ltc3869gn-2 Linear Technology Corporation, ltc3869gn-2 Datasheet - Page 19

• Manufacturer Part Number: ltc3869gn-2
• Description: Ltc3869/ltc3869-2 - Dual, 2-phase Synchronous Step-down Dc/dc Controllers
• Manufacturer: Linear Technology Corporation

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
the LTC3869’s output 1 as a master channel and V
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
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
TK/SS2
V
OUT1
PIN
TO
OUT1
(5a) Coincident Tracking
(6a) Coincident Tracking Setup
must be set higher than V
TIME
R3
R4
R1
R2
TO
V
PIN
FB1
Figure 5. Two Different Modes of Output Voltage Tracking
Figure 6. Setup for Coincident and Ratiometric Tracking
OUT1
3869 F05a
V
PIN
FB2
TO
V
V
OUT1
OUT1
OUT2
and connect
refers to
R3
R4
OUT2
OUT2
V
OUT2
.
To implement the ratiometric tracking in Figure 6b, the
ratio of the V
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 tradeoffs 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.
V
TK/SS2
OUT1
PIN
(6b) Ratiometric Tracking Setup
TO
OUT2
R1
R2
LTC3869/LTC3869-2
TO
V
PIN
FB1
divider should be exactly the same as
(5b) Ratiometric Tracking
V
PIN
FB2
TO
TIME
R3
R4
3869 F09
V
OUT2
3869 F08b
V
V
OUT1
OUT2
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