June 2005

ASM2I9940L

rev 1.0

Low Voltage 1:18 Clock Distribution Chip

8 of 13

Notice: The information in this document is subject to change without notice.

Power Consumption of the ASM2I9940L and

Thermal Management

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.

Table 11. Die junction temperature and MTBF

Junction temperature

(°C)

MTBF (Years)

100

20.4

110

9.1

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.

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

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

the clock signal duty cycle. If transmission lines are used

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

consumption.

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.

()

()

[]

()

4

1

3

2

1

1

2

Equation

V

I

R

T

T

V

N

C

f

Equation

R

P

T

T

Equation

V

I

DC

V

V

I

DC

C

C

N

f

V

I

V

P

Equation

V

C

C

N

f

V

I

P

CC

CCQ

thja

A

JMAX

CC

PD

CLOCKMAX

thja

TOT

A

J

P

OL

OL

Q

OH

CC

OH

Q

M

L

PD

CLOCK

CC

CCQ

CC

TOT

CC

M

L

PD

CLOCK

CC

CCQ

TOT

















⋅

−

−

⋅

⋅

⋅

=

⋅

+

=

⋅

⋅

−

+

−

⋅

+

























+

⋅

⋅

⋅

+

⋅

=

⋅

























+

⋅

⋅

⋅

+

=

∑

∑

∑