Datasheet

LTM4637
15
4637fc
For more information www.linear.com/LTM4637
where I
0
is a process and geometry dependent current, (I
0
is typically around 20k orders of magnitude larger than I
S
at room temperature) and V
G0
is the band gap voltage of
1.2V extrapolated to absolute zero or –273°C.
If we take the I
S
equation and substitute into the V
D
equa-
tion, then we get:
V
D
= V
G0
–
kT
q






ln
I
0
I
D






, V
T
=
kT
q
The expression shows that the diode voltage decreases
(linearly if I
0
were constant) with increasing temperature
and constant diode current. Figure 6 shows a plot of V
D
vs Temperature over the operating temperature range of
the LTM4637.
If we take this equation and differentiate it with respect to
temperature T, then:
dV
D
dT
= –
V
G0
– V
D
T
This dV
D
/dT term is the temperature coefficient equal to
about –2mV/K or –2mV/°C. The equation is simplified for
the first order derivation.
Solving for T, T = –(V
G0
– V
D
)/(dV
D
/dT) provides the
temperature.
1st Example: Figure 6 for 27°C, or 300K the diode
voltage is 0.598V, thus, 300K = –(1200mV – 598mV)/
–2.0 mV/K)
2nd
Example: Figure 6 for 75°C, or 350K the diode
voltage is 0.50V, thus, 350K = –(1200mV – 500mV)/
–2.0mV/K)
Converting the Kelvin scale to Celsius is simply taking the
Kelvin temp and subtracting 273 from it.
A typical forward voltage is given in the electrical charac
-
teristics section
of the data sheet, and Figure 6 is the plot
of this forward voltage. Measure this forward voltage at
applicaTions inForMaTion
27°C to establish a reference point. Then using the above
expression while measuring the forward voltage over
temperature will provide a general temperature monitor.
Connect a resistor between TEMP and V
IN
to set the cur-
rent to 100µA. See Figure 22 for an example.
Figure 6. Diode Voltage V
D
vs Temperature T(°C)
TEMPERATURE (°C)
–50 –25
0.3
DIODE VOLTAGE (V)
0.5
0.8
0
50
75
0.4
0.7
0.6
25
100
4637 F06
125
I
D
= 100µA
Overtemperature Protection
The internal overtemperature protection monitors the
internal temperature of the module and shuts off the
regulator at ~130°C to 137°C. Once the regulator cools
down the regulator will restart.
Run Enable
The RUN pin is used to enable the power module or se
-
quence the power module. The threshold is 1.25V, and
the
pin has an internal 5.1V Zener to protect the pin. The
RUN pin can be used as an undervoltage lockout (UVLO)
function by connecting a resistor divider from the input
supply to the RUN pin:
V
UVLO
= ((R1+R2)/R2) • 1.25V
See Figure 1, Simplified Block Diagram.