853001AG 13 REV. A NOVEMBER 24, 2008
ICS853001
1:1, DIFFERENTIAL LVPECL-TO-
2.5V, 3.3V, 5V LVPECL/ECL 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.
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
– 1.005V
(V
CC_MAX
- V
OH_MAX
)
= 1.005
• For logic low, V
OUT
= V
OL_MAX
= V
CC_MAX
– 1.78V
(V
CC_MAX
- V
OL_MAX
)
= 1.78V
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 - 1.005V)/50Ω] * 1.005V = 20mW
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 - 1.78V)/50Ω] * 1.78V = 7.83mW
Total Power Dissipation per output pair = Pd_H + Pd_L = 27.83mW
FIGURE 7. LVPECL DRIVER CIRCUIT AND T ERMINATION
VOUT
Q1
VCC - 2V
RL
50
VCC
