REVISION B 10/15/15 13 FEMTOCLOCK
®
CRYSTAL-TO-3.3V LVPECL CLOCK GENERATOR
843051 DATA SHEET
3. Calculations and Equations.
The purpose of this section is to calculate the power dissipation for the LVPECL output pair.
LVPECL output driver circuit and termination are shown in Figure 6.
Figure 6. LVPECL Driver Circuit and Termination
To calculate worst case power dissipation into the load, use the following equations which assume a 50 load, and a termination voltage of
V
CC
– 2V.
• For logic high, V
OUT
= V
OH_MAX
= V
CC_MAX
– 0.9V
(V
CC_MAX
– V
OH_MAX
) = 0.9V
• For logic low, V
OUT
= V
OL_MAX
= V
CC_MAX
– 1.7V
(V
CC_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
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 – 0.9V)/50] * 0.9V = 19.8mW
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 – 1.7V)/50] * 1.7V = 10.2mW
Total Power Dissipation per output pair = Pd_H + Pd_L = 30mW
V
OUT
V
CC
V
CC
- 2V
Q1
RL
50Ω
