Datasheet

LT4430
17
4430fc
For more information www.linear.com/LT4430
applicaTions inForMaTion
Figure 6b. Frequency Compensation with Opto-Coupler Common-Collector Configuration
–
+
–
+
V
CC
V
REF
V
C
R
E
R
K
PRIMARY-SIDE
ERROR AMP
OPTO
DRIVER
OPTO
OPTO
LT4430
COMP
1.1V
0.6V
FB
–
+
ERROR
AMP
R3
R4
15k
R5
90k
4430 F06b
C
K
ISOLATION
BARRIER
V
OUT
FB
C3
R1
R2
C1
C2
R
C
C
C
In this example, the error amplifier is typically a trans-
conductance amplifier
with high output impedance and
R
C
dominates the impedance at the V
C
node. Frequency
compensation for this feedback loop is directly affected by
the output transistor’s collector-to-base capacitance as it
introduces a pole into the feedback loop. This pole varies
considerably with the transistor’s operating conditions. In
many cases, this pole limits the achievable loop bandwidth.
Cascoding the output transistor significantly reduces the
effects of this capacitance and increases achievable loop
bandwidth. However, not all designs have the voltage
headroom required for the cascode connection or can
tolerate the additional circuit complexity. The open loop
transfer function from the output voltage to the primary-
side error amplifier’s output is:
V
C
V
OUT
=
−A •
R2
R1+R2
⎛
⎝
⎜
⎞
⎠
⎟
•(1+s+R1• C1) • (1+ s •R3 • C3)
[s • A • R1•(C2+C3)]• 1+s •R3 •
(C2 • C3)
(C2+C3)
⎛
⎝
⎜
⎞
⎠
⎟
•
6 •
(1+ s •R
K
• C
K
)
1+s •
(R
K
•R
D
)
(R
K
+R
D
)
• C
K
⎛
⎝
⎜
⎞
⎠
⎟
•
CTR •R
C
(R
K
+R
D
)
•
1
1+s •r
π
•
(CTR • R
C
)
(R
K
+R
D
)
• C
CB
+C
BE
⎡
⎣
⎢
⎤
⎦
⎥
⎛
⎝
⎜
⎞
⎠
⎟
•
1
(1+ s •R
C
• C
C
)
where:
A = LT4430 open loop DC Gain
R
D
= Opto-coupler diode equivalent small-signal
resistance
CTR = Opto-coupler AC current transfer ratio
C
CB
= Opto-coupler nonlinear collector-to-base
capacitor
C
BE
= Opto-coupler nonlinear base-to-emitter
capacitor
r
π
= Opto-coupler small-signal base-to-emitter
resistor
Figure 6
a and its transfer function illustrate most of the
possible poles and zeroes that can be set and are shown
for the sake of completeness. In a practical application, the
transfer function simplifies considerably because not all
the poles and zeroes are used. Also, different combinations
of poles and zeroes can result in the same small signal
gain-phase characteristics but demonstrate dramatically
different large-signal behavior.
The common-collector configuration eliminates the miller
effect of the output transistor’s collector-to-base capaci
-
tance and
generally increases achievable loop bandwidth.
Figure
6b illustrates the common-collector design with the
output transistor’s emitter connected to the inverting input
of the primary-side controller’s error amplifier.