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 50Ω 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)/50Ω] * 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)/50Ω] * 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
50Ω
