LTM4636
17
4636f
applicaTions inForMaTion
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
•
4.99k
=
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 4.99k. R
TA
is derived from equation:
R
TA
=
0.6V
V
FB
4.99k
+
V
FB
R
FB1
–
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 4.99k
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
= 4.99k, and R
TA
= 4.99k in
Figure 5.
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
= 6.19k. Solve for R
TA
to equal 4.22k.
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.
Default Overcurrent and Overvoltage Protection
The LTM4636 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.
Temperature Monitoring
Measuring the absolute temperature of a diode is pos-
sible due to the relationship between current, voltage
and temperature described by the classic diode equation:
I
D
=
I
S
•
e
V
D
η
•
V
T
or
V
D
= η
•
V
T
•
In
I
D
I
S
where I
D
is the diode current, V
D
is the diode voltage,
η
is the ideality factor (typically close to 1.0) and I
S
(satura-
tion current) is a process dependent parameter. V
T
can
be broken out to:
V
T
=
k
•
T
q
where T is the diode junction temperature in Kelvin, q is
the electron charge and k is Boltzmann’s constant. V
T
is
approximately 26mV at room temperature (298K) and
scales linearly with Kelvin temperature. It is this linear
temperature relationship that makes diodes suitable tem-
perature sensors. The I
S
term in the previous equation is
the extrapolated current through a diode junction when
the diode has zero volts across the terminals. The I
S
term
varies from process to process, varies with temperature,
and by definition must always be less than I
D
. Combining
all of the constants into one term:
K
D
= η
•
k
q
where K
D
= 8.62
−5
, and knowing ln(I
D
/I
S
) is always posi-
tive because I
D
is always greater than I
S
, leaves us with
the equation that:
V
D
=
T KELVIN
(
)
•
K
D
•
In
I
D
I
S