Datasheet

LTM4608A
16
4608afd
applicaTions inForMaTion
The track pin of the master can be controlled by an external
ramp or by R
SR
and C
SR
in Figure 5 referenced to V
IN
. The
RC ramp time can be programmed using equation:
t = – ln 1–
0.596V
V
IN






• R
SR
• C
SR








Ratiometric tracking can be achieved by a few simple
calculations and the slew rate value applied to the mas-
ter’s track pin. As mentioned above, the TRACK pin has
a control range from 0V to 0.596V. The master’s TRACK
pin slew rate is directly equal to the master’s output slew
rate in Volts/Time:
MR
SR
• 10k = R
FB3
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
FB3
is equal the 10k. R
FB4
is derived from equation:
R
FB4
=
0.596V
V
FB
10k
+
V
FB
R
FB2
–
V
TRACK
R
FB3
where V
FB
is the feedback voltage reference of the regulator
and V
TRACK
is 0.596V. Since R
FB3
is equal to the 10k top
feedback resistor of the slave regulator in equal slew rate
or coincident tracking, then R
FB4
is equal to R
FB2
with V
FB
=
V
TRACK
. Therefore R
FB3
= 10k and R
FB4
= 6.65k in Figure 5.
In ratiometric tracking, a different slew rate maybe desired
for the slave regulator. R
FB3
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 it final value before the master output.
For example: MR = 3.3V/ms and SR = 1.5V/ms. Then
R
FB3
= 22.1k. Solve for R
FB4
to equal to 4.87k.
For applications that do not require tracking or sequencing,
simply tie the TRACK pin to SV
IN
to let RUN control the
turn on/off. Connecting TRACK to SV
IN
also enables the
~100µs of internal soft-start during start-up. Load current
needs to be present during track down.
Power Good
The PGOOD pin is an open-drain pin that can be used to
monitor valid output voltage regulation. This pin monitors
a ±10% window around the regulation point. As shown
in Figure 20, the sequencing function can be realized in a
dual output application by controlling the RUN pins and the
PGOOD signals from each other. The 1.5V output begins
its soft starting after the PGOOD signal of 3.3V output
becomes high, and 3.3V output starts its shut down after
the PGOOD signal of 1.5V output becomes low. This can
be applied to systems that require voltage sequencing
between the core and sub-power supplies.
Figure 6. Output Voltage Coincident Tracking
OUTPUT VOLTAGE (V)
TIME
MASTER OUTPUT
SLAVE OUTPUT
4608A F06