ICS844008I-15 Datasheet, PDF (9/12 Pages) Integrated Circuit Systems – FEMTOCLOCKS™ CRYSTAL-TO-LVDS FREQUENCY SYNTHESIZER

Integrated
Circuit
Systems, Inc.
PRELIMINARY
ICS844008I-15
FEMTOCLOCKS™ CRYSTAL-TO-
LVDS FREQUENCY SYNTHESIZER
POWER CONSIDERATIONS
This section provides information on power dissipation and junction temperature for the ICS844008I-15.
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the ICS844008I-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 * (122mA + 11mA) = 460.85mW
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 = 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 flow of 200 linear feet per minute and a multi-layer board, the appropriate value is 42.1°C/W per
Table 7 below.
Therefore, Tj for an ambient temperature of 85°C with all outputs switching is:
85°C + 0.461W * 42.1°C/W = 104.4°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 (single layer or multi-layer).
TABLE 7. THERMAL RESISTANCE θ_JA FOR 32-LEAD LQFP, FORCED CONVECTION
θ_JA by Velocity (Linear Feet per Minute)
Single-Layer PCB, JEDEC Standard Test Boards
Multi-Layer PCB, JEDEC Standard Test Boards
0
67.8°C/W
47.9°C/W
200
55.9°C/W
42.1°C/W
500
50.1°C/W
39.4°C/W
NOTE: Most modern PCB designs use multi-layered boards. The data in the second row pertains to most designs.
844008AYI-15
www.icst.com/products/hiperclocks.html
9
REV. A FEBRUARY 2, 2006