AK4180 Datasheet, PDF (10/17 Pages) Asahi Kasei Microsystems – Touch Screen Controller

[ASAHI KASEI]
[AK4180]
„ Temperature Measurement
Equation <1> describes the forward characteristics of the diode.
i_D=I₀exp(v_D/V_T) ( V_T = kT/q)
<1>
I₀: 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 characteristics is approximately showed As a diode junction voltage is theoretically proportional to the temperature, the
ambient temperature can be predicted by knowing this voltage.
Temp.
I
Sensor
82I
TEMP0
TEMP0
Figure 4 Temperature Measurement
As the AK4180 has two different fixed current circuits and a diode, 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(NI) – V(I)
T°C = 2.63×10³ × ∆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)* v_D/ln(i_D/I₀)
<2>
<MS0195-E-01>
10
2003/02