Datasheet

LTC3829
27
3829fa
For more information www.linear.com/LTC3829
APPLICATIONS INFORMATION
The LTC3829 allows the user to program how its output
ramps up and down by means of the TK/SS pins. Through
these pins, the output can be set up to either coincidentally
or ratiometrically track another supply’s output, as shown
in Figure 11. In the following discussions, V
OUT1
refers
to the LTC3829’s output as a master and V
OUT2
refers to
another supply output as a slave. To implement the coinci-
dent tracking in Figure 11a, connect an additional resistive
divider to V
OUT1
and connect its mid-point to the TK/SS pin
of the slave controller. The ratio of this divider should be
the same as that of the slave controller’s feedback divider
shown in Figure 12a. In this tracking mode, V
OUT1
must
be set higher than V
OUT2
. To implement the ratiometric
tracking in Figure 11b, the ratio of the V
OUT2
divider should
be exactly the same as the master controller’s feedback
divider shown in Figure 12b . By selecting different resis-
tors, the LTC3829 can achieve different modes of tracking
including the two in Figure 11.
So which mode should be programmed? While either
mode in Figure 11 satisfies most practical applications,
some trade-offs exist. The ratiometric mode saves a pair
of resistors, but the coincident mode offers better output
regulation. Under ratiometric tracking, when the master
controller’s output experiences dynamic excursion (under
load transient, for example), the slave controller output
will be affected as well. For better output regulation, use
the coincident tracking mode instead of ratiometric.
Figure 11. Two Different Modes of Output Voltage Tracking
Figure 12. Setup and Coincident and Ratiometric Tracking
TIME
(11a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
3829 F11a
V
OUT1
V
OUT2
TIME
3829 F11b
(11b) Ratiometric Tracking
OUTPUT VOLTAGE
R3 R1
R4 R2
R3
V
OUT2
R4
(12a) Coincident Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3829 F12
(12b) Ratiometric Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1