LTC3773EG-PBF LINER [Linear Technology], LTC3773EG-PBF Datasheet - Page 21

• Manufacturer Part Number: LTC3773EG-PBF
• Description: Triple Output Synchronous 3-Phase DC/DC Controller with Up/Down Tracking
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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
and the TRACK DOWN switch as shown in Figure 5a
and 5b.
With a simple confi guration, TRACK allows V
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
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
0V TO 2V
TRACK
DOWN
SOURCE
Figure 5a. Ratiometric Tracking. TRACK1 Functions
as a Soft-Start Pin; V
with Ratiometric Start-Up Slew Rate
RAMP
10k
1MΩ
FB
C
MASTER
SLEW
V
pin. With a ratiometric confi guration,
OUT
0.1s/DIV
RM2
RM1
OUT2
V
OUT1
R12
R11
and V
C
SS
OUT3
V
TRACK2
TRACK3
TRACK1
FB1
TRACK 1
0.5V/DIV
2.5V V
1.8V V
1.2V V
LTC3773
Track V
OUT1
OUT2
OUT3
V
V
3773 F04a
3773 F05a
FB2
FB3
1V/DIV
1V/DIV
1V/DIV
V
OUT1
OUT2
R22
R21
OUT
V
OUT3
R32
R31
start-
SLEW
0V TO 2V
TRACK
DOWN
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
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
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
higher than V
tion produces the same slew rate at the three outputs,
so that the lowest output voltage channel reaches its
SOURCE
RAMP
Figure 5b. Coincident Tracking. TRACK1 Functions
as a Soft-Start Pin; V
with the Same Start-Up Slew Rate
10k
1MΩ
C
MASTER
OUT2
SLEW
V
OUT
R12
R11
and V
2.5V
1.8V
1.2V
C
SS
V
V
V
OUT1
OUT2
OUT3
OUT2
OUT3
TRACK 1
1V/DIV
1V/DIV
1V/DIV
1V/DIV
R32
R31
and V
. The coincident confi gura-
R22
R21
V
OUT1
OUT3
R12
R11
Track V
FB
V
TRACK2
TRACK3
TRACK1
FB1
LTC3773
OUT1
LTC3773
dividers across
0.1s/DIV
OUT1
must be set
3773 F05b
V
V
FB2
FB3
V
OUT2
21
R22
R21
V
3773 F05b
OUT3
3773fb
R32
R31