NX2142A MICROSEMI [Microsemi Corporation], NX2142A Datasheet - Page 11

• Manufacturer Part Number: NX2142A
• Description: SINGLE CHANNEL PWM CONTROLLER WITH FEEDFORWARD AND 5V BIAS REGULATOR
• Manufacturer: MICROSEMI [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 ca-
pacitors have to be chosen after the test. Typically, for
high frequency capacitor such as high quality POSCAP
especially ceramic capacitor, 20% to 100% (for ce-
ramic) 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 param-
eters.
Compensator Design
the power stage, the power system has 180
shift , and therefore, is unstable by itself. In order to
achieve accurate output voltage and fast transient re-
sponse, 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 fre-
quency, phase margin greater than 50
crossing 0dB with -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 crossover frequency. Otherwise type
III compensator should be chosen.
NX2142 to compensate the output voltage variation
caused by input voltage changing. The feedforward
funtion is realized by using VIN pin voltage to program
the oscillator ramp voltage V
voltage, which provides nearly constant power stage
gain under wide voltage input range.
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 com-
pensate the system with type III compensator. The
Rev. 1.1
10/28/07
It should be considered that the proposed equa-
Due to the double pole generated by LC filter of
Voltage feedforward compensation is used in
For low ESR output capacitors, typically such as
OSC
at about 1/10 of V
o
and the gain
o
phase
IN
following figures and equations show how to realize
the type III compensator by transconductance ampli-
fier.
the compensator.
transconductance amplifier is given by:
compensator is
the compensator of transconductance amplifier must
satisfy this condition: R4>>2/gm. And it would be de-
sirable if R1||R2||R3>>1/gm can be met at the same
time,
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
Zin
Figure 9 - Type III compensator using
e
e
C3
R3
F
F
F
F
Z1
Z2
P1
P2
1 g
Z1
Z
Z
,F
in
2
2
2
2
Vout
f
Z2
m
R2
1 g
R1
,F
transconductance amplifier
P1
Z
1
1
R
R
(R
R
m
in
and F
4
3
4
2
1
1
NX2142/2142A
Z
Vref
C
Z / R
C
C
C
Fb
f
C2
R ) C
in
2
3
1
1
P2
3
C
are poles and zeros in
C
C1
1
2
Zf
2
gm
3
R4
...(13)
...(14)
...(11)
...(12)
Ve
11