Datasheet

LTC3869/LTC3869-2
20
38692fa
For more information www.linear.com/LTC3869
applicaTions inForMaTion
Output Voltage Tracking
The LTC3869 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 5. In the following discussions, V
OUT1
refers to
the LTC3869’s output 1 as a master channel and V
OUT2
refers to the LTC3869’s output 2 as a slave channel. In
practice, though, either phase can be used as the master.
To implement the coincident tracking in Figure 5a, con-
nect an additional resistive divider to V
OUT1
and connect
its midpoint to the TK/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 6a. In
this tracking mode, V
OUT1
must be set higher than V
OUT2
.
To implement the ratiometric tracking in Figure 6b, the
ratio of the V
OUT2
divider should be exactly the same as
the master channel’s feedback divider shown in Figure 6b.
By selecting different resistors, the LTC3869 can achieve
different modes of tracking
including the two in Figure 5.
So which mode should be programmed? While either
mode in Figure 5 satisfies most practical applications,
some trade-offs exist. The ratiometric mode saves a pair
of resistors, but the coincident mode offers better output
regulation.
When the master channel’s output experiences dynamic
excursion (under load transient, for example), the slave
channel output will be affected as well. For better output
regulation, use the coincident tracking mode instead of
ratiometric.
Figure 5. Tw o Different Modes of Output Voltage Tracking
Figure 6. Setup for Coincident and Ratiometric Tracking
TIME
(5a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
3869 F05a
V
OUT1
V
OUT2
TIME
3869 F08b
(5b) Ratiometric Tracking
OUTPUT VOLTAGE
R3 R1
R4 R2
R3
V
OUT2
R4
(6a) Coincident Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3869 F09
(6b) Ratiometric Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1