Datasheet

ADSP-21483/ADSP-21486/ADSP-21487/ADSP-21488/ADSP-21489
Rev. B | Page 57 of 68 | March 2013
Note that the thermal characteristics values provided in
Table 56 and Table 57 are modeled values.
Thermal Diode
The ADSP-2148x processors incorporate thermal diode/s to
monitor the die temperature. The thermal diode of 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 temperature sensor (such as ADM 1021A 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:
where:
n = multiplication factor close to 1, depending on process
variations
k = Boltzmann’s 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 micro Amperes
to 300 micro Amperes for the common temperature sensor
chips available.
Table 58 contains the thermal diode specifications using the
transistor model.
Table 56. Thermal Characteristics for 100-Lead LQFP_EP
Parameter Condition Typical Unit

JA
Airflow = 0 m/s 17.8 °C/W

JMA
Airflow = 1 m/s 15.4 °C/W

JMA
Airflow = 2 m/s 14.6 °C/W

JC
2.4 °C/W

JT
Airflow = 0 m/s 0.24 °C/W

JMT
Airflow = 1 m/s 0.37 °C/W

JMT
Airflow = 2 m/s 0.51 °C/W
Table 57. Thermal Characteristics for 176-Lead LQFP_EP
Parameter Condition Typical Unit

JA
Airflow = 0 m/s 16.9 °C/W

JMA
Airflow = 1 m/s 14.6 °C/W

JMA
Airflow = 2 m/s 13.8 °C/W

JC
2.3 °C/W

JT
Airflow = 0 m/s 0.21 °C/W

JMT
Airflow = 1 m/s 0.32 °C/W

JMT
Airflow = 2 m/s 0.41 °C/W
V
BE
n
kT
q
------
In(N)=
Table 58. Thermal Diode Parameters – Transistor Model
1
Symbol Parameter Min Typ Max Unit
I
FW
2
Forward Bias Current 10 300 A
I
E
Emitter Current 10 300 A
n
Q
3, 4
Transistor Ideality 1.012 1.015 1.017
R
T
3, 5
Series Resistance 0.12 0.2 0.28 
1
See Engineer-to-Engineer Note “Using the On-Chip Thermal Diode on Analog Devices Processors” (EE-346).
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: I
C
= I
S
× (e
qVBE/nqkT
–1) where I
S
= saturation current,
q = electronic charge, V
BE
= voltage across the diode, k = Boltzmann Constant, and T = absolute temperature (Kelvin).
5
The series resistance (R
T
) can be used for more accurate readings as needed.