LTC3829_15 Datasheet, PDF (27/40 Pages) Linear Technology – 3-Phase, Single Output Synchronous Step-Down DC/DC Controller with Diffamp

LTC3829
APPLICATIONS INFORMATION
when there is no load. After TK/SS drops below 0.1V, the
controller operates in discontinuous mode.
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
coincident 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 feed-
back 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 control-
ler’s feedback divider shown in Figure 12b . By selecting
different resistors, 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
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
3829 F11a
(11a) Coincident Tracking
TIME
3829 F11b
(11b) Ratiometric Tracking
Figure 11. 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
V_OUT1
TO
TK/SS2
PIN
R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R2
V_OUT2
R3
R4
(12a) Coincident Tracking Setup
3829 F12
(12b) Ratiometric Tracking Setup
Figure 12. Setup and Coincident and Ratiometric Tracking
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