NX2715
pensate the system with type III compensator. The
following figures and equations show how to realize
the type III compensator by transconductance ampli-
fier.
The number of capacitors has to satisfy both ripple
and transient requirement. Overall, we choose N=2.
It should be considered that the proposed equa-
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.
1
FZ1
FZ2
=
=
=
=
...(11)
...(12)
...(13)
...(14)
2´ p ´ R4 ´ C2
1
2´ p ´ (R2 + R3 )´ C3
1
F
P1
2´ p ´ R3 ´ C3
1
F
P2
C1 ´ C2
2´ p ´ R4 ´
C1 + C2
where FZ1,FZ2,FP1 and FP2 are poles and zeros in
the compensator.
Compensator Design
The transfer function of type III compensator for
transconductance amplifier is given by:
Due to the double pole generated by LC filter of
the power stage, the power system has 180o phase
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 50o and the gain
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.
Ve
1- gm ´ Zf
=
VOUT
1+ gm ´ Zin + Zin /R1
For the voltage amplifier, the transfer function of
compensator is
Ve
- Zf
=
VOUT
Zin
To achieve the same effect as voltage amplifier,
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.
Zf
Vout
Zin
R3
C3
C1
Voltage feedforward is used in NX2715 to com-
pensate 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 VOSC at about 1/10 of VIN voltage, which pro-
vides nearly constant power stage gain under wide volt-
age input range.
C2
R4
R2
R1
Fb
Ve
gm
Vref
A. Type III compensator design
For low ESR output capacitors, typically such as
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-
Figure 11 - Type III compensator using
transconductance amplifier
Rev. 1.4
01/08/08
13
