LTC3867_15 Datasheet, PDF (25/36 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3867
APPLICATIONS INFORMATION
the soft-start phase expires, the LTC3867 is forced into
continuous mode of operation as soon as V_FB is below
the undervoltage threshold of 0.555V regardless of the
setting on the MODE pin. However, the LTC3867 should
always be set in forced continuous mode tracking down
when there is no load. After TK/SS drops below 0.1V, the
controller operates in discontinuous mode.
The LTC3867 allows the user to program how its output
ramps up and down by means of the TK/SS pin. Through
these pins, the output can be set up to either coincidentally
or ratiometrically track another supply’s output, as shown
in Figure 10. In the following discussions, V_OUT2 refers to
the LTC3867’s output as a slave and V_OUT1 refers to another
supply output as a master. To implement the coincident
tracking in Figure 10a, connect an additional resistive di-
vider 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 11a. In this tracking mode, V_OUT1 must
be set higher than V_OUT2. To implement the ratiometric
tracking in Figure 10b, the ratio of the V_OUT2 divider should
be exactly the same as the master controller’s feedback
divider shown in Figure 11b . By selecting different resis-
tors, the LTC3867 can achieve different modes of tracking
including the two in Figure 10.
So which mode should be programmed? While either
mode in Figure 10 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.
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
(10a) Coincident Tracking
TIME
3867 F10
(10b) Ratiometric Tracking
Figure 10. 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
(11a) Coincident Tracking Setup
3867 F11
(11b) Ratiometric Tracking Setup
Figure 11. Setup and Coincident and Ratiometric Tracking
3867f
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