LT4430HS6 LINER [Linear Technology], LT4430HS6 Datasheet - Page 17

• Manufacturer Part Number: LT4430HS6
• Description: Secondary-Side Opto-Coupler Driver Ground-Referenced Opto-Coupler Drive
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
In this example, the error amplifi er is typically a trans-
conductance amplifi er with high output impedance and
R
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 signifi cantly 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 amplifi er’s output is:
V
C
V
OUT
C
dominates the impedance at the V
=
6
– •
⎛
⎜
⎝
(
[ • •
1
V
s A R
1
A
•
C
+
+
⎛
⎜
⎝
1
s R
⎛
⎜
⎝
s r
C
+
PRIMARY-SIDE
C
•
R
• •
ERROR AMP
(
1
1
s
1
π
R
1
C
+
•
+
• (
2
•
(
R
(
R
s R
⎡
⎢
⎣
R
Figure 6b. Frequency Compensation with Opto-Coupler Common-Collector Confi guration
C
C
+
–
2
R
(
K
•
(
2
K
CTR R
R
C
C
⎞
⎟
⎠
FB
+
)
K
+
•
• (
K
R
V
R R
+
C
REF
1
•
D
•
D
C
1
+
3
R
)
)
)]•
K
D
s R C
C
•
)
)
)
•
C
⎛
⎜
⎝
•
K
1
C
1 1 1
⎞
⎟
⎠
V
+
• )• (
CB
CC
ISOLATION
•
BARRIER
R
s R
E
(
CTR R
R
C
•
+
K
node. Frequency
C
3
+
BE
•
+
•
R
(
R
⎤
⎥
⎦
(
C
OPTO
s R
C
D
K
⎞
⎟
⎠
C
2
•
)
2
•
•
+
•
3
C
C
C
•
3
K
3
C
)
)
3
OPTO
⎞
⎟
⎠
)
•
90k
R5
where:
Figure 6a 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 simplifi es 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 confi guration 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 amplifi er.
DRIVER
OPTO
A = LT4430 open loop DC Gain
R
resistance
CTR = Opto-coupler AC current transfer ratio
C
capacitor
C
capacitor
r
resistor
π
CB
BE
D
+
–
= Opto-coupler small-signal base-to-emitter
= Opto-coupler diode equivalent small-signal
= Opto-coupler nonlinear base-to-emitter
= Opto-coupler nonlinear collector-to-base
1.1V
15k
R4
LT4430
COMP
C3
ERROR
C2
AMP
R3
+
–
FB
0.6V
4430 F06b
V
OUT
R1
R2
LT4430
C1
17
4430fb