IDT™ / ICS™ 3.3V LVPECL FREQUENCY SYNTHESIZER

10

ICS843204AGI REV. A MARCH 18, 2009

ICS843204I

FEMTOCLOCK™ CRYSTAL-TO-3.3V LVPECL FREQUENCY SYNTHESIZER

POWER CONSIDERATIONS

This section provides information on power dissipation and junction temperature for the ICS843204I.

Equations and example calculations are also provided.

1. Power Dissipation.

The total power dissipation for the ICS843204I 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 + 10% = 3.63V, 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

= 3.63V * 170mA = 617.10mW

• Power (outputs)

MAX

= 30mW/Loaded Output pair

If all outputs are loaded, the total power is 4 * 30mW = 120mW

Total Power

_MAX

(3.63V, with all outputs switching) = 617.10mW + 120mW = 737.10mW

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 + TA

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 1 meter per second and a multi-layer board, the appropriate value is 51°C/W per Table 6 below.

Therefore, Tj for an ambient temperature of 85°C with all outputs switching is:

85°C + 0.737W * 51°C/W = 122.6°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 6. THERMAL RESISTANCE

θθθθθ

JA

FOR

48-PIN TSSOP, FORCED CONVECTION

θθθθθ

JA

by Velocity (Meters Per Second)

0

1

2.5

Multi-Layer PCB, JEDEC Standard Test Boards

54.8°C/W

51.0°C/W

49.1°C/W