ICS810251 datasheet(11/15 Pages) IDT

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ICS810251I Data Sheet

VCXO AND SYNCHRONOUS ETHERNET JITTER ATTENUATOR

ICS810251AGI REVISION B OCTOBER 5, 2012

Power Considerations

This section provides information on power dissipation and junction temperature for the ICS810251I.

Equations and example calculations are also provided.

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 VDD = 3.3V + 5% = 3.465V, which gives worst case results.

•

Power (core)MAX = VDD_MAX * (IDD + IDDA + IDDO) = 3.465V *(40mA + 7mA + 5mA) = 180.18mW

•

Output Impedance ROUT Power Dissipation due to Loading 50 to VDD/2

Output Current IOUT = VDD_MAX / [2 * (50 + ROUT)] = 3.465V / [2 * (50 + 15)] = 26.7mA

•

Power Dissipation on the ROUT per LVCMOS output

Power (ROUT) = ROUT * (IOUT)

2 = 15

 * (26.7mA)2 = 10.7mW per output

Dynamic Power Dissipation at 25MHz

Power (25MHz) = CPD * Frequency * (VDD)

2 = 8pF * 25MHz * (3.465V)2 = 2.4mW per output

Total Power Dissipation

•

Total Power

= Power (core)MAX + Power (ROUT) + 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 + TA

Tj = Junction Temperature



JA = 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



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

2.5

Multi-Layer PCB, JEDEC Standard Test Boards

92.4°C/W

88.0°C/W

85.9°C/W

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