SNOS412O February 2000 – June 2020 LM1117
When an integrated circuit operates with an appreciable current, its junction temperature is elevated. It is important to quantify its thermal limits in order to achieve acceptable performance and reliability. This limit is determined by summing the individual parts consisting of a series of temperature rises from the semiconductor junction to the operating environment. A one-dimensional steady-state model of conduction heat transfer is demonstrated in Figure 22. The heat generated at the device junction flows through the die to the die attach pad, through the lead frame to the surrounding case material, to the printed circuit board, and eventually to the ambient environment. Below is a list of variables that may affect the thermal resistance and in turn the need for a heatsink.
|RθJC (COMPONENT VARIABLES)||RθJA (APPLICATION VARIABLES)|
|Leadframe Size and Material||Mounting Pad Size, Material, and Location|
|No. of Conduction Pins||Placement of Mounting Pad|
|Die Size||PCB Size and Material|
|Die Attach Material||Traces Length and Width|
|Molding Compound Size and Material||Adjacent Heat Sources|
|Volume of Air|
|Shape of Mounting Pad|
The LM1117 regulators have internal thermal shutdown to protect the device from over-heating. Under all possible operating conditions, the junction temperature of the LM1117 must be within the range of 0°C to 125°C. A heatsink may be required depending on the maximum power dissipation and maximum ambient temperature of the application. To determine if a heatsink is needed, the power dissipated by the regulator, PD , must be calculated:
Figure 23 shows the voltages and currents which are present in the circuit.
The next parameter which must be calculated is the maximum allowable temperature rise, TR(max):
Using the calculated values for TR(max) and PD, the maximum allowable value for the junction-to-ambient thermal resistance (RθJA) can be calculated:
For the maximum allowable value for θJA, refer to the Thermal Information table.
Figure 26 and Figure 27 shows the maximum allowable power dissipation vs. ambient temperature for the SOT-223 and TO-252 device. Figure 28 and Figure 29 shows the maximum allowable power dissipation vs. copper area (in2) for the SOT-223 and TO-252 devices. Please see AN1028 for power enhancement techniques to be used with SOT-223 and TO-252 packages.
The AN-1187 Leadless Leadframe Package (LLP) application note discusses improved thermal performance and power dissipation for the WSON.
|LAYOUT||COPPER AREA||THERMAL RESISTANCE|
|Top Side (in2)(1)||Bottom Side (in2)||(θJA,°C/W) SOT-223||(θJA,°C/W) TO-252|