1-TO-21, DIFFERENTIAL HCSL FANOUT BUFFER

12

REVISION C 11/03/15

851021 DATA SHEET

Power Considerations

This section provides information on power dissipation and junction temperature for the 851021.



Equations and example calculations are also provided.

1.

Power Dissipation.

The total power dissipation for the 851021 is the sum of the core power plus the power dissipated in the load(s). The following is the power

dissipation for VDD = 3.3V + 5% = 3.465V, which gives worst case results.

•

Power (core)MAX = VDD_MAX * IDD_MAX = 3.465V * 105mA = 363.825mW

•

Power (outputs)MAX = 44.5mW/Loaded Output Pair

If all outputs are loaded, the total power is 21 * 44.5mW = 934.5mW



Total Power_MAX (3.465V, with all outputs switching) = 363.825mW + 934.5mW = 1298.325mW

•

2. Junction Temperature.

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

maximum recommended junction temperature is 125°C. Limiting the internal transistor junction temperature, Tj, to 125°C ensures that the bond

wire and bond pad temperature remains below 125°C.

The equation for Tj is as follows: Tj =



JA * Pd_total + TA

Tj = Junction Temperature



JA = Junction-to-Ambient Thermal Resistance

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

TA = Ambient Temperature

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



JA must be used. Assuming no air flow and

a multi-layer board, the appropriate value is 31.8°C/W per Table 4 below.

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

70°C + 1.298W * 31.8°C/W = 111.3°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 (multi-layer).

Table 4. Thermal Resistance JA for 64 Lead TQFP, E-Pad Forced Convection



JA by Velocity

Meters per Second

01

2.5

Multi-Layer PCB, JEDEC Standard Test Boards

31.8°C/W

25.8°C/W

24.2°C/W