Datasheet

LTC3838
30
3838fa
APPLICATIONS INFORMATION
When a channel is configured to soft-start by itself, a ca-
pacitor should be connected to its TRACK/SS pin. TRACK/
SS is pulled low until the RUN pin voltage exceeds 1.2V
and UVLO is released, at which point an internal current
of 1µA charges the soft-start capacitor, C
SS
, connected
to the TRACK/SS pin. Current-limit foldback is disabled
during this phase to ensure smooth soft-start or track-
ing. The soft-start or tracking range is defined to be the
voltage range from 0V to 0.6V on the TRACK/SS pin. The
total soft-start time can be calculated as:
t
SS
(SEC)= 0.6(V)•
C
SS
(µF)
1(µA)
When one particular channel is configured to track an
external supply, a voltage divider can be used from the
external supply to the TRACK/SS pin to scale the ramp
rate appropriately. Two common implementations are co-
incidental tracking and ratiometric tracking. For coincident
tracking, make the divider ratio from the external supply
the same as the divider ratio for the differential feedback
voltage. Ratiometric tracking could be achieved by using
a different ratio than the differential feedback.
Note that the 1µA soft-start capacitor charging current is
still flowing, producing a small offset error. To minimize
this error, select the tracking resistive divider values to be
small enough to make this offset error negligible.
The LTC3838 allows the user to program how its two
channels’ outputs track each other ramping up or down.
In the following discussions, V
OUT1
refers to the LTC3838’s
output 1 as a master channel and V
OUT2
refers to the
LTC3838’s output 2 as a slave channel. In practice though,
either channel can be used as the master.
By selecting different resistors, the LTC3838 can achieve
different modes of tracking including the two in Figure 9.
To implement the coincident tracking, connect an addi-
tional resistive divider to V
OUT1
and connect its midpoint
to the TRACK/SS pin of the slave channel. The ratio of this
divider should be the same as that of the slave channel’s
feedback divider shown in Figure 9b. In this tracking mode,
Figure 9a. Two Different Modes of Output Tracking
Figure 9b. Setup for Coincident and Ratiometric Tracking
TIME
Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
V
OUT1
V
OUT2
TIME
3838 F09a
Ratiometric Tracking
OUTPUT VOLTAGE
R
FB2(1)
R
FB1(1)
R
FB2(2)
R
FB1(2)
R
FB2(2)
V
OUT2
R
FB1(2)
Coincident Tracking Setup
TO
V
OUTSENSE1
+
PIN
TO
V
OUTSENSE1
–
PIN
TO
TRACK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
R
FB2(1)
R
FB1(1)
R
FB2(2)
V
OUT2
R
FB1(2)
3838 F09b
Ratiometric Tracking Setup
TO
V
OUTSENSE1
+
PIN
TO
V
OUTSENSE1
–
PIN
TO
TRACK/SS2
PIN
TO
V
FB2
PIN
V
OUT1