Datasheet

LT4430
18
4430fc
For more information www.linear.com/LT4430
applicaTions inForMaTion
In this example, the error amplifier is typically a voltage
error amplifier configured as a transimpedance amplifier.
The opto-coupler transistor’s emitter provides feedback
information directly to the FB pin and the resistor R
E
from
FB to GND sets the DC bias condition for the opto-coupler.
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
• C
BE
( )
•
1
(1+ s •R
C
• C
C
)
Figure 6b 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.
In both configurations, the terms R
D
, CTR, r
π
, C
CB
and C
BE
.
vary from part to part and also change with bias current.
For most opto-couplers, R
D
is 50Ω at a DC bias of 1mA,
and 25Ω at a DC bias of 2mA. CTR is the small signal AC
current transfer ratio. As an example, the Fairchild MOC207
opto-coupler has an AC CTR around 1, even though the
DC CTR is much lower when biased at 1mA or 2mA. Most
opto-coupler data sheets do not specify the terms C
CB
,
C
BE
and r
π
and values must be obtained from empirical
measurements.
This frequency compensation discussion only addresses
the transfer function from the output back to the control
node on the primary-side. Compensation of the entire
feedback loop must combine this transfer function with
the transfer function of the power processing circuitry,
commonly referred to as the modulator. In an isolated
power
supply, the modulator’s transfer function depends
on
topology (flyback, forward, push-pull, bridge), current
or voltage mode control, operation in discontinuous or
continuous mode, input/output voltage, transformer turns
ratio and output load current. It is beyond this data sheet’s
scope to detail the transfer functions for all of the vari
-
ous combinations. However, the power supply designer
must fully characterize and understand the modulator’s
transfer function to successfully frequency compensate
the feedback loop for all operating conditions.
Opto-Couplers
Opto-couplers are available in a wide variety of package
styles and performance criteria including isolation rating,
CTR, output transistor breakdown voltage, output transistor
current capability, and response time. Table 1 lists several
manufacturers of opto-coupler devices, although this is
by no means a complete list.
Table 1. Opto-Coupler Vendors
VENDOR PHONE URL
Agilent Technologies 800-235-0312 www.agilent.com
Fairchild Semiconductor 207-775-8100 www.fairchildsemi.com
Isocom 214-495-0755 www.isocom.com
Kodenshi Korea Corp. 82-63-839-2111 www.kodenshi.co.kr
NEC 81-44-435-1588 www.ncsd.necel.com
Sharp Microelectronics 877-343-2181 www.sharpsma.com
Toshiba 949-455-2000 www.toshiba.com
Vishay 402-563-6866 www.vishay.com