LMK00308 Datasheet, PDF (20/25 Pages) Texas Instruments – 3-GHz 8-Output Differential Clock Buffer/Level Translator

14.4 Power Supply and Thermal Considerations
14.4.1 Current Consumption and Power Dissipation Calculations
The current consumption values specified in Section 11.0 Electrical Characteristics can be used to calculate the total power dis-
sipation and IC power dissipation for any device configuration. The total V_CC core supply current (I_{CC_TOTAL}) can be calculated
using Equation 5:
I_{CC_TOTAL} = I_{CC_CORE} + I_{CC_BANK_A} + I_{CC_BANK_B} + I_{CC_CMOS}
(5)
Where:
• I_{CC_CORE} is the current for core logic and input blocks and depends on selected input (CLKinX or OSCin).
• I_{CC_BANK_A} is the current for Bank A and depends on output type (I_{CC_PECL}, I_{CC_LVDS}, I_{CC_HCSL}, or 0 mA if disabled).
• I_{CC_BANK_B} is the current for Bank B and depends on output type (I_{CC_PECL}, I_{CC_LVDS}, I_{CC_HCSL}, or 0 mA if disabled).
• I_{CC_CMOS} is the current for the LVCMOS output (or 0 mA if REFout is disabled).
Since the output supplies (V_CCOA, V_CCOB, V_CCOC) can be powered from 3 independent voltages, the respective output supply
currents (I_{CCO_BANK_A}, I_{CCO_BANK_B}, and I_{CCO_CMOS}) should be calculated separately.
I_{CCO_BANK} for either Bank A or B can be directly taken from the corresponding output supply current spec (I_{CCO_PECL}, I_{CCO_LVDS}, or
I_{CCO_HCSL}) provided the output loading matches the specified conditions. Otherwise, I_{CCO_BANK} should be calculated as fol-
lows:
I_{CCO_BANK} = I_{BANK_BIAS} + (N * I_{OUT_LOAD})
(6)
Where:
• I_{BANK_BIAS} is the output bank bias current (fixed value).
• I_{OUT_LOAD} is the DC load current per loaded output pair.
• N is the number of loaded output pairs per bank (N = 0 to 4).
Table 5 shows the typical I_{BANK_BIAS} values and I_{OUT_LOAD} expressions for LVPECL, LVDS, and HCSL.
For LVPECL, it is possible to use a larger termination resistor (R_T) to ground instead of terminating with 50 Ω to V_TT = Vcco - 2 V;
this technique is commonly used to eliminate the extra termination voltage supply (V_TT) and potentially reduce device power
dissipation at the expense of lower output swing. For example, when Vcco is 3.3 V, a R_T value of 160 Ω to ground will eliminate
the 1.3 V termination supply without sacrificing much output swing. In this case, the typical I_{OUT_LOAD} is 25 mA, so I_{CCO_PECL} for a
fully-loaded bank reduces to 126.5 mA (vs. 132 mA with 50 Ω resistors to Vcco - 2 V).
Current Parameter
I_{BANK_BIAS}
I_{OUT_LOAD}
TABLE 5. Typical Output Bank Bias and Load Currents
LVPECL
26.5 mA
(V_OH - V_TT)/R_T + (V_OL - V_TT)/R_T
LVDS
26 mA
0 mA
(No DC load current)
HCSL
4.8 mA
V_OH/R_T
Once the current consumption is calculated for each supply, the total power dissipation (P_TOTAL) can be calculated as:
P_TOTAL = (V_CC*I_{CC_TOTAL}) + (V_CCOA*I_{CCO_BANK_A}) + (V_CCOB*I_{CCO_BANK_B}) + (V_CCOC*I_{CCO_CMOS})
(7)
If the device configuration is configured with LVPECL and/or HCSL outputs, then it is also necessary to calculate the power
dissipated in any termination resistors (P_{RT_ PECL} and P_{RT_HCSL}) and in any LVPECL termination voltages (P_{VTT_PECL}). The external
power dissipation values can be calculated as follows:
P_{RT_PECL} (per LVPECL pair) = (V_OH - V_TT)²/R_T + (V_OL - V_TT)²/R_T
(8)
P_{VTT_PECL} (per LVPECL pair) = V_TT * [(V_OH - V_TT)/R_T + (V_OL - V_TT)/R_T]
(9)
P_{RT_HCSL} (per HCSL pair) = V_OH2 / R_T
(10)
Finally, the IC power dissipation (P_DEVICE) can be computed by subtracting the external power dissipation values from P_TOTAL as
follows:
Where:
P_DEVICE = P_TOTAL - N₁*(P_{RT_PECL} + P_{VTT_PECL}) - N₂*P_{RT_HCSL}
(11)
• N₁ is the number of LVPECL output pairs with termination resistors to V_TT (usually Vcco - 2 V or GND).
• N₂ is the number of HCSL output pairs with termination resistors to GND.
www.ti.com
20