Datasheet
LTC3766
30
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
Note that the ratios of the resistor dividers on the SW
and V
S
+
pins must be the same for ripple cancellation to
operate properly. This requires that:
K
R
=
R2
R1+ R2
=
69k •R4
69k •R4+R3 69k+ R 4
( )
where the 69k accounts for the internal resistance on the
V
S
+
and V
S
–
pins.
LOAD CURRENT (A)
0
0
OUTPUT VOLTAGE (V)
1
2
3
4
6
5
10 15 20
3766 F16
25
30
5
V
IN
= 72V
V
IN
= 36V
Figure 16. Typical Current Limit Performance
Figure 15. Setting R
IPK
for High V
OUT
Applications
Figure 14a. Setting the Average Current Limit (R
IPK
)
Figure 14b. Setting R
IPK
with No Differential Amplifier
V
S
+
V
S
–
R
IPK
LTC3766
MAIN
XFMR
V
OUT
3766 F14a
V
SW
V
SOUT
V
FB
SW
I
PK
R
B
R
LOAD
R
A
•
•
V
S
+
V
S
–
R
IPK
LTC3766
MAIN
XFMR
V
OUT
3766 F14
V
SW
V
SOUT
SW
I
PK
•
•
V
S
–
160k
V
S
+
GND
R
IPK
LTC3766
MAIN
XFMR
V
OUT
3766 F15
V
SW
V
SOUT
SW
R1
R2
I
PK
120k
R3
R4
•
•
For resistor sense mode, place a resistor on the I
PK
pin
that is chosen using:
R
IPK
=
K
R
L
IPK
17.6nF
( )
R
SENSE
where L
IPK
is the inductance of the output inductor at I =
I
LIM(AVG)
. For low V
OUT
applications where no SW node
divider is needed, K
R
= 1. For current transformer mode,
use:
R
IPK
=
K
R
L
IPK
1.32nF
( )
K
CT
R
SENSE
•
N
P
N
S
When the LTC3766 is in current limit and the output volt-
age is very low, the control of the output current will be
limited by the minimum on-time of the converter. Once this
minimum on-time has been reached, further decreases in
output voltage during current limit will result in an inductor
current that continues to rise, until the over
current limit
is reached. This will cause the LTC3766 to shut down and
attempt a restart, resulting in a hiccup mode of operation.
Typical average current limit performance is illustrated
in Figure 16. Note that the average current delivered to
the load is held substantially constant as the output volt
-
age is decreased down to a low level, at which point the
converter will enter hiccup mode. Depending upon the
particular application,
hiccup mode is entered either due
to the loss of secondary-side bias voltage (UVLO) or due
to an overcurrent fault.