Datasheet

LTC3855
23
3855f
applicaTions inForMaTion
producing a small offset error. To minimize this error, select
the tracking resistive divider value to be small enough to
make this error negligible.
In order to track down another channel or supply after
the soft-start phase expires, the LTC3855 is forced into
continuous mode of operation as soon as V
FB
is below the
undervoltage threshold of 0.54V regardless of the setting
on the MODE/PLLIN pin. However, the LTC3855 should
always be set in force continuous mode tracking down
when there is no load. After TK/SS drops below 0.1V, its
channel will operate in discontinuous mode.
Output Voltage Tracking
The LTC3855 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 8. In the following discussions, V
OUT1
refers to
the LTC3855’s output 1 as a master channel and V
OUT2
refers to the LTC3855’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 8a, 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 9a. In this
tracking mode, V
OUT1
must be set higher than V
OUT2
. To
implement the ratiometric tracking in Figure 9b, the ratio of
the V
OUT2
divider should be exactly the same as the master
channel’s feedback divider shown in Figure 9b. By select-
ing different resistors, the LTC3855 can achieve different
modes of tracking including the two in Figure 8.
So which mode should be programmed? While either
mode in Figure 8 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.
Figure 8. Two Different Modes of Output Voltage Tracking
Figure 9. Setup for Coincident and Ratiometric Tracking
TIME
(8a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
3855 F08a
V
OUT1
V
OUT2
TIME
3855 F08b
(8b) Ratiometric Tracking
OUTPUT VOLTAGE
R3 R1
R4 R2
R3
V
OUT2
R4
(9a) Coincident Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3855 F09
(9b) Ratiometric Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1