IDT / ICS 3.3V LVPECL FREQUENCY SYNTHESIZER
11
ICS843204AGI REV. A MARCH 18, 2009
ICS843204I
FEMTOCLOCK CRYSTAL-TO-3.3V LVPECL FREQUENCY SYNTHESIZER
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 5.
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
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
FIGURE 5. LVPECL DRIVER CIRCUIT AND TERMINATION
Q1
V
OUT
V
CCO
RL
50
V
CCO - 2V