LM1117-2.85-252 View Datasheet(PDF) - First Components International

Part Name

Description

Manufacturer

LM1117-2.85-252

800mA Low-dropout Linear Regulator

First Components International 

LM1117 800mA Low-dropout Linear Regulator

APPLICATION NOTE (Continued)

into the output of the regulator. The discharge current de-

pends on the value of the capacitor, the output voltage of the

regulator, and rate of decrease of V IN. In the LM1117 regu-

lators, the internal diode between the output and input pins

can withstand microsecond surge currents of 10A to 20A.

With an extremely large output capacitor (≥1000 µF), and

with input instantaneously shorted to ground, the regulator

could be damaged.

In this case, an external diode is recommended between the

output and input pins to protect the regulator, as shown in

Figure 4.

FIGURE 5. Cross-sectional view of Integrated Circuit

Mounted on a printed circuit board. Note that the case

temperature is measured at the point where the leads

contact with the mounting pad surface

The LM1117 regulators have internal thermal shutdown to

protect the device from over-heating. Under all possible op-

erating 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 deter-

mine if a heatsink is needed, the power dissipated by the

regulator, PD , must be calculated:

IIN = IL + IG

PD = (VIN-VOUT)I L + VINIG

Figure 6 shows the voltages and currents which are present

in the circuit.

FIGURE 4. Regulator with Protection Diode

5.0 Heatsink Requirements

When an integrated circuit operates with an appreciable cur-

rent, its junction temperature is elevated. It is important to

quantify its thermal limits in order to achieve acceptable per-

formance 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 envi-

ronment. A one-dimensional steady-state model of conduc-

tion heat transfer is demonstrated in Figure 5. The heat gen-

erated 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 af-

fect the thermal resistance and in turn the need for a heat-

sink.

RθJC(Component Vari-

ables)

RθCA (Application Vari-

ables)

Leadframe Size &

Material

No. of Conduction Pins

Die Size

Die Attach Material

Molding Compound Size

and Material

Mounting Pad Size,

Material, & Location

Placement of Mounting

Pad

PCB Size & Material

Traces Length & Width

Adjacent Heat Sources

Volume of Air

Ambient Temperatue

Shape of Mounting Pad

9

FIGURE 6. Power Dissipation Diagram

Share Link: