April 2007
rev 1.2
ASM2I9940L
Power Consumption of the ASM2I9940L and
Thermal Management
Where ICCQ is the static current consumption of the
ASM2I9940L, CPD is the power dissipation capacitance
per output, (M)∑CL represents the external capacitive
output load, N is the number of active outputs (N is
always 12 in case of the ASM2I9940L). The ASM2I9940L
supports driving transmission lines to maintain high signal
integrity and tight timing parameters. Any transmission
line will hide the lumped capacitive load at the end of the
board trace, therefore, ∑CL is zero for controlled
transmission line systems and can be eliminated from
equation 1. Using parallel termination output termination
results in equation 2 for power dissipation.
The ASM2I9940L AC specification is guaranteed for the
entire operating frequency range up to 250MHz. The
ASM2I9940L power consumption and the associated
long-term reliability may decrease the maximum
frequency limit, depending on operating conditions such
as clock frequency, supply voltage, output loading,
ambient temperature, vertical convection and thermal
conductivity of package and board. This section
describes the impact of these parameters on the junction
temperature and gives a guideline to estimate the
ASM2I9940L die junction temperature and the associated
device reliability.
In equation 2, P stands for the number of outputs with a
parallel or thevenin termination, VOL, IOL, VOH and IOH are
a function of the output termination technique and DCQ is
the clock signal duty cycle. If transmission lines are used
∑CL is zero in equation 2 and can be eliminated. In
general, the use of controlled transmission line
techniques eliminates the impact of the lumped capacitive
loads at the end lines and greatly reduces the power
dissipation of the device. Equation 3 describes the die
junction temperature TJ as a function of the power
consumption.
Table 11. Die junction temperature and MTBF
Junction temperature
MTBF (Years)
(°C)
100
110
120
130
20.4
9.1
4.2
2.0
Increased power consumption will increase the die
junction temperature and impact the device reliability
(MTBF). According to the system-defined tolerable
MTBF, the die junction temperature of the ASM2I9940L
needs to be controlled and the thermal impedance of the
board/package should be optimized. The power
dissipated in the ASM2I9940L is represented in
equation 1.
Where Rthja is the thermal impedance of the package
(junction to ambient) and TA is the ambient temperature.
According to Table 11, the junction temperature can be
used to estimate the long-term device reliability. Further,
combining equation 1 and equation 2 results in a
maximum operating frequency for the ASM2I9940L in a
series terminated transmission line system, equation 4.
P
= ICCQ + VCC ⋅ fCLOCK ⋅ N ⋅ C
+
C
⋅VCC
Equation1
L
∑
TOT
PD
M
P
=VCC ⋅ ICCQ + VCC ⋅ fCLOCK ⋅ N ⋅ C
+
C
+
[
∑
P
DC ⋅ I
(
VCC −VOH
)
+
(
1− DCQ
)
⋅ IOL ⋅VOL
]
Equation 2
Equation3
L
∑
TOT
PD
Q
OH
M
TJ = TA + P ⋅ Rthja
TOT
TJMAX − TA
1
(
)
fCLOCKMAX
=
⋅
− ICCQ ⋅VCC
Equation 4
CPD ⋅ N ⋅VC2C
Rthja
Low Voltage 1:18 Clock Distribution Chip
8 of 13
Notice: The information in this document is subject to change without notice.
PULSECORE [ PulseCore Semiconductor ]
