843034-06 Datasheet, PDF (18/24 Pages) Integrated Device Technology – FemtoClock™ Multi-Rate 3.3V LVPECL Frequency Synthesizer

843034-06 DATA SHEET
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 devices is 125°C.
The equation for Tj is as follows: Tj = θJA * Pd_total + TA
Tj = Junction Temperature
θJA = 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 θJA must be used. Assuming no air flow
and a multi-layer board, the appropriate value is 65.7°C/W per Table 9 below.
Therefore, Tj for an ambient temperature of 75°C with all outputs switching is:
75°C + 0.666W * 65.7°C/W = 118°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).
TABLE 9. THERMAL RESISTANCE θJA FOR 48-PIN LQFP, FORCED CONVECTION
θJA by Velocity (Meters per Second)
Multi-Layer PCB, JEDEC Standard Test Boards
0
65.7°C/W
1
55.9°C/W
2.5
52.4°C/W
REVISION B 8/17/15
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FemtoClock™ Multi-Rate 3.3V LVPECL Frequency Synthesizer