11.2.1 Power Dissipation Example #1: Separate Vcc and Vcco Supplies with Unused Outputs
This example shows how to calculate IC power dissipation for a configuration with separate VCC and VCCO supplies and unused outputs. Because some outputs are not used, the ICCO_PECL value specified in Electrical Characteristics cannot be used directly, and output bank current (ICCO_BANK) should be calculated to accurately estimate the IC power dissipation.
- VCC = 3.3 V, VCCOA = 3.3 V, VCCOB = 2.5 V. Typical ICC and ICCO values.
- CLKin0/CLKin0* input is selected.
- Bank A is configured for LVPECL: 4 pairs used with RT = 50 Ω to VT = Vcco - 2 V (1 pair unused).
- Bank B is configured for LVDS: 3 pairs used with RL = 100 Ω differential (2 pairs unused).
- REFout is disabled.
- TA = 85 °C
Using the current and power calculations from the previous section, we can compute PTOTAL and PDEVICE.
- From Equation 5: ICC_TOTAL = 8.5 mA + 20 mA + 26 mA + 0 mA = 54.5 mA
- From Table 6: IOUT_LOAD (LVPECL) = (1.6 V - 0.5 V)/50 Ω + (0.75 V - 0.5 V)/50 Ω = 27 mA
- From Equation 6: ICCO_BANK_A = 33 mA + (4 * 27 mA) = 141 mA
- From Equation 7: PTOTAL = (3.3 V * 54.5 mA) + (3.3 V * 141 mA) + (2.5 V * 34 mA)] = 730 mW
- From Equation 8: PRT_PECL = ((2.4 V - 1.3 V)2/50 Ω) + ((1.55 V - 1.3 V)2/50 Ω) = 25.5 mW (per output pair)
- From Equation 9: PVTT_PECL = 0.5 V * [ ((2.4 V - 1.3 V) / 50 Ω) + ((1.55 V - 1.3 V) / 50 Ω) ] = 13.5 mW (per output pair)
- From Equation 10: PRT_HCSL = 0 mW (no HCSL outputs)
- From Equation 11: PDEVICE = 730 mW - (4 * (25.5 mW + 13.5 mW)) - 0 mW = 574 mW
In this example, the IC device will dissipate about 574 mW or 79% of the total power (730 mW), while the remaining 21% will be dissipated in the emitter resistors (102 mW for 4 pairs) and termination voltage (54 mW into Vcco - 2 V).
Based on the thermal resistance junction-to-case (RθJA) of 28.5 °C/W, the estimated die junction temperature would be about 16.4 °C above ambient, or 101.4 °C when TA = 85 °C.