Datasheet
LTC3867
25
3867f
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.
Figure 10. Two Different Modes of Output Voltage Tracking
Figure 11. Setup and Coincident and Ratiometric Tracking
TIME
(10a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
V
OUT1
V
OUT2
TIME
3867 F10
(10b) Ratiometric Tracking
OUTPUT VOLTAGE
R3 R1
R4 R2
R3
V
OUT2
R4
(11a) Coincident Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3867 F11
(11b) Ratiometric Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1