ICS889831 Datasheet, PDF (14/18 Pages) Integrated Circuit Systems – LOW SKEW, 1-TO-4 DIFFERENTIAL LVPECL-TO-LVPECL/ECL FANOUT BUFFER

Integrated
Circuit
Systems, Inc.
ICS889831
LOW SKEW, 1-TO-4
DIFFERENTIAL LVPECL-TO-LVPECL/ECL FANOUT BUFFER
POWER CONSIDERATIONS
This section provides information on power dissipation and junction temperature for the ICS839831.
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the ICS889831 is the sum of the core power plus the power dissipated in the load(s).
The following is the power dissipation for V_CC = 3.63V, 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 * 60mA = 217.8mW
• Power (outputs) = 30.94mW/Loaded Output pair
MAX
If all outputs are loaded, the total power is 4 * 30.94mW = 123.8mW
• Power Dissipation at built-in terminations: Assume the input is driven by a 3.3V SSTL driver as shown in Figure 5E
and estimated approximately 1.75V drop across IN and nIN.
Total Power Dissipation for the two 50Ω built-in terminations is: (1.75V)² / (50Ω + 50Ω) = 30.6mW
Total Power_{_MAX} (3.63V, with all outputs switching) = 217.8mW + 123.8mW + 30.6mW = 372.2mW
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
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 0 linear feet per minute and a multi-layer board, the appropriate value is 51.5°C/W per Table 6 below.
Therefore, Tj for an ambient temperature of 85°C with all outputs switching is:
85°C + 0.372W * 51.5°C/W = 104°C. This is well 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 θ_JA FOR 16-PIN VFQFN, FORCED CONVECTION
θ_JAvs. 0 Velocity (Linear Feet per Minute)
Multi-Layer PCB, JEDEC Standard Test Boards
0
51.5°C/W
889831AK
www.icst.com/products/hiperclocks.html
14
REV. A JUNE 16, 2005