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MPC7448 RISC Microprocessor Hardware Specifications, Rev. 4

52 Freescale Semiconductor

System Design Information

9.7.4 Temperature Diode

The MPC7448 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 MPC7448’s internal diode.

The following are the specifications of the MPC7448 on-board temperature diode:

> 0.40 V

< 0.90 V

Operating range 2–300 μA

Diode leakage < 10 nA @ 125°C

Ideality factor over 5–150 μA at 60°C: n = 1.0275 ± 0.9%

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

Another useful equation is:

Where:

= Forward current

= Saturation current

= Voltage at diode

= Voltage forward biased

= Diode voltage while I

is flowing

= Diode voltage while I

is flowing

= Larger diode bias current

= Smaller diode bias current

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)

The ratio of I

to I

is usually selected to be 10:1. The previous equation simplifies to the following:

= I

e – 1

f___

nKT

– V

= n

– 1

H__

– V

= 1.986 × 10

–4

× nT

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