C
f
in
2
3
1
1
P2
3
C1
C
C
are poles and zeros in
1
Zf
gm
2
2
R4
3
...(13)
...(14)
...(11)
...(12)
Ve
11