Datasheet

13
LTC1698
1698f
supply must cross the isolation boundary. Coupling this
signal requires an element that will withstand the isolation
potentials and still transfer the loop error signal.
Optocouplers are widely used for this function due to their
ability to couple DC signals. To properly apply them, a
number of factors must be considered. The gain, or
current transfer ratio (CTR) through an optocoupler is
loosely specified and is a strong function of the input
current through the diode. It changes considerably as a
function of time (aging) and temperature. The amount of
aging accelerates with higher operating current. This
variation directly affects the overall loop gain of the sys-
tem. To be an effective optical detector, the output transis-
tor of the optocoupler must have a large base area to
collect the light energy. This gives it a large collector to
base capacitance which can introduce a pole into the
feedback loop. This pole varies considerably with the
current and interacts with the overall loop frequency
compensation network.
The common collector optocoupler configuration removes
the miller effect due to the parasitic capacitance and
increases the frequency response. Figure 7 shows the
optocoupler feedback circuitry using the common collec-
tor approach. Note that the terms R
D
, CTR, C
DE
and r
π
vary
from part to part. They also change with bias current. The
dominant pole of the opto feedback is due to R
F
and C
F
. The
feedforward capacitor C
K
at the optocoupler creates a low
frequency zero. This zero should be chosen to provide a
phase boost at the loop crossover frequency. The parallel
combination of R
K
and R
D
form a high frequency pole with
C
K
. For most optocouplers, R
D
is 50Ω at a DC bias of 1mA,
and 25Ω at a DC bias of 2mA. The CTR term is the small
signal AC current transfer ratio. For the QT Optoelectron-
ics MOC207 optocoupler used here, the AC CTR is around
1, even though the DC CTR is much lower when biased at
1mA or 2mA. The first denominator term in the V
C
/V
OUT
equation has been simplified and assumes that C
FB
<<C
C
.
The actual term is:
sR C C sR
CC
CC
CFB C
CFB
CFB
••( )• ••
•
21++
+






APPLICATIO S I FOR ATIO
WUUU
V
V
sC R
sR C sR C
sR C
sC
RR
RR
R CTR
RR srC sRC
C
OUT
CC
CCFB
KK
K
KD
KD
F
DK DE FF
=
+
+
+
+
+






+






++
π
–
(••)
(••)•( •• )
••
(••)
••
•
•
•
•
(••)
•
(••)
1
21
5
1
1
1
1
1
1
wherewhere
R Optocoupler diode equivalent small signalresis ce
CTR Optocoupler current transfer ratio
C Optocoupler nonlinear capacitor across base to emitter
r Optocoupler small signalresis ce across the base emitter
D
DE
:
tan
tan
=−
=
=
=−
π
20k
V
REF
V
COMP
C
C
C
FB
R
C
V
REF
R2
V
OUT
LTC1698
V
FB
1698 F07
100k
OPTODRV
R
K
C
K
MOC207
–
+
–
+
V
REF
LT3781
V
FB
V
CC
C
F
V
C
R
F
R1
R
E
–
+
Figure 7. Error Signal Feedback