11©2016 Integrated Device Technology, Inc. Revision E, February 18, 2016
85311 Datasheet
Power Considerations
This section provides information on power dissipation and junction temperature for the 85311.
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the 85311 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 * 25mA = 86.6mW
• Power (outputs)
MAX
= 30mW/Loaded Output pair
If all outputs are loaded, the total power is 2 * 30mW = 60mW
Total Power_
MAX
(3.3V, with all outputs switching) = 86.6mW + 60mW = 146.6mW
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 for devices is 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 103°C/W per Table 5 below.
Therefore, Tj for an ambient temperature of 70°C with all outputs switching is:
70°C + 0.147W * 103°C/W = 85.14°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 5. Thermal Resistance
JA
for 8 Lead SOIC, Forced Convection
JA
vs. Air Flow
Linear Feet per Minute 0200500
Multi-Layer PCB, JEDEC Standard Test Boards 103°C/W 94°C/W 89°C/W
