11

ICS843242AG REV. A OCTOBER 2, 2007

ICS843242

FEMTOCLOCKS™ CRYSTAL-TO-3.3V LVPECL FREQUENCY SYNTHESIZER

PRELIMINARY

This section provides information on power dissipation and junction temperature for the ICS843242.

Equations and example calculations are also provided.

1. Power Dissipation.

The total power dissipation for the ICS843242 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.

MAX

= V

CC_MAX

* I

EE_MAX

= 3.465V * 125mA = 433mW

MAX

= 30mW/Loaded Output pair

If all outputs are loaded, the total power is 2 * 30mW = 60mW

Total Power

_MAX

(3.465V, with all outputs switching) = 433mW + 60mW = 493mW

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

The equation for Tj is as follows: Tj =

θ

Tj = Junction Temperature

θ

Pd_total = Total Device Power Dissipation (example calculation is in section 1 above)

T

In order to calculate junction temperature, the appropriate junction-to-ambient thermal resistance

θ

air flow and a multi-layer board, the appropriate value is 92.4°C/W per Table 7 below.

Therefore, Tj for an ambient temperature of 70°C with all outputs switching is:

70°C + 0.493W * 92.4°C/W = 115.5°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 flow,

and the type of board (single layer or multi-layer).

θθθθθ

JA

FOR

0

1

2.5

Multi-Layer PCB, JEDEC Standard Test Boards

92.4°C/W

88.0°C/W

85.9°C/W

θθθθθ

JA

by Velocity (Meters per Second)