ASM2I9940L Datasheet, PDF (8/10 Pages) Alliance Semiconductor Corporation – Low Voltage 1:18 Clock Distribution Chip

ASM2I9940L
Power Consumption of the ASM2I9940L and Thermal
Management
The ASM2I9940L AC specification is guaranteed for the
entire operating frequency range up to 250 MHz. 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.
Where I_CCQ is the static current consumption of the
ASM2I9940L, C_PD is the power dissipation capacitance per
output, (M)SC_L 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, SC_L 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.
In Equation 2, P stands for the number of outputs with a
Table 11. DIE JUNCTION TEMPERATURE AND
MTBF
parallel or thevenin termination, V_OL, I_OL, V_OH and I_OH are
a function of the output termination technique and DC_Q is
the clock signal duty cycle. If transmission lines are used
Junction Temperature (°C)
100
MTBF (Years)
20.4
SC_L 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
110
9.1
greatly reduces the power dissipation of the device.
120
4.2
130
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.
Equation 3 describes the die junction temperature T_J as a
function of the power consumption.
Where R_qja is the thermal impedance of the package
(junction−to−ambient) and T_A 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.
ƪ ǒ ^S Ǔƫ P_TOT + I_CCQ ) V_CC @ f_CLOCK @ N @ C_PD ) _M C_L @ V_CC
(eq. 1)
ƪ ǒ ^S Ǔƫ ^{S ƪ ƫ} P_TOT + V_CC @ I_CCQ ) V_CC @ f_CLOCK @ N @ C_PD ) _M C_L ) _P DC_Q @ I_OH^ǒV_CC * V_OH^Ǔ ) ^ǒ1 * DC_Q^Ǔ @ I_OL @ V_OL
(eq. 2)
T_J + T_A ) P_TOT @ R_qJA
(eq. 3)
ƪ ƫ ^{ǒ Ǔ} f_CLOCKMAX
+
C_PD
@
1
N@
V_CC
2
@
T_JMAX * T_{A *}
R_qJA
I_CCQ @ V_CC
(eq. 4)
T_J,MAX should be selected according to the MTBF system
requirements and Table 11. R_qja can be derived from
Table 12. The R_qja represent data based on 1S2P boards,
using 2S2P boards will result in a lower thermal impedance
than indicated below.
Table 12. THERMAL PACKAGE IMPEDANCE OF
THE 32LQFP
Convection,
LFPM
R_thja (1P2S
board), °C/W
R_thja (2P2S
board), °C/W
Still air
86
61
100 lfpm
76
56
200 lfpm
71
54
300 lfpm
68
53
400 lfpm
66
52
500 lfpm
60
49
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