FEMTOCLOCK™ CRYSTAL-TO-LVDS
FREQUENCY SYNTHESIZER
844008I-15 DATA SHEET
10 REVISION B 7/2/15
POWER CONSIDERATIONS
This section provides information on power dissipation and junction temperature for the 844008I-15.
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the 844008I-15 is the sum of the core 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.
• Power (core)
MAX
= V
DD_MAX
* (I
DD_MAX
+ I
DDA_MAX
) = 3.465V * (150mA + 15mA) = 571.73mW
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 HiPerClockS
TM
devices is 125°C.
The equation for Tj is as follows: Tj = θJA * Pd_total + TA
Tj = Junction Temperature
θ
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 θ
JA
must be used. Assuming no air fl ow
and a multi-layer board, the appropriate value is 65.7°C/W per Table 7 below.
Therefore, Tj for an ambient temperature of 85°C with all outputs switching is:
85°C + 0.572W * 65.7°C/W = 122.5°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 fl ow, and the
type of board (single layer or multi-layer).
TABLE 7. THERMAL RESISTANCE θ
JA
FOR 32-LEAD LQFP, FORCED CONVECTION
θ
JA
by Velocity (Meters per Second)
0 1 2.5
Multi-Layer PCB, JEDEC Standard Test Boards 65.7°C/W 55.9°C/W 52.4°C/W
