LTC1698 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1698' PDF : 24 Pages View PDF
LTC1698
APPLICATIO S I FOR ATIO
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 RD, CTR, CDE and rπ vary
from part to part. They also change with bias current. The
dominant pole of the opto feedback is due to RF and CF. The
feedforward capacitor CK 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 RK and RD form a high frequency pole with
CK. For most optocouplers, RD 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 VC/VOUT
equation has been simplified and assumes that CFB<<CC.
The actual term is:
s
• R2
• (CC
+
CFB ) •
1+
s •RC
•
CC
CC
• CFB
+ CFB
VC
VOUT
=–
(1+ s • CC •RC)
• 5 • (1+ s •RK • CK)
(s •R2 • CC)• (1+ s •RC • CFB)
1+
s
•
CK
•
RK
RK
• RD
+ RD
•
RF • CTR
RD + RK
•
(1+
s
•
1
rπ
•
C DE )
•
(1+
s
•
1
RF
•
CF)
where:
RD = Optocoupler diode equivalent small − signal resistance
CTR = Optocoupler current transfer ratio
CDE = Optocoupler nonlinear capacitor across base to emitter
rπ = Optocoupler small − signal resistance across the base emitter
LT3781
VC
CF
VREF
VFB
RF
VCC
RE
OPTODRV
RK
CK
MOC207
100k
VREF
20k
LTC1698
VREF
VFB
VOUT
R2
VCOMP
R1
CC
RC
CFB
1698 F07
Figure 7. Error Signal Feedback
1698f
13
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