LTC1698 LINER [Linear Technology], LTC1698 Datasheet - Page 14

• Manufacturer Part Number: LTC1698
• Description: Isolated Secondary Synchronous Rectifier Controller
• Manufacturer: LINER [Linear Technology]

LTC1698
APPLICATIO S I FOR ATIO
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 0.1 F. 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
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
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
14
DD
– 2.5V). This should meet the design consideration
V
OUT
SECSEN
R
CILM
C
CILM
. The current limit transconductance
13
5
U
LTC1698
OPTODRV
I
COMP
U
100k
LIM
W
OPTO
or OVP comparator
M
+
–
ILIM
Figure 8. Secondary Average Current Limit
R
3k
ILIM
V
20k
REF
U
V
COMP
I
LIM
–
+
amplifier I
Figure 8, if the secondary current is small, the I
goes low and the transistor M
at V
ary current is large, I
sistor M
overrides the voltage feedback and forces OPTODRV high
and injects maximum current into the external optocoupler.
The R
the current sensed and the OPTODRV output.
The I
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:
where:
R
I
f
DRIVE
LMAX
SW
SECSEN
R
R
25mV
+
COMP
= Forward converter switching frequency
SECSEN
6
SNS
I
ILIM
= Maximum allowed secondary current
SNSGND
ILIM
I
SNS
2
= Secondary current sense resistor
CG
determines the OPTODRV output. If the second-
FG
and I
resistor provides a linear relationship between
• •
LIM
16
12
11
to turn on hard. Thus the current limit circuit
2
25
I
C
SNSGND
has a – 25mV threshold. As shown in
LMAX
1
2
mV
•
C2
f
COMP
10
SW
pins allow a true Kelvin current
R
(OPTIONAL)
DIV
pulls high and forces the tran-
ILIM
R6
R6
shuts off. The potential
Q4
T1
Q3
R
1698 F08
SECSEN
COMP
1698f
pin