Datasheet

REV. C
ADSP-21065L
–38–
POWER DISSIPATION
Total power dissipation has two components: one due to inter-
nal circuitry and one due to the switching of external output
drivers. Internal power dissipation depends on the sequence in
which instructions execute and the data operands involved. See
I
DDIN
calculation in Electrical Characteristics section. Internal
power dissipation is calculated this way:
P
INT
= I
DDIN
¥ V
DD
The external component of total power dissipation is caused by
the switching of output pins. Its magnitude depends on:
– the number of output pins that switch during each cycle (O)
– the maximum frequency at which the pins can switch (f)
– the load capacitance of the pins (C)
– the voltage swing of the pins (V
DD
).
The external component is calculated using:
P
EXT
= O ¥ C ¥ V
DD
2
¥ f
The load capacitance should include the processor’s package
capacitance (C
IN
). The frequency f includes driving the load
high and then back low. Address and data pins can drive high
and low at a maximum rate of 1/t
CK
while in SDRAM burst
mode.
Example:
Estimate P
EXT
with the following assumptions:
–a system with one bank of external memory (32-bit)
–two 1M ¥ 16 SDRAM chips, each with a control signal load
of 3 pF and a data signal load of 4 pF
– external data writes occur in burst mode, two every 1/t
CK
cycles, a potential frequency of 1/t
CK
cycles/s. Assume 50%
pin switching
– the external SDRAM clock rate is 60 MHz (2/t
CK
).
The P
EXT
equation is calculated for each class of pins that can
drive:
Table V. External Power Calculations
Pin # of %
Type Pins Switching ⴛ C ⴛ f ⴛ V
DD
2
= P
EXT
Address 11 50 ¥ 10.7 ¥ 30 MHz ¥ 10.9 V = 0.019 W
MS
0
10 ¥ 10.7 — ¥ 10.9 V = 0.000 W
SDWE 10 ¥ 10.7 — ¥ 10.9 V = 0.000 W
Data 32 50 ¥ 7.7 ¥ 30 MHz ¥ 10.9 V = 0.042 W
SDRAM CLK 1 – ¥ 10.7 ¥ 30 MHz ¥ 10.9 V = 0.007 W
P
EXT
= 0.068 W
A typical power consumption can now be calculated for these
conditions by adding a typical internal power dissipation. (I
DDIN
see calculation in Electrical Characteristics section):
P
TOTAL
= P
EXT
+ (I
DDIN
¥ V
DD
)
Note that the conditions causing a worst-case P
EXT
differ from
those causing a worst-case P
INT
. Maximum P
INT
cannot occur
while 100% of the output pins are switching from all ones (1s)
to all zeros (0s). Note also that it is not common for an appli-
cation to have 100% or even 50% of the outputs switching
simultaneously.
ENVIRONMENTAL CONDITIONS
Thermal Characteristics
The ADSP-21065L is offered in a 208-lead MQFP and a 196-
ball Mini-BGA package.
The ADSP-21065L is specified for a case temperature (T
CASE
)
.
To ensure that T
CASE
is not exceeded, an air flow source may be
used.
T
CASE
= T
AMB
+ (PD ¥ q
CA
)
T
CASE
=Case temperature (measured on top surface of package)
PD = Power Dissipation in W (this value depends upon the
specific application; a method for calculating PD is
shown under Power Dissipation)
q
JC
= 7.1∞C/W for 208-lead MQFP
q
JC
= 5.1∞C/W for 196-ball Mini-BGA
Airflow
Table VI. Thermal Characteristics (208-Lead MQFP)
(Linear Ft./Min.) 0 100 200 400 600
q
CA
(∞C/W) 24 20 19 17 13
Table VII. 196-Ball Mini-BGA
(Linear Ft./Min.) 0 200 400
q
CA
(∞C/W) 38 29 23