LMK00301 Datasheet, PDF (21/26 Pages) Texas Instruments – 3-GHz, 10-Output Differential Fanout 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. Because the LMK00301 has dual supplies, the total V_CC supply
current (I_{CC_TOTAL}) and total V_CCO supply current (I_{CCO_TOTAL}) should be calculated separately as follows:
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).
I_{CCO_TOTAL} = I_{CCO_BANK_A} + I_{CCO_BANK_B} + I_{CCO_CMOS}
(6)
Where:
• I_{CCO_BANK_A} is the current for Bank A and depends on output type (or 0 mA if disabled).
• I_{CCO_BANK_B} is the current for Bank B and depends on output type (or 0 mA if disabled).
• I_{CCO_CMOS} is the current for LVCMOS output (or 0 mA if REFout is disabled).
I_{CCO_BANK} consists of a fixed bias current (I_{BANK_BIAS}) plus the DC load current (I_{OUT_LOAD}) for each output pair. I_{CCO_PECL},
I_{CCO_LVDS}, or I_{CCO_HCSL} specified in Section 11.0 Electrical Characteristics can be directly applied to I_{CCO_BANK} only if the same
exact loading conditions are used.
If X is the number of loaded output pairs per bank, then I_{CCO_BANK} can be calculated:
I_{CCO_BANK} = I_{BANK_BIAS} + (X * I_{OUT_LOAD})
(7)
Table 5 shows the typical bias current values and load current expressions for the differential output types. 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 will provide some power savings 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 157.5
mA (vs. 165 mA with 50 Ω resistors to Vcco – 2 V).
Parameter
I_{BANK_BIAS}
I_{OUT_LOAD}
TABLE 5. Output Bank Bias and Load Currents
LVPECL
33 mA
(V_OH – V_TT)/R_T + (V_OL – V_TT)/R_T
LVDS
34 mA
0 mA
(No DC load current)
HCSL
6 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_CCO*I_{CCO_TOTAL})
(8)
If the device configuration has LVPECL or HCSL outputs, then it is also necessary to calculate the power dissipated in any termi-
nation resistors (P_{RT_ PECL} and P_{RT_HCSL}) and in any termination voltages (P_VTT). 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
(9)
P_{VTT_PECL} (per LVPECL pair) = V_TT * [(V_OH – V_TT)/R_T + (V_OL – V_TT)/R_T]
(10)
P_{RT_HCSL} (per HCSL pair) = V_OH2 / R_T
(11)
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}
(12)
• 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.
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