10.2.1 Power Dissipation Example: Worst-Case Dissipation
This example shows how to calculate IC power dissipation for a configuration to estimate worst-case power dissipation. In this case, the maximum supply voltage and supply current values specified in Electrical Characteristics are used.
- Max VCC = VCCO = 3.465 V. Max ICC and ICCO values.
- CLKin0/CLKin0* input is selected.
- Banks A and B are configured for LVPECL: all outputs terminated with 50 Ω to VT = Vcco – 2 V.
- REFout is enabled with 5-pF load.
- TA = 85°C
Using the power calculations from the previous section and maximum supply current specifications, we can compute PTOTAL and PDEVICE.
- From Equation 5: ICC_TOTAL = 10.5 mA + 48 mA + 5.5 mA = 64 mA
- From ICCO_PECL max spec: ICCO_BANK = 50% of ICCO_PECL = 81.5 mA
- From Equation 7: PTOTAL = (3.465 V × 64 mA) + (3.465 V × 81.5 mA)+ (3.465 V × 81.5 mA) + (3.465 V × 10 mA) = 821 mW
- From Equation 8: PRT_PECL = ((2.57 V – 1.47 V)2/50 Ω) + ((1.72 V – 1.47 V)2/50 Ω) = 25.5 mW (per output pair)
- From Equation 9: PVTT_PECL = 1.47 V × [ ((2.57 V – 1.47 V) / 50 Ω) + ((1.72 V – 1.47 V) / 50 Ω) ] = 39.5 mW (per output pair)
- From Equation 10: PRT_HCSL = 0 mW (no HCSL outputs)
- From Equation 11: PDEVICE = 821 mW – (4 × (25.5 mW + 39.5 mW)) – 0 mW = 561 mW
In this worst-case example, the IC device will dissipate about 561 mW or 68% of the total power (821 mW), while the remaining 32% will be dissipated in the emitter resistors (102 mW for 4 pairs) and termination voltage (158 mW into Vcco – 2 V). Based on θJA of 38.1°C/W, the estimate die junction temperature would be about 21.4°C above ambient, or 106.4°C when TA = 85°C.