ICS843001I-23 Datasheet, PDF (19/24 Pages) Integrated Circuit Systems – FEMTOCLOCKS™ CRYSTAL-TO-3.3V LVPECL/LVCMOS FREQUENCY SYNTHESIZER

ICS843001I-23 Data Sheet
FEMTOCLOCK^® CRYSTAL/LVCMOS-TO-LVPECL/LVCMOS FREQUENCY SYNTHESIZER
Power Considerations
This section provides information on power dissipation and junction temperature for the ICS843001I-23.
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the ICS843001I-23 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.
• Power (core)_MAX = V_{CC_MAX} * I_{EE_MAX} = 3.465V * 140mA = 485.1mW
• Power (outputs)_MAX = 30mW/Loaded Output pair
LVCMOS Output Power Dissipation
• Output Impedance R_OUT Power Dissipation due to Loading 50Ω to V_DDO/2
Output Current I_OUT = V_{DDO_MAX} / [2 * (50Ω + R_OUT)] = 3.465V / [2 * (50Ω + 21Ω)] = 24.4mA
• Power Dissipation on the R_OUT per LVCMOS output
Power (R_OUT) = R_OUT * (I_OUT)² = 21Ω * (24.4mA)² = 12.5mW per output
Total Power Dissipation
• Total Power
= Power (core) + Power (LVPECL output) + Power (R_OUT)
= 485.1mW + 30mW + 12.5mW = 527.6mW
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 is 125°C. Limiting the internal transistor junction temperature, Tj, to 125°C ensures that the bond
wire and bond pad temperature remains below 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 no air flow and
a multi-layer board, the appropriate value is 32.1°C/W per Table 7 below.
Therefore, Tj for an ambient temperature of 85°C with all outputs switching is:
85°C + 0.528W * 32.1°C/W = 102°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 7. Thermal Resitance θ_JA for 24 Lead TSSOP, E-Pad Forced Convection
θ_JA vs. Air Flow
Meters per Second
0
Multi-Layer PCB, JEDEC Standard Test Boards
32.1°C/W
1
25.5°C/W
2.5
24.0°C/W
ICS843001CGI-23 REVISION A OCTOBER 4, 2011
19
©2011 Integrated Device Technology, Inc.