ICS8S89874I Datasheet, PDF (15/19 Pages) Integrated Device Technology – 1:2 Differential-to-LVPECL Buffer/Divider

ICS8S89874I Data Sheet
1:2 DIFFERENTIAL-TO-LVPECL BUFFER/DIVIDER
3. Calculations and Equations.
The purpose of this section is to calculate the power dissipation for the LVPECL output pairs.
LVPECL output driver circuit and termination are shown in Figure 7.
V_CC
Q1
V_OUT
RL
50Ω
V_CC - 2V
Figure 7. LVPECL Driver Circuit and Termination
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} – 0.82V
(V_{CC_MAX} – V_{OH_MAX}) = 0.82V
• For logic low, V_OUT = V_{OL_MAX} = V_{CC_MAX} – 1.58V
(V_{CC_MAX} – V_{OL_MAX}) = 1.58V
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 – 0.82V)/50Ω] * 0.82V = 19.35mW
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_{CO_MAX} – V_{OL_MAX}) =
[(2V – 1.58V)/50Ω] * 1.58V = 13.27mW
Total Power Dissipation per output pair = Pd_H + Pd_L = 32.62mW
ICS8S89874BKI REVISION A OCTOBER 22, 2010
15
©2010 Integrated Device Technology, Inc.