Datasheet

LTC3773
21
3773fb
V
FB1
TRACK2
TRACK3
TRACK1
V
FB2
V
FB3
LTC3773
V
OUT1
V
OUT2
V
OUT3
R12
R11
R22
R21
R32
R31
R22
R21
R32
R31
3773 F05b
1MΩ
C
SS
MASTER
V
OUT
R12
R11
10k
C
SLEW
RAMP
SOURCE
TRACK
DOWN
0V TO 2V
2.5V V
OUT1
1V/DIV
1.8V V
OUT2
1V/DIV
1.2V V
OUT3
1V/DIV
TRACK 1
1V/DIV
0.1s/DIV
3773 F05b
Figure 5a. Ratiometric Tracking. TRACK1 Functions
as a Soft-Start Pin; V
OUT2
and V
OUT3
Track V
OUT1
with Ratiometric Start-Up Slew Rate
Figure 5b. Coincident Tracking. TRACK1 Functions
as a Soft-Start Pin; V
OUT2
and V
OUT3
Track V
OUT1
with the Same Start-Up Slew Rate
V
FB1
TRACK2
TRACK3
TRACK1
V
FB2
V
FB3
LTC3773
V
OUT1
1MΩ
C
SS
V
OUT2
V
OUT3
R12
R11
MASTER
V
OUT
RM2
RM1
R22
R21
R32
R31
10k
3773 F04a
C
SLEW
RAMP
SOURCE
TRACK
DOWN
0V TO 2V
2.5V V
OUT1
1V/DIV
1.8V V
OUT2
1V/DIV
1.2V V
OUT3
1V/DIV
TRACK 1
0.5V/DIV
0.1s/DIV 3773 F05a
An internal 1μA current source pull-up at the TRACK pin
programs the output to take about 600ms/μF to reach its
steady state value. The output voltage ramp down slew
rate can be controlled by the external capacitor C
SLEW
and the TRACK DOWN switch as shown in Figure 5a
and 5b.
With a simple confi guration, TRACK allows V
OUT
start-
up to track the master channel as shown qualitatively
in Figures 5a and 5b. The LTC3773 can be confi gured
for two different up/down tracking modes:coincident or
ratiometric.
To implement the ratiometric tracking shown in Figure 5a,
no extra divider is needed; simply connect the TRACK2
and TRACK3 pins to the TRACK1 pin. Do not connect
TRACK to the V
FB
pin. With a ratiometric confi guration,
the LTC3773 produces three different output slew rates.
Because each channel’s slew rate is proportional to its
corresponding output voltage, the three output voltages
reach their steady-state values at about the same time.
If any of the channel SDB pins are asserted, its TRACK
pin will be internally pulled low and all channels will be
disabled.
To implement coincident tracking, connect extra resistor
dividers to the output of channel 1. These resistor dividers
are selected to be the same as the V
FB
dividers across
the outputs of channels 2 and 3. TRACK2 and TRACK3
are connected to these extra resistor dividers as shown
in Figure 5b. In this tracking scheme, V
OUT1
must be set
higher than V
OUT2
and V
OUT3
. The coincident confi gura-
tion produces the same slew rate at the three outputs,
so that the lowest output voltage channel reaches its
steady state fi rst.
The TRACK pin 1μA internal pull-up current performs the
soft-start action, but in tracking mode it introduces an
error term in the resistive divider. To minimize this error,
build the resistive divider with smaller value resistors, or
APPLICATIONS INFORMATION