844004AGI-04 www.icst.com/products/hiperclocks.html REV. A FEBRUARY 25, 2009
10
Integrated
Circuit
Systems, Inc.
ICS844004I-04
FEMTOCLOCKS™ C RYSTAL/LVCMOS-TO-
LVDS FREQUENCY SYNTHESIZER
PRELIMINARY
POWER CONSIDERATIONS
This section provides information on power dissipation and junction temperature for the ICS844004I-04.
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the ICS844004I-04 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 * (80mA + 8mA) = 304.92mW
• Power (outputs)
MAX
= V
DDO_MAX
* I
DDO_MAX
= 3.465V * 87mA = 301.45mW
Total Power
_MAX
= 304.92mW + 301.45mW = 606.37mW
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
q
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 1 meter per second and a multi-layer board, the appropriate value is 65°C/W per Table 7
below.
Therefore, Tj for an ambient temperature of 85°C with all outputs switching is:
85°C + 0.606W * 65°C/W = 124°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
θθ
θθ
θ
JA
FOR 24-LEAD TSSOP, FORCED CONVECTION
θθ
θθ
θ
JA
by Velocity (Meters per Second)
0 1 2.5
Multi-Layer PCB, JEDEC Standard Test Boards 70°C/W 65°C/W 62°C/W
