LTC3773 Datasheet, PDF (21/32 Pages) Linear Technology – Triple Output Synchronous 3-Phase DC/DC Controller with Up/Down Tracking

LTC3773
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_SLEW
and the TRACK DOWN switch as shown in Figure 5a
and 5b.
With a simple configuration, TRACK allows V_OUT start-
up to track the master channel as shown qualitatively
in Figures 5a and 5b. The LTC3773 can be configured
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 configuration,
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 configura-
tion produces the same slew rate at the three outputs,
so that the lowest output voltage channel reaches its
steady state first.
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
RAMP
SOURCE
MASTER
V_OUT
V_OUT1
RM2
R12
RM1
R11
C_SLEW
V_OUT2
R22
V_FB1
V_FB2
LTC3773
TRACK2
TRACK3
TRACK1 V_FB3
3773 F04a
R21
V_OUT3
R32
R31
1MΩ
TRACK
DOWN
10k
C_SS
0V TO 2V
RAMP
SOURCE
MASTER
V_OUT
R12
R11
V_OUT1
V_OUT2
R12
R22
V_FB1
V_FB2
R11
R22
LTC3773
R21
R32
TRACK2
V_FB3
TRACK3
R31
R21
V_OUT3
R32
R31
C_SLEW
C_SS
1MΩ
TRACK1
3773 F05b
TRACK 1
0.5V/DIV
TRACK
DOWN
10k
0V TO 2V
TRACK 1
1V/DIV
2.5V V_OUT1 1V/DIV
1.8V V_OUT2 1V/DIV
1.2V V_OUT3 1V/DIV
0.1s/DIV
3773 F05a
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
2.5V V_OUT1 1V/DIV
1.8V V_OUT2 1V/DIV
1.2V V_OUT3 1V/DIV
0.1s/DIV
3773 F05b
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
3773fb
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