Datasheet

LTM4644/LTM4644-1
16
Rev. F
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APPLICATIONS INFORMATION
Temperature Monitoring
A diode connected PNP transistor is used for the TEMP
monitor function by monitoring its voltage over tempera
-
ture. The temperature dependence of this diode voltage
can be understood in the equation:
V
D
=nV
T
ln
I
D
I
S
⎛
⎝
⎜
⎞
⎠
⎟
where V
T
is the thermal voltage (kT/q), and n, the ideality
factor, is 1 for the diode connected PNP transistor be-
ing used
in the LTM4644. I
S
is expressed by the typical
empirical equation:
I
S
=I
0
exp
–V
G0
V
T
⎛
⎝
⎜
⎞
⎠
⎟
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 LTM4644.
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 7 for 27°C, or 300K the diode
voltage is 0.598V, thus, 300K = –(1200mV – 598mV)/
–2.0 mV/K)
2
nd Example: Figure 7 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 7 is the plot
of this forward voltage. Measure this forward voltage at
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 35 for an example.
Figure 7. 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 F07
125
I
D
= 100µA
Thermal Considerations and Output Current Derating
The thermal resistances reported in the Pin Configura-
tion section
of the data sheet are consistent with those
parameters defined by JESD 51-12 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 per-
formed on a µModule package mounted to a hardware
test board: defined by JESD 51-9 (“Test Boards for Area
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