LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 30

• Manufacturer Part Number: LTC3855
• Description: Dual, Fast, Accurate Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3838
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
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:
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
30
t
SS
(SEC) = 0.6(V)•
TRACK/SS2
V
OUT1
PIN
TO
R
R
C
FB2(2)
FB1(2)
SS
1(µA)
Coincident Tracking Setup
(µF)
R
R
FB2(1)
FB1(1)
Coincident Tracking
Figure 9b. Setup for Coincident and Ratiometric Tracking
TIME
TO
V
PIN
TO
V
PIN
OUTSENSE1
OUTSENSE1
Figure 9a. Two Different Modes of Output Tracking
+
–
SS
V
, connected
PIN
FB2
TO
V
V
OUT1
OUT2
R
R
FB2(2)
FB1(2)
V
OUT2
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
output 1 as a master channel and V
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
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,
TRACK/SS2
V
OUT1
PIN
TO
Ratiometric Tracking
Ratiometric Tracking Setup
R
R
FB2(1)
FB1(1)
TIME
TO
V
PIN
TO
V
PIN
OUTSENSE1
OUTSENSE1
OUT1
OUT1
+
–
and connect its midpoint
3838 F09a
V
PIN
FB2
TO
refers to the LTC3838’s
V
V
OUT1
OUT2
OUT2
R
R
3838 F09b
FB2(2)
FB1(2)
refers to the
V
OUT2
3838fa