Datasheet

Rev. 0 | Page 62 of 72 | June 2010
ADSP-21469
Values of 
JB
are provided for package comparison and PCB
design considerations. Note that the thermal characteristics val-
ues provided in Table 56 are modeled values.
Thermal Diode
The ADSP-21469 processors incorporate thermal diodes 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 V
BE
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 μA to 300 μA for
the common temperature sensor chips available.
Table 57 contains the thermal diode specifications using the
transistor model. Note that Measured Ideality Factor already
takes into effect variations in beta ().
Table 56. Thermal Characteristics for 324-Lead CSP_BGA
Parameter Condition Typical Unit

JA
Airflow = 0 m/s 22.7 °C/W

JMA
Airflow = 1 m/s 20.4 °C/W

JMA
Airflow = 2 m/s 19.5 °C/W

JC
6.6 °C/W

JT
Airflow = 0 m/s 0.11 °C/W

JMT
Airflow = 1 m/s 0.19 °C/W

JMT
Airflow = 2 m/s 0.24 °C/W
V
BE
n
kT
q
------
In(N)=
Table 57. 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
4, 5
Series Resistance 0.12 0.2 0.28 
1
See the Engineer-to-Engineer Note EE-346.
2
Analog Devices does not recommend operation of the thermal diode under reverse bias.
3
Not 100% tested. 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.