873991

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REV. A 8/25/15

12

873991

This section provides information on power dissipation and junction temperature for the 873991.

Equations and example calculations are also provided.

1. Power Dissipation.

The total power dissipation for the 873991 is the sum of the core power plus the power dissipated in the load(s).

The following is the power dissipation for V

NOTE: Please refer to Section 3 for details on calculating power dissipated in the load.

If all outputs are loaded, the total power is 14 * 30mW = 420mW

Total Power

2. Junction Temperature.

Junction temperature, Tj, is the temperature at the junction of the bond wire and bond pad and directly affects the reliability of

the device. The maximum recommended junction temperature for the devices is 125°C.

The equation for Tj is as follows: Tj =

Tj = Junction Temperature

Pd_total = Total Device Power Dissipation (example calculation is in section 1 above)

In order to calculate junction temperature, the appropriate junction-to-ambient thermal resistance

a moderate air flow of 200 linear feet per minute and a multi-layer board, the appropriate value is 47.1°C/W per Table 7 below.

Therefore, Tj for an ambient temperature of 70°C with all outputs switching is:

70°C + 0.992W * 47.1°C/W = 116.7°C. This is below the limit of 125°C.

This calculation is only an example. Tj will obviously vary depending on the number of loaded outputs, supply voltage, air flow,

and the type of board (single layer or multi-layer).

0

200

500

Single-Layer PCB, JEDEC Standard Test Boards

58.0°C/W

47.1°C/W

42.0°C/W

Multi-Layer PCB, JEDEC Standard Test Boards

42.3°C/W

36.4°C/W

34.0°C/W

NOTE: Most modern PCB designs use multi-layered boards. The data in the second row pertains to most designs.