ICS844256 Datasheet, PDF (12/16 Pages) Integrated Circuit Systems – FEMTOCLOCKS™ CRYSTAL-TO-LVDS FREQUENCY SYNTHESIZER W/INTEGRATED FANOUT BUFFER

ICS844256
FEMTOCLOCKS™ CRYSTAL-TO-LVDS FREQUENCY SYNTHESIZER W/INTEGRATED FANOUT BUFFER PRELIMINARY
POWER CONSIDERATIONS
This section provides information on power dissipation and junction temperature for the ICS844256.
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the ICS844256 is the sum of the core power plus the power dissipated in the load(s).
The following is the power dissipation for V = 3.3V + 5% = 3.465V, which gives worst case results.
DD
• Power (core) = V * (I + I ) = 3.465V * (132mA + 8mA) = 485.1mW
MAX
DD_MAX
DD_MAX
DDA_MAX
• Power (outputs) = V
*I
= 3.465V * 120mA = 415.8mW
MAX
DDO_MAX DDO_MAX
Total Power = 485.1mW + 415.8mW = 900.9mW
_MAX
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 * Pd_total + T
JA
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 = Ambient Temperature
A
In order to calculate junction temperature, the appropriate junction-to-ambient thermal resistance θ_JA must be used. Assuming no air
flow and a multi-layer board, the appropriate value is 32.1°C/W per Table 7 below.
Therefore, Tj for an ambient temperature of 70°C with all outputs switching is:
70°C + 0.901W * 32.1°C/W = 98.9°C. This is well 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 FOR 24-LEAD TSSOP, E-PAD, FORCED CONVECTION
JA
θ by Velocity (Meters per Second)
JA
Multi-Layer PCB, JEDEC Standard Test Boards
0
32.1°C/W
1
25.5°C/W
2.5
24.0°C/W
IDT^™ / ICS^™ LVDS FREQUENCY SYNTHESIZER W/FANOUT BUFFER 12
ICS844256BG REV. B NOVEMBER 19, 2007