May 2005

ASM2I99447

rev 0.3

3.3V/2.5V 1:9 LVCMOS Clock Fanout Buffer

9 of 14

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

Power Consumption of the ASM2I99447 and

Thermal Management

The ASM2I99447 AC specification is guaranteed for the

entire operating frequency range up to 350MHz. The

ASM2I99447 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

ASM2I99447 die junction temperature and the associated

device reliability.

Table 9. 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 ASM2I99447

needs to be controlled and the thermal impedance of the

board/package

should

be

optimized.

The

power

dissipated in the

ASM2I99447

is represented in

capacitance per output,

capacitive output load, N is the number of active outputs

(N is always 12 in case of the ASM2I99447). The

ASM2I99447 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,

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

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.

Where Rthja is the thermal impedance of the package

(junction to ambient) and TA is the ambient temperature.

According to Table 9, 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 ASM2I99447 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

















⋅

−

−

⋅

⋅

⋅

=

⋅

+

=

⋅

⋅

−

+

−

⋅

+

























+

⋅

⋅

⋅

+

⋅

=

⋅

























+

⋅

⋅

⋅

+

=

∑

∑

∑