MPC7447 datasheet(52/60 Pages) FREESCALE

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MPC7448 RISC Microprocessor Hardware Specifications, Rev. 4

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 VBE variation of each MPC7448’s internal diode.

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

Vf > 0.40 V

Vf < 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:

Ifw = Forward current

Is = Saturation current

Vd = Voltage at diode

Vf = Voltage forward biased

VH = Diode voltage while IH is flowing

VL = Diode voltage while IL is flowing

IH = Larger diode bias current

IL = Smaller diode bias current

q = Charge of electron (1.6 x 10 –19 C)

n = Ideality factor (normally 1.0)

K = Boltzman’s constant (1.38 x 10–23 Joules/K)

T = Temperature (Kelvins)

The ratio of IH to IL is usually selected to be 10:1. The previous equation simplifies to the following:

Ifw = Is e

– 1

qVf

___

nKT

VH – VL = n

– 1

VH – VL = 1.986 × 10

–4 × nT

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