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ICS843246BGI REV. A AUGUST 2, 2007

ICS843246I

FEMTOCLOCKS™ CRYSTAL-TO-3.3V LVPECL FREQUENCY SYNTHESIZER W/FANOUT BUFFER

3. Calculations and Equations.

The purpose of this section is to derive the power dissipated into the load.

LVPECL output driver circuit and termination are shown in Figure 7.

Ω load, and a termination

voltage of V

CCO

– 2V.

•

For logic high, V

OUT

= V

OH_MAX

= V

CCO_MAX

– 0.9V

(V

CCO_MAX

– V

OH_MAX

) = 0.9V

•

For logic low, V

OUT

= V

OL_MAX

= V

CCO_MAX

– 1.7V

(V

CCO_MAX

– V

OL_MAX

) = 1.7V

Pd_H is power dissipation when the output drives high.

Pd_L is the power dissipation when the output drives low.

Pd_H = [(V

OH_MAX

– (V

CCO_MAX

– 2V))/R

L

] * (V

CCO_MAX

– V

OH_MAX

) = [(2V – (V

CCO_MAX

– V

OH_MAX

))/R

L

] * (V

CCO_MAX

– V

OH_MAX

) =

[(2V – 0.9V)/50

Ω] * 0.9V = 19.8mW

Pd_L = [(V

OL_MAX

– (V

CCO_MAX

– 2V))/R

L

] * (V

CCO_MAX

– V

OL_MAX

) = [(2V – (V

CCO_MAX

– V

OL_MAX

))/R

L

] * (V

CCO_MAX

– V

OL_MAX

) =

[(2V – 1.7V)/50

Ω] * 1.7V = 10.2mW

Total Power Dissipation per output pair = Pd_H + Pd_L = 30mW

Q1

V

OUT

V

CCO

RL

50

V