ASAHI KASEI
[AK4181A]
Temperature Measurement
Equation <1> describes the forward characteristics of the diode.
i
D
=I
0
exp(v
D
/V
T
) ( V
T
= kT/q)
<1>
I
0
: reverse saturation current
q : 1.602189×10
-19
(electron charge)
k : 1.38054×10
-23
(Boltzmann’s constant)
v
D
: voltage across diode
T: absolute temperature K
The diode characteristic is approximately shown as a diode junction voltage. That is theoretically proportional to the temperature;
the ambient temperature can be predicted by knowing this voltage.
I
Temp.
Sensor
82I
TEMP0
TEMP1
Figure 5 Temperature Measurement
As the AK4181A has two different fixed current circuits and a diode (temperature sensor), the temperature can be measured by
using two different methods.
The first method needs two conversions, but can derive the temperature directly without knowing the voltage at a specific
temperature.
From equation <1>
(i
D2
/ i
D1
) = exp{(v
(NI)
- v
(I)
)/V
T
}
N = (i
D2
/ i
D1
) = 82 (ratio of the current)
T°C = (∆Vbe * q)/(k * ln N) – 273
∆Vbe
= V(82I) – V(I)
T°C = 2.63×10
3
× ∆Vbe
– 273
The second method needs only one conversion as the following equation, but requires knowing the junction voltage at the specific
temperature.
T = (k/q)* v
D
/ln(i
D
/I
0
)
<2>
MS0313-E-00
10
2004/06
AKM [ ASAHI KASEI MICROSYSTEMS ]
