IDT™ / ICS™ LVPECL FANOUT BUFFER

13

ICS853S12AKI REV. A MAY 21, 2008

ICS853S12I

LOW SKEW, 1-TO-12, DIFFERENTIAL-TO-3.3V, 2.5V LVPECL 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 6.

To calculate worst case power dissipation into the load, use the following equations which assume a 50

Ω load, and a termination

voltage of V

CC

- 2V.

•

For logic high, V

OUT

= V

OH_MAX

= V

CC_MAX

– 0.8V

(V

CC_MAX

- V

OH_MAX

) = 0.8V

•

For logic low, V

OUT

= V

OL_MAX

= V

CC_MAX

– 1.6V

(V

CC_MAX

- V

OL_MAX

) = 1.6V

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

CC_MAX

- 2V))/R

L

] * (V

CC_MAX

- V

OH_MAX

) = [(2V - (V

CC_MAX

- V

OH_MAX

))/R

L

] * (V

CC_MAX

- V

OH_MAX

) =

[(2V - 0.8V)/50

Ω] * 0.8V = 19.2mW

Pd_L = [(V

OL_MAX

– (V

CC_MAX

- 2V))/R

L

] * (V

CC_MAX

- V

OL_MAX

) = [(2V - (V

CC_MAX

- V

OL_MAX

))/R

L

] * (V

CC_MAX

- V

OL_MAX

) =

[(2V - 1.6V)/50

Ω] * 1.6V = 12.8mW

Total Power Dissipation per output pair = Pd_H + Pd_L = 32mW

FIGURE 6. LVPECL DRIVER CIRCUIT AND TERMINATION

V

OUT

V

CC

V

CC - 2V

Q1

RL

50

Ω