LTM4636
18
4636f
where V
D
appears to increase with temperature. It is com-
mon knowledge that a silicon diode biased with a current
source has an approximate –2mV/°C temperature rela-
tionship (Figure 7), which is at odds with the equation. In
fact, the I
S
term increases with temperature, reducing the
ln(I
D
/I
S
) absolute value yielding an approximate –2mV/°C
composite diode voltage slope.
applicaTions inForMaTion
To obtain a linear voltage proportional to temperature
we cancel the I
S
variable in the natural logarithm term to
remove the I
S
dependency from the equation 1. This is
accomplished by measuring the diode voltage at two cur-
rents I
1
, and I
2
, where I
1
= 10 •
I
2
) and subtracting we get:
∆V
D
=
T(KELVIN)
•
K
D
•
IN
I
1
I
S
– T(KELVIN)
•
K
D
•
IN
I
2
I
S
Combining like terms, then simplifying the natural log
terms yields:
∆V
D
= T(KELVIN
) •
K
D
•
lN(10)
and redefining constant
K'
D
=
K
D
•
IN(10)
=
198µV
K
yields
∆V
D
= K'
D
•
T(KELVIN)
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
4636 F07
125
Solving for temperature:
T(KELVIN)
=
∆V
D
K'
D
(
°
CELSIUS)
=
T(KELVIN)– 273.15
where
300°K = 27°C
means that is we take the difference in voltage across the
diode measured at two currents with a ratio of 10, the
resulting voltage is 198μV per Kelvin of the junction with
a zero intercept at 0 Kelvin.
The diode connected NPN transistor at the TEMP pin can be
used to monitor the internal temperature of the LTM4636.
Figure 8. The Two Images Show the LTM4636 Operating at 1V at
40A and 3.3V at 35A from a 12V Input. Both Images Reflect Only
a 40°C to 45°C Rise Above Ambient at Full Load Current with
200LFM.
8b.
8a.
V
IN
V
OUT
I
OUT
AIR FLOW
12
1
40
200 LFM
V
IN
V
OUT
I
OUT
AIR FLOW
12
3.3
35
200 LFM
