Datasheet
LTM4603/LTM4603-1
14
4603fb
control the ramp of the internal reference and the output
voltage. The total soft-start time can be calculated as:
t
SOFTSTART
= 0.8 • 0.6V ± V
OUT(MARGIN)
( )
•
C
SS
1.5µA
When the RUN pin falls below 1.5V, then the TRACK/SS
pin is reset to allow for proper soft-start control when the
regulator is enabled again. Current foldback and forced
continuous mode are disabled during the soft-start pro-
cess. The soft-start function can also be used to control
the output ramp up time, so that another regulator can
be easily tracked to it.
Output Voltage Tracking
Output voltage tracking can be programmed externally
using the TRACK/SS pin. The output can be tracked up
and down with another regulator. Figure 5 shows an ex-
ample of coincident tracking where the master regulator’s
output is divided down with an external resistor divider
that is the same as the slave regulator’s feedback divider.
The master output must be greater than the slave output
for the tracking to work. Figure 6 shows the coincident
output tracking characteristics.
Ratiometric tracking can be achieved by a few simple
calculations and the slew rate value applied to the master’s
TRACK/SS pin. The TRACK/SS pin has a control range
from 0V to 0.6V. The master’s TRACK/SS pin slew rate
is directly equal to the master’s output slew rate in volts/
time. The equation:
MR
SR
• 60.4k = R
T
where MR is the master’s output slew rate and SR is the
slave’s output slew rate in volts/time. When coincident
tracking is desired, then MR and SR are equal, thus R
TB
is equal to 60.4k. R
B
is derived from equation:
R
B
=
0.6V
V
FB
60.4k
+
V
FB
R
FB
(Slave)
–
V
TRACK
R
TB
where V
FB
is the feedback voltage reference of the regula-
tor, and V
TRACK
is 0.6V.
In ratiometric tracking, a different slew rate maybe desired
for the slave regulator. R
T
can be solved for when SR is
slower than MR. Make sure that the slave supply slew
rate is chosen to be fast enough so that the slave output
voltage will reach its final value before the master output.
For example, MR = 1.5V/ms, and SR = 1.2V/ms. Then R
T
= 75k. Solve for R
B
to equal 51.1k.
For applications that do not require tracking or sequencing,
simply tie the TRACK/SS pin to INTV
CC
to let RUN control
the turn on/off. When the RUN pin is below its threshold
or V
IN
is below the undervoltage lockout threshold, then
TRACK/SS is pulled low.
Figure 5. Coincident Tracking Schematic
Figure 6. Coincident Output Tracking Characteristics
OUTPUT
VOLTAGE
TIME
4603 F06
MASTER OUTPUT
SLAVE OUTPUT
applications inForMation
V
OUT
V
FB
MARG0
MARG1
V
OUT_LCL
DIFFV
OUT
V
OSNS
+
V
OSNS
–
PGOOD
MPGM
RUN
COMP
INTV
CC
DRV
CC
TRACK/SS
TRACK CONTROL
PLLIN
LTM4603
R
SET
40.2k
100k
R
B
40.2k
MASTER
OUTPUT
R
T
60.4k
C
OUT
SLAVE OUTPUT
4603 F05
C
IN
V
IN
f
SETPGNDSGND
V
IN