nx2154 Microsemi Corporation, nx2154 Datasheet - Page 8

• Manufacturer Part Number: nx2154
• Description: 300khz Synchronous Pwm Controller
• 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 fre-
quency between1/10 and 1/5 of the switching frequency,
phase margin greater than 50
0dB with -20dB/decade. Power stage output capacitors
usually decide the compensator type. If electrolytic
capacitors are chosen as output capacitors, type II com-
pensator can be used to compensate the system, be-
cause the zero caused by output capacitor ESR is lower
than crossover frequency. Otherwise type III compensa-
tor 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.1.2
02/26/07
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
o
phase shift ,
the compensator. Their locations are shown in figure 4.
transconductance amplifier is given by:
compensator is
the compensator of transconductance amplifier must
satisfy this condition: R
able if R
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
V
V
OUT
OUT
Figure 3 - Type III compensator using
1
e
e
Zin
||R
F
F
F
F
C3
R3
Z1
Z2
P1
P2
2
||R
1 g
Z1
Z
Z
3
2
2
,F
in
2
2
>>1/gm can be met at the same time.
f
Vout
Z2
m
1 g
transconductance amplifier
R2
,F
R1
P1
Z
1
1
R
R
(R
R
m
in
4
and F
4
3
4
>>2/gm. And it would be desir-
2
1
1
Z
C
NX2154/2154A
C
Z / R
C
Vref
C
R ) C
f
Fb
in
2
3
C2
1
1
P2
3
C
C
are poles and zeros in
1
C1
2
2
Zf
gm
3
R4
...(13)
...(14)
...(11)
...(12)
Ve
8