Datasheet
LTM4637
13
4637fc
For more information www.linear.com/LTM4637
applicaTions inForMaTion
2V and a low level below 0.8V. See the Typical Applica-
tions section for synchronization examples. The LTM4637
minimum
on-time
is limited to approximately 100ns.
Guardband the on-time to 110ns. The on-time can be
calculated as:
t
ON(MIN)
=
1
FREQ
•
V
OUT
V
IN
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. The master regulator’s
output is divided down with an external resistor divider
that is the same as the slave regulator’s feedback divider
to implement coincident tracking. The LTM4637 uses an
accurate 60.4k resistor internally for the top feedback
resistor. Figure 4 shows an example of coincident tracking.
V
OUT(SLAVE)
= 1+
60.4k
R
TA
• V
TRACK
V
TRACK
is the track ramp applied to the slave’s track pin.
V
TRACK
has a control range of 0V to 0.6V, or the internal
reference voltage. When the master’s output is divided
down with the same resistor values used to set the slave’s
output, then the slave will coincident track with the master
until it reaches its final value. The master will continue
to its final value from the slave’s regulation point (see
Figure 5). Voltage tracking is disabled when V
TRACK
is
more than 0.6V. R
TA
in Figure 4 will be equal to R
FB
for
coincident tracking.
The TRACK/SS pin of the master can be controlled by an
external ramp or the soft-start function of that regulator can
be used to develop that master ramp. The LTM4637 can
be used as a master by setting the ramp rate on its track
pin using a soft-start capacitor. A 1.2µA current source
is used to charge the soft-start capacitor. The following
equation can be used:
t
SOFT-START
= 0.6V •
C
SS
1.2µA
Ratiometric tracking can be achieved by a few simple
calculations and the slew rate value applied to the master’s
TRACK/SS pin. As mentioned above, 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
TB
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
TA
is derived from equation:
R
TA
=
0.6V
V
FB
60.4k
+
V
FB
R
FB
–
V
TRACK
R
TB
where V
FB
is the feedback voltage reference of the regula-
tor, and V
TRACK
is 0.6V. Since R
TB
is equal to the 60.4k
top feedback resistor of the slave regulator in equal slew
rate or coincident tracking, then R
TA
is equal to R
FB
with
V
FB
= V
TRACK
. Therefore R
TB
= 60.4k, and R
TA
= 60.4k in
Figure 4.
In ratiometric tracking, a different slew rate maybe desired
for the slave regulator. R
TB
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
TB
= 75k. Solve for R
TA
to equal 51.1k.
For applications that do not require tracking or sequenc-
ing, 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 the V
IN
undervoltage lockout, then
TRACK/SS is pulled low.
Overcurrent and Overvoltage Protection
The LTM4637 has overcurrent protection (OCP) in a
short circuit. The internal current comparator threshold
folds back during a
short to reduce the output current. An
overvoltage condition (OVP) above 10% of the regulated
output voltage will force the top MOSFET off and the bottom
MOSFET on until the condition is cleared. Foldback current
limiting is disabled during soft-start or tracking start-up.