F75393
6.9 ADC Noise Filtering
The ADC is integrating type with inherently good noise rejection. Micro-power operation places constraints on high-frequency noise
rejection; therefore, careful PCB board layout and suitable external filtering are required for high-accuracy remote measurement in
electronically noisy environment. High frequency EMI is best filtered at D+ and D- with an external 2200pF capacitor. Too high
capacitance may introduce errors due to the rise time of the switched current source. Nearly all noise sources tested cause the ADC
measurement to be higher than the actual temperature, depending on the frequency and amplitude.
6.10 Beta Compensation
The F75393 is configured to detect the temperature of diodes (e.g. 2N3906) or CPU thermal diodes. The diode can be connected in
different way as below Figure.
D+
D-
D+
D-
Substrate PNP transistor
i.e. CPU
Discrete PNP transistor
i.e. 2N3906
The basic of the temperature sensor follows mathematical formula as below:
∆
V
BE
1
+
β
1
β
Ic
1
KT
Ie
1
KT
1
=
×
ln
=
×
ln
q
Ie
2
q
1
+
β
2
β
Ic
2
2
The F75393 measures temperature from the thermal diodes by the basic. In traditional case, the F75393 outputs dual currents to a
thermal diode. Then the F75393 calculates the absolute temperature by
△
V
BE
. For discrete transistor (i.e. 2N3906), the beta is
normally very high such that the percent change in beta is very small. For example, 15% variation in beta for two forced I
E
currents
and the beta is 50 would contribute about 0.32
℃
error per 100
℃
. For Substrate PNP transistor (i.e. CPU), the beta is very small
such that the proportional beta variation will very high, and it will cause large error in temperature measurement. For example, 15%
variation in beta for two forced I
E
currents and the beta is 0.5 would contribute about 11.12
℃
error per 100
℃
.
In Order to solve the second issue, the F75393 provides a beta compensation solution for accurate temperature sensing. There is a
register (CR30h bit7) for external thermal diode selection by Beta variation. If this bit is enabled, the beta compensation will
automate to measure the temperature from substrate transistor (i.e. CPU). The F75393 can support the beta range from 0.05~1.8 for
beta compensation. In this new method, the F75393 will provide two I
E
currents, and feedback two I
B
currents. The F75393 will
F75393
-7-
May, 2008
V0.16P
FINTEK [ FEATURE INTEGRATION TECHNOLOGY INC. ]
