Linear ANALOG DEVICES LTM4644EY PBF Series Manual page 16

LTM4644/LTM4644-1
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:
⎛ ⎞
I
D
V ln
=nV
⎜ ⎟
D T
I
⎝ ⎠
S
where V is the thermal voltage (kT/q), and n, the ideality
T
factor, is 1 for the diode connected PNP transistor be-
ing used in the LTM4644. I
empirical equation:
⎛ ⎞
–V
G0
I exp
=I
⎜ ⎟
S 0
V
⎝ ⎠
T
where I is a process and geometry dependent current, (I
0
is typically around 20k orders of magnitude larger than I
at room temperature) and V
1.2V extrapolated to absolute zero or –273°C.
If we take the I equation and substitute into the V
S
tion, then we get:
⎛ ⎞
⎛ kT ⎞ I
0
V = V – ⎟ ln ⎟ , V
⎜
⎜
D G0
q I
⎝ ⎠
⎝ ⎠
D
The expression shows that the diode voltage decreases
(linearly if I were constant) with increasing temperature
0
and constant diode current. Figure 6 shows a plot of V
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 V – V
D G0 D
= –
dT T
This dV /dT term is the temperature coefficient equal to
D
about –2mV/K or –2mV/°C. The equation is simplified for
the first order derivation.
16
is expressed by the typical
S
is the band gap voltage of
G0
equa-
D
kT
=
T
q
For more information
Solving for T, T = –(V
temperature.
1st Example: Figure 7 for 27°C, or 300K the diode
voltage is 0.598V, thus, 300K = –(1200mV – 598mV)/
–2.0 mV/K)
2nd 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.
0
Connect a resistor between TEMP and V
S
rent to 100µA. See Figure 35 for an example.
0.8
0.7
0.6
0.5
0.4
D
0.3
–50 –25
Figure 7. Diode Voltage V
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
www.analog.com
– V )/(dV /dT) provides the
G0 D D
to set the cur-
IN
I = 100µA
D
50
0 25 75 100 125
TEMPERATURE (°C)
4637 F07
vs Temperature T(°C)
D
Rev. G