ADSP-21477KCPZ-1A Datasheet, PDF (67/76 Pages) Analog Devices – SHARC Processor

ADSP-21477/ADSP-21478/ADSP-21479
Thermal Diode
The processors incorporate thermal diode/s to monitor the die
temperature. The thermal diode is a grounded collector, PNP
bipolar junction transistor (BJT). The THD_P pin is connected
to the emitter, and the THD_M pin is connected to the base of
the transistor. These pins can be used by an external tempera-
ture sensor (such as ADM1021A or LM86 or others) to read the
die temperature of the chip.
The technique used by the external temperature sensor is to
measure the change in VBE when the thermal diode is operated
at two different currents. This is shown in the following
equation:
VBE=
n  ^k----^T-  In(N)
q
where:
n = multiplication factor close to 1, depending on process
variations
k = Boltzmann constant
T = temperature (°C)
q = charge of the electron
N = ratio of the two currents
The two currents are usually in the range of 10 μA to 300 μA for
the common temperature sensor chips available.
Table 60 contains the thermal diode specifications using the
transistor model.
Table 60. Thermal Diode Parameters—Transistor Model¹
Symbol
Parameter
Min
Typ
Max
Unit
I_FW2
Forward Bias Current
10
300
μA
I_E
n_Q3, ⁴
R_T3, ⁵
Emitter Current
Transistor Ideality
Series Resistance
10
300
μA
1.012
1.015
1.017
0.12
0.2
0.28
Ω
1 _{Analog Devices does not recommend operation of the thermal diode under reverse bias.}
2 _{Analog Devices does not recommend operation of the thermal diode under reverse bias.}
3 _{Specified by design characterization.}
4 _{The ideality factor, nQ, represents the deviation from ideal diode behavior as exemplified by the diode equation: IC = IS × (e} qVBE/nqkT _{– 1) where IS = saturation current,}
q = electronic charge, V_BE = voltage across the diode, k = Boltzmann constant, and T = absolute temperature (Kelvin).
5 _{The series resistance (RT) can be used for more accurate readings as needed.}
Rev. C | Page 67 of 76 | July 2013