Compatible Systems RISC 2800i Bedienungsanleitung PDF herunterladen (Seite 49)

MPC7447A RISC Microprocessor Hardware Specifications, Rev. 5

Freescale Semiconductor 49

System Design Information

9.8.4 Temperature Diode

The MPC7447A has a temperature diode on the microprocessor that can be used in conjunction with other

system temperature monitoring devices (such as Analog Devices, ADT7461™). These devices use the

negative temperature coefficient of a diode operated at a constant current to determine the temperature of

the microprocessor and its environment. For proper operation, the monitoring device used should

auto-calibrate the device by canceling out the V

variation of each MPC7447A’s internal diode.

The following are the specifications of the MPC7447A on-board temperature diode:

0.40 V <V

<0.90 V

Operating range 2–300 μA

Diode leakage < 10 nA @ 125 C

Ideality factor (n) over 5–150 μA @ 60 C: 1.0275 ± 0.9 %

Ideality factor is defined as the deviation from the ideal diode equation:

Where:

= Forward current

= Saturation current

= Voltage at diode BM: this does not show up in any equations.

= Voltage forward biased

q = Charge of electron (1.6 x 10

-19

n = Ideality factor (normally 1.0)

K = Boltzman’s constant (1.38 x 10

-23

Joules/K)

T = Temperature (Kelvins)

Another useful equation is :

Where:

= Diode voltage while I

is flowing

= Diode voltage while I

is flowing

= Larger diode bias current

= Smaller diode bias current

= I

e – 1

f___

nKT

– V

= n

H__

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