PCI Express™ Jitter Attenuator

874005 DATA SHEET

10

REVISION B 7/20/15

POWER CONSIDERATIONS

This section provides information on power dissipation and junction temperature for the 874005.

Equations and example calculations are also provided.

1. Power Dissipation.

The total power dissipation for the 874005 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 = VDD_MAX * (IDD_MAX + IDDA_MAX) = 3.465V * (85mA + 15mA) = 346.5mW

• Power (outputs)

MAX = VDDO_MAX * IDDO_MAX = 3.465V * 115mA = 398.48mW

Total Power

_MAX

= 3.465mW + 398.48mW = 745mW

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 the devices is 125°C.

The equation for Tj is as follows: Tj =

θJA * Pd_total + TA

Tj = Junction Temperature

qJA = Junction-to-Ambient Thermal Resistance

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

TA = Ambient Temperature

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

θJA must be used. Assuming

a moderate air flow of 200 linear feet per minute and a multi-layer board, the appropriate value is 63°C/W per Table 6 below.

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

70°C + 0.745W * 63°C/W = 117°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 (multi-layer).

TABLE 6. THERMAL RESISTANCE

θJA FOR 24-LEAD TSSOP, FORCED CONVECTION

θJA by Velocity (Linear Feet per Minute)

0

200

500

Multi-Layer PCB, JEDEC Standard Test Boards

70°C/W

63°C/W

60°C/W

NOTE: Most modern PCB designs use multi-layered boards. The data in the second row pertains to most designs.