ICS810251 Datasheet, PDF (11/15 Pages) Integrated Device Technology – VCXO and Synchronous Ethernet Jitter Attenuator

ICS810251I Data Sheet
VCXO AND SYNCHRONOUS ETHERNET JITTER ATTENUATOR
Power Considerations
This section provides information on power dissipation and junction temperature for the ICS810251I.
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the ICS810251I is the sum of the core power plus the analog 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 + I_DDA + I_DDO) = 3.465V *(40mA + 7mA + 5mA) = 180.18mW
• Output Impedance R_OUT Power Dissipation due to Loading 50 to V_DD/2
Output Current I_OUT = V_{DD_MAX} / [2 * (50 + R_OUT)] = 3.465V / [2 * (50 + 15)] = 26.7mA
• Power Dissipation on the R_OUT per LVCMOS output
Power (R_OUT) = R_OUT * (I_OUT)² = 15 * (26.7mA)² = 10.7mW per output
Dynamic Power Dissipation at 25MHz
Power (25MHz) = C_PD * Frequency * (V_DD)² = 8pF * 25MHz * (3.465V)² = 2.4mW per output
Total Power Dissipation
• Total Power
= Power (core)_MAX + Power (R_OUT) + Power (25MHz)
= 180.18mW + 10.7mW + 2.4mW
= 193.28mW
2. Junction Temperature.
Junction temperature, Tj, is the temperature at the junction of the bond wire and bond pad and it 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 = _JA * Pd_total + T_A
Tj = Junction Temperature
_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 _JA must be used. Assuming no air flow and
a multi-layer board, the appropriate value is 92.4°C/W per Table 5 below.
Therefore, Tj for an ambient temperature of 85°C with all outputs switching is:
85°C + 0.193W *92.4°C/W = 102.8°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 (multi-layer).
Table 5. Thermal Resistance _JA for 16 Lead TSSOP, Forced Convection
_JA by Velocity
Meters per Second
0
Multi-Layer PCB, JEDEC Standard Test Boards
92.4°C/W
1
88.0°C/W
2.5
85.9°C/W
ICS810251AGI REVISION B OCTOBER 5, 2012
11
©2012 Integrated Device Technology, Inc.