Datasheet
14
LTC1698
1698f
A series RC network can be added in parallel with R2
(Figure 7) to provide a zero for the feedback loop fre-
quency compensation.
The opto driver will drive a capacitive load up to 0.1µF. For
optocouplers with a base pin, switching signal noise can
get into this high impedance node. Connect a large resis-
tor, 1M or 2M between the base and the emitter. This
increases the diode current and the overall feedback
bandwidth slightly, and decreases the optocoupler gain.
When designing the resistor in series with the optocoupler
diode, it is important to consider the part to part variations
in the current transfer ratio and its reduction over tem-
perature and aging. The bigger the biasing current, the
faster the aging. The LTC1698 opto driver is designed to
source up to 10mA of current and swing between 0.4V to
(V
DD
– 2.5V). This should meet the design consideration
of most optocouplers.
Besides the voltage feedback function, the LTC1698 opto
driver couples fault signals to the primary controller and
prevents catastrophic damage to the circuit. Upon current
limit or an overvoltage fault, the I
LIM
or OVP comparator
overrides the error amplifier output and forces the
OPTODRV pin high. This sources maximum current into
the external optodiode and reduces the forward converter
duty cycle.
Average Current Limit
The secondary current limit function is implemented by
measuring the negative voltage across the current sense
resistor R
SECSEN
. The current limit transconductance
amplifier I
LIM
has a –25mV threshold. As shown in
Figure 8, if the secondary current is small, the I
COMP
pin
goes low and the transistor M
ILIM
shuts off. The potential
at V
COMP
determines the OPTODRV output. If the second-
ary current is large, I
COMP
pulls high and forces the tran-
sistor M
ILIM
to turn on hard. Thus the current limit circuit
overrides the voltage feedback and forces OPTODRV high
and injects maximum current into the external optocoupler.
The R
ILIM
resistor provides a linear relationship between
the current sensed and the OPTODRV output.
The I
SNS
and I
SNSGND
pins allow a true Kelvin current
sense measurement and offer true differential measure-
ment across the sense resistor. A differential lowpass
filter formed by R6 and C2 removes the pulse-to-pulse
inductor current ripple and generates the average sec-
ondary current which is equal to the load current. The
lowpass corner frequency is typically set to 1 to 2 orders
of magnitude below the switching frequency and follows
the relationship:
R
mV
I
R
C
f
SECSEN
LMAX
SW
=
=
π
25
6
1
22
10
•• •
where:
R
SECSEN
= Secondary current sense resistor
I
LMAX
= Maximum allowed secondary current
f
SW
= Forward converter switching frequency
APPLICATIO S I FOR ATIO
WUUU
–
+
–
+
12
11
2
16
OPTO
I
LIM
+
OPTODRV
V
OUT
R
CILM
C
CILM
V
REF
V
COMP
LTC1698
I
SNSGND
I
SNS
FG
CG
DRIVE
25mV
20k
100k
M
ILIM
R
ILIM
3k
5
I
COMP
13
C2
Q4
Q3
T1
R
SECSEN
1698 F08
R6
R6
R
DIV
(OPTIONAL)
Figure 8. Secondary Average Current Limit