pc87591l National Semiconductor Corporation, pc87591l Datasheet - Page 187

• Manufacturer Part Number: pc87591l
• Description: Lpc Mobile Embedded Controllers
• Manufacturer: National Semiconductor Corporation

Revision 1.07
Embedded Controller Modules
Remote Diode Selection
Temperature accuracy depends on a good quality transistor used as a diode-connected, small-signal transistor. Accuracy
has been experimentally verified for the devices, among them Motorola, listed in Table 25. A temperature sensor can directly
measure the die temperature of CPUs with on-board temperature-sensing diodes.
The transistor must be a small-signal type with a relatively high forward voltage; otherwise, the A/D input voltage range might
be violated. The forward voltage must be greater than 0.25V at 10 A for the highest expected temperature. The forward
voltage must be less than 0.95V at 100 A for the lowest expected temperature. Large power transistors do not work. Also,
ensure that the base resistance is less than 100 . Tight specifications for forward-current gain (+50 to +150, for example)
indicate devices with consistent VBE characteristics.
Thermal mass can seriously degrade the temperature sensor’s effective accuracy. The use of smaller packages for remote
sensors, such as SOT23s, improves the situation.
Filtering the Noise on Temperature Input Signals
Noise may be coupled to the input signal for various reasons, including close proximity to digital circuits. The ADC conver-
sion technology has good noise rejection especially for low-frequency signals, such as power supply hum. Rejection of high-
frequency noise can be improved using an external filter capacitor and PC board layout.
High-frequency noise is best filtered by placing a capacitor (C
imity to the PC87591x pins. The connection to the diode should use a twisted wire pair or a PCB that is read out using the
guidelines below.
It is most important to prevent any leak to ground, especially from the DP pin and serial resistance on the current loop to the
diode.
C
a. NTC Thermistor
F
AVCC
Figure 63. Measuring Temperature Using Thermistors
T
o
Table 25. Remote Sensor Transistor Manufacturers
Thermistor
National Semiconductor (USA)
Central Semiconductor (USA)
Rohm Semiconductor (Japan)
AGND
R
NTC
Siemens (Germany)
Samsung (Korea)
Zetex (England)
Motorola (USA)
ADn
Manufacturer
(Continued)
187
C
of 2.2 nF) between pins DP and DN (see Figure 64), in prox-
FMMT3904CT-ND
KST3904-TF
MMBT3904
MMBT3904
CMPT3904
SMBT3904
SST3904
Model
C
F
b. PTC Thermistor
AVCC
T
o
Thermistor
AGND
R
PTC
ADn
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