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
= V
DD_MAX
* (I
DD_MAX
+ I
DDA_MAX
) = 3.465V * (85mA + 15mA) = 346.5mW
• Power (outputs)
MAX
= V
DDO_MAX
* I
DDO_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 = 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 fl ow 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 fl ow,
and the type of board (multi-layer).
TABLE 6. THERMAL RESISTANCE θ
JA
FOR 24-LEAD TSSOP, FORCED CONVECTION
θ
JA
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.
