Datasheet

LTM4620A
21
4620afb
For more information www.linear.com/LTM4620A
APPLICATIONS INFORMATION
I
S
at room temperature, so I
o
is much larger than typical
values of I
D
), and
VG0
is the band gap voltage of 1.2V
extrapolated to absolute zero of –273°C Kelvin. Figure 8
shows a
plot of the diode temperature characteristic of
the
diode connected PNP transistor biased with a 100µa
current source. This plot would extend to the left back to
1.2V at –273°C Kelvin. This curve is stop at –55°C due to
the test system limits.
If we take the I
S
equation and substitute into the V
D
equa-
tion, then we get:
VD = VG0 –
Kt
q




ln
I0
ID


, V
T
=
kT
q


 

The expression shows that the junction voltage of the PNP
connected diode decreases linearly if I
o
were constant
from a value
VG0
of 1.2V at absolute zero to a decreasing
value with increased temperature.
If we take this equation and differentiate it with respect to
temperature T, then:
dVD
dT
= – VG0 – VD
(
)
/ T This
dVD
dT
change as a function of temperature is the typical ~–2mV/°C.
This equation is simplified for the first order derivation.
Solving for T, T = –(VG0 – VD)/ dVD provide the temperature.
1st Example: Figure 4 for 27°C, or 300°C Kelvin the diode
voltage is 0.598V, thus, 300°C = –(1200mv – 598mv)/
–2mv/°C)
2nd Example: Figure 4 for 75°C, or 350°C Kelvin the diode
voltage is 0.50V, thus, 350°C = –(1200mv – 500mv)/
–2mv/°C)
Converting the Kelvin scale to Celsius is simply taking the
Kelvin temp and subtracting –273°C Kelvin from it.
A typical forward voltage is measured and placed in the
electrical characteristics section of the data sheet, and
Figure 8 is the plot of this forward voltage. Measure this
forward voltage at 27°C to establish a reference point.
Then use the above expression while measuring the
forward voltage over temperature will provide a general
temperature monitor.
Figure 8. Diode Voltage V
D
vs Temperature
T(°C) for Different Bias Currents
The diode connected PNP transistor can be pulled up to
V
IN
with a resistor to set the current to 100µA for using
this diode connected transistor as a general temperature
monitor by monitoring the diode voltage drop with tem
-
perature. See Figure 27 for an example.
Thermal Considerations and Output Current Derating
The thermal resistances reported in the Pin Configuration
section of the data sheet are consistent with those param
-
eters defined
by JESD51-9 and are intended for use with
finite element analysis (FEA) software modeling tools that
leverage the outcome of thermal modeling, simulation,
and correlation to hardware evaluation performed on a
µModule package mounted to a hardware test board—also
defined by JESD51-9 (“Test Boards for Area Array Surface
Mount Package Thermal Measurements”). The motivation
for providing these thermal coefficients in found in JESD
51-12 (“Guidelines for Reporting and Using Electronic
Package Thermal Information”).
Many designers may opt to use laboratory equipment
and a test vehicle such as the demo board to anticipate
the µModule regulator’s thermal performance in their ap
-
plication at various electrical and environmental operating
TEMPERATURE (°C)
–50 –25
0.3
TEMP PIN DIODE VOLTAGE (V)
0.5
0.8
0
50
75
0.4
0.7
0.6
25
100
4620A F08
125
I
D
= 100µA