Datasheet

LTM4625
12
Rev D
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APPLICATIONS INFORMATION
Soft-Start And Output Voltage Tracking
The TRACK/SS pin provides a means to either soft start
the regulator or track it to a different power supply. A
capacitor on the TRACK/SS pin will program the ramp
rate of the output voltage. An internal A current source
will charge up the external soft-start capacitor towards
INTV
CC
voltage. When the TRACK/SS voltage is below
0.6V, it will take over the internal 0.6V reference voltage
to control the output voltage. The total soft-start time can
be calculated as:
t
SS
= 0.6
C
SS
2µA
where C
SS
is the capacitance on the TRACK/SS pin. Cur-
rent foldback and forced continuous mode are disabled
during the soft-start process.
Output voltage tracking can also be programmed externally
using the TRACK/SS pin. The output can be tracked up
and down with another regulator. Figure 4 and Figure 5
show an example waveform and schematic of ratiometric
tracking where the slave regulators output slew rate is
proportional to the master’s.
Since the slave regulators TRACK/SS is connected to
the masters output through a R
TR(TOP)
/R
TR(BOT)
resistor
divider and its voltage used to regulate the slave output
voltage when TRACK/SS voltage is below 0.6V, the slave
output voltage and the master output voltage should satisfy
the following equation during start-up:
V
OUT(SL)
R
FB(SL)
R
FB(SL)
+ 60.4k
=
V
OUT(MA)
R
TR(BOT)
R
TR(TOP)
+R
TR(BOT)
Figure 4. Output Ratiometric Tracking Waveform
Figure 5. Example Schematic of Ratiometric Output Voltage Tracking
TIME
SLAVE OUTPUT
MASTER OUTPUT
OUTPUT VOLTAGE
4625 F04
FREQ
V
IN
SV
IN
RUN
INTV
CC
MODE
TRACK/SS
PGOOD
V
OUT
FB
COMP
GND SGND
R
FB(MA)
40.2k
LTM4625
10µF
25V
V
IN
4V TO 20V
V
OUT(MA)
1.5V
5A
47µF
6.3V
R
TR(BOT)
40.2k
R
FB(SL)
60.4k
R
TR(TOP)
60.4k
FREQ
V
IN
SV
IN
RUN
INTV
CC
MODE
TRACK/SS
PGOOD
V
OUT
FB
COMP
GND SGND
4625 F05
LTM4625
10µF
25V
V
OUT(SL)
1.2V
5A
47µF
6.3V
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