POWER DISSIPATION
INTRODUCTION TO STD80/STDM80
STD80/STDM80
1-8
SEC ASIC
Power Dissipation in STD80/STDM80
This section describes the equations on how to
estimate the power dissipation in STD80/STDM80. As
explained in the previous section, the total power
dissipation (P
TOTAL
) consists of static power dissipation
(P
DC
) and dynamic power dissipation (P
AC
).
P
TOTAL
= P
DC
+ P
AC
Since only output buffers contribute to the static power
dissipation,
P
DC
= P
DC_OUTPUT
,where P
DC
output is the static power dissipated
when output buffers source or sink.
The dynamic power dissipation is caused by three
components: input buffers (P
AC_INPUT
), output buffers
(P
AC_OUTPUT
), and internal cells (P
AC_INTERNAL
).
P
AC
= P
AC_ INPUT
+ P
AC_OUTPUT
+ P
AC_INTERNAL
Each term mentioned above is characterized by the
following equations:
In STD80,
P
DC_OUTPUT
= 150 x I
OL
x N_output [
μ
W]
P
AC_INPUT
= 23 x N_input x F x S [
μ
W]
P
AC_OUTPUT
= 25 x N_output x F x S x C [
μ
W]
P
AC_INTERNAL
= 2.3 x N_internal x F x S [
μ
W]
In STDM80,
P
DC_OUTPUT
= 150 x I
OL
x N_output [
μ
W]
P
AC_INPUT
= 9.8 x N_input x F x S [
μ
W]
P
AC_OUTPUT
= 25 x N_output x F x S x C [
μ
W]
P
AC_INTERNAL
= 1.2 x N_internal x F x S [
μ
W]
,where
I
OL
is source and sink current of output buffers in
mA,
N_output is the number of output buffers used,
N_input is the number of input buffers used,
N_internal is the number of internal cells used,
F is the maximum operation frequency in MHz,
S is the estimated degree of a switching activity
(typically 0.2),
C is the output load capacitance in pF.
Temperature and Power Dissipation
The total power dissipation, P
TOTAL
can be used to find
out the device temperature by the following equation:
θ
JA
= (T
J
– T
A
) / P
TOTAL
,where
θ
JA
is the thermal impedance,
T
J
is the junction temperature of the device,
T
A
is the ambient temperature.
Thermal impedances of the SEC packages are given
in the following table. The junction temperature,
obtained by multiplying P
TOTAL
by the appropriate
θ
JA
and adding T
A
, determines the derating factor for the
propagation delays and also indicates the reliability
measures. Hence, designers can achieve the desired
derating factor and reliability targets by choosing
appropriate packages and system cooling methods.
Table 1-1.
Thermal Impedances of SEC
Packages
QFP
Maximum Junction Temperature (T
J
)
The allowable maximum junction temperatures for
plastic and ceramic packages are as follows:
Junction temperature for plastic package
≤
125
°
C
Junction temperature for ceramic package
≤
150
°
C.
Pin
Number
θ
JA
[
°
C/W]
64
80
100
120
160
208
240
60
60
60
50
50
40
40