ASAHI KASEI
[AK4182A]
Temperature Measurement
Equation <1> describes the forward characteristics of the diode.
iD=I0exp(vD/VT) ( VT = kT/q)
<1>
I0: reverse saturation current
q : 1.602189×10-19 (electron charge)
k : 1.38054×10-23 (Boltzmann’s constant)
vD: 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.
Temp.
iD1
= I
Sensor
iD2
= 91 x I
TEMP0
TEMP1
Figure 5 Temperature Measurement
As the AK4182A 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>
(iD2 / iD1) = exp {(v(91 x I) - v(I))/VT} N = (iD2 / iD1) = 91 (ratio of the current)
T°C = (∆Vbe * q)/(k * ln N) – 273
∆Vbe = V (91 x I) – V (I)
T°C = 2.573×103 × ∆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)* vD/ln (iD/I0)
<2>
MS0482-E-00
10
2006/03
AKM [ ASAHI KASEI MICROSYSTEMS ]
