841N254B Datasheet, PDF (18/24 Pages) Integrated Device Technology – FemtoClock NG Crystal-to-LVDS/ HCSL Clock Synthesizer

841N254B Datasheet
Power Considerations
This section provides information on power dissipation and junction temperature for the 841N254B. 
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the 841N254B is the sum of the core power plus the analog power plus the power dissipated in the load(s). 
The following is the power dissipation for V_DD = 3.3V + 5% = 3.465V, which gives worst case results.
NOTE: Please refer to Section 3 for details on calculating power dissipated in the load.
• Power (core)_MAX = V_{DD_MAX} * (I_{DD_MAX} + I_{DDA_MAX} + I_{DDOA&B_MAX} ) = 3.465V *(113mA + 30mA + 72mA) = 744.98mW
• Power (HCSL_output)_MAX = 44.5mW * 2 = 89.0mW
Total Power__MAX = (3.465V, with all outputs switching) = 744.98mW + 89.0mW = 833.98mW
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 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 + T_A
Tj = Junction Temperature
_JA = Junction-to-Ambient Thermal Resistance
Pd_total = Total Device Power Dissipation (example calculation is in section 1 above)
T_A = 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 37.7°C/W per Table 7 below.
Therefore, Tj for an ambient temperature of 85°C with all outputs switching is:
85°C + 0.834W * 37.7°C/W = 116.4°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 7. Thermal Resistance _JA for 32-Lead VFQFN, Forced Convection
_JA Vs. Air Flow
Meters per Second
0
Multi-Layer PCB, JEDEC Standard Test Boards
37.7°C/W
1
32.9°C/W
2.5
29.5°C/W
©2016 Integrated Device Technology, Inc.
18
Revision B, May 23, 2016