FEMTOCLOCKS
®
CRYSTAL-TO-
3.3V LVPECL CLOCK GENERATOR
843011 DATA SHEET
8 REVISION B 9/25/15
POWER CONSIDERATIONS
This section provides information on power dissipation and junction temperature for the 843011.
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the 843011 is the sum of the core power plus the power dissipated in the load(s).
The following is the power dissipation for V
CC
= 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
CC_MAX
* I
EE_MAX
= 3.465V * 93mA = 322.2mW
• Power (outputs)
MAX
= 30mW/Loaded Output pair
Total Power
_MAX
(3.465V, with all outputs switching) = 322.2mW + 30mW = 352.2mW
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 = θ
JA
* Pd_total + T
A
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
a moderate air fl ow of 1 meter per second and a multi-layer board, the appropriate value is 90.5°C/W per Table 6 below.
Therefore, Tj for an ambient temperature of 85°C with all outputs switching is:
85°C + 0.352W * 90.5°C/W = 116.9°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 6. THERMAL RESISTANCE θ
JA
FOR 8-PIN TSSOP, FORCED CONVECTION
θ
JA
by Velocity (Meters per Second)
0 1 2.5
Multi-Layer PCB, JEDEC Standard Test Boards 101.7°C/W 90.5°C/W 89.8°C/W
