LTC3869-2_15 Datasheet, PDF (20/42 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down DC/DC Controllers

LTC3869/LTC3869-2
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.
V_OUT1
V_OUT2
V_OUT1
V_OUT2
20
TIME
3869 F05a
(5a) Coincident Tracking
TIME
3869 F08b
(5b) Ratiometric Tracking
Figure 5. Two Different Modes of Output Voltage Tracking
V_OUT1
TO
TK/SS2
PIN
R3 R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R4 R2
V_OUT2
R3
R4
(6a) Coincident Tracking Setup
V_OUT1
TO
TK/SS2
PIN
R1
TO
TO
V_FB1
PIN
V_FB2
PIN
R2
V_OUT2
R3
R4
3869 F09
(6b) Ratiometric Tracking Setup
Figure 6. Setup for Coincident and Ratiometric Tracking
For more information www.linear.com/LTC3869
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