f75395 Feature Integration Technology Inc., f75395 Datasheet - Page 11

• Manufacturer Part Number: f75395
• Description: ?1o C Temperature Sensor With ? Compensation
• Manufacturer: Feature Integration Technology Inc.

6.8 Thermal Mass and Self Heating
6.9 ADC Noise Filtering
6.10 Beta Compensation
F75395
Thermal mass effect can seriously degrade the F75395’s effective accuracy. The thermal time constant of the MSOP package is
about 140 in still air. For the F75395 junction temperature to settle to within +1°C after a sudden +100°C change requires about five
time constants or 12 minutes. The use of smaller packages for remote sensors such as SOT23, improves the situation. Take care to
account for thermal gradients between the heat source and the sensor package do not interfere with measurement accuracy.
Sel-heating does not significantly affect measurement accuracy. Remote sensor self-heating due to the diode current source is
negligible. For the local diode, the worst case error occurs when auto-converting at the fastest rate and simultaneously sinking
maximum current at the ALERT# output. For instance, at an 64Hz rate and ALERT# sink around 0.7mA when pull up resistor 4.7K
ohm to 3.3VCC, the typical power dissipation is VCC x 220 uA plus 0.4V x 0.7mA. Package θ
3.3V and no copper PC board heat-sinking, the resulting temperature rise is:
Even with these contrived circumstances, it is difficult to introduce significant self-heating errors.
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.
The F75395 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.
The basic of the temperature sensor follows mathematical formula as below:
Substrate PNP transistor
i.e. CPU
dT = 1.01mW x 120 °C/W = 0.12 °C
D+
D-
-7-
Discrete PNP transistor
i.e. 2N3906
JA
is about 120 °C/W, so with VCC =
D+
D-
F75395
May, 2008
V0.15P