83918 Datasheet, PDF (14/19 Pages) Integrated Device Technology – Maximum output frequency

83918 DATA SHEET
Power Considerations
This section provides information on power dissipation and junction temperature for the 83918.
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the 83918 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 + I_DDO) = 3.465V *(24mA + 27mA) = 176.7mW
Dynamic Power Dissipation at 200MHz
Power (200MHz) = C_PD * Frequency * (V_DD)² * number of outputs = 9pF * 200MHz * (3.465V)² * 18 = 389mW
Total Power Dissipation
• Total Power
= Power (core)_MAX + Power (200MHz)
= 176.7mW + 389mW
= 565.7mW
2. Junction Temperature.
Junction temperature, Tj, is the temperature at the junction of the bond wire and bond pad 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 53.5°C/W per Table 6 below.
Therefore, Tj for an ambient temperature of 85°C with all outputs switching is:
85°C + 0.566W * 53.5°C/W = 115.3°C. This is well below the limit of 125°C.
This calculation is only an example. Tj will vary depending on the number of loaded outputs, supply voltage, air flow and the type of board
(multi-layer).
Table 6. Thermal Resistance _JA for 32 Lead LQFP, Forced Convection
_JA by Velocity
Meters per Second
0
Multi-Layer PCB, JEDEC Standard Test Boards
53.5°C/W
1
48.0°C/W
2.5
44.0°C/W
Rev B 3/25/15
14
LOW SKEW, 1:18 CRYSTAL-TO-LVCMOS/LVTTL FANOUT BUFFER