Datasheet

19
LTC3736-1
37361f
The ramp time for V
OUT2
to rise from 0V to its final value
is:
tt
R
RA
RA RB
RR
SS SS
TRACKA
TRACKA TRACKB
21
1
11
=
+
+
••
For coincident tracking,
tt
V
V
SS SS
OUT F
OUT F
21
2
1
= •
where V
OUT1F
and V
OUT2F
are the final, regulated values of
V
OUT1
and V
OUT2
. V
OUT1
should always be greater than
V
OUT2
when using the TRACK pin. If no tracking function
is desired, then the TRACK pin may be tied to V
IN
. How-
ever, in this situation there would be no (internal nor
external) soft-start on V
OUT2
.
When using tracking with spread spectrum operation
enabled, the tracking resistors R
TRACKA
and R
TRACKB
should have value at least 10 times smaller than corre-
sponding feedback resistors R2A and R2B.
Fault Condition: Short Circuit and Current Limit
To prevent excessive heating of the bottom MOSFET,
foldback current limiting can be added to reduce the
current in proportion to the severity of the fault.
Foldback current limiting is implemented by adding di-
odes D
FB1
and D
FB2
between the output and the I
TH
pin as
shown in Figure 9. In a hard short (V
OUT
= 0V), the current
will be reduced to approximately 50% of the maximum
output current.
Low Supply Operation
Although the LTC3736-1 can function down to below
2.4V, the maximum allowable output current is reduced as
V
IN
decreases below 3V. Figure 10 shows the amount of
APPLICATIO S I FOR ATIO
WUUU
LTC3736-1
V
FB2
V
OUT2
V
OUT1
V
FB1
TRACK
R2B
R2A
37361 F08a
R1B
R1A
R
TRACKA
R
TRACKB
Figure 8a. Using the TRACK Pin
TIME
(8b) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
TIME
37361 F08b,c
(8c) Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
Figures 8b and 8c. Two Different Modes of Output
Voltage Tracking
+
1/2 LTC3736-1
V
FB
I
TH
R2
D
FB1
V
OUT
D
FB2
37361 F09
R1
Figure 9. Foldback Current Limiting
INPUT VOLTAGE (V)
75
NORMALIZED VOLTAGE OR CURRENT (%)
85
95
105
80
90
100
2.2 2.4 2.6 2.8
37361 F10
3.02.12.0 2.3 2.5 2.7 2.9
V
REF
MAXIMUM
SENSE VOLTAGE
Figure 10. Line Regulation of V
REF
and
Maximum Sense Voltage for Low Input Supply