LM1117 datasheet(10/21 Pages) NSC

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Application Note (Continued)

When the adjustable regulator is used (Figure 3), the best

performance is obtained with the positive side of the resistor

R1 tied directly to the output terminal of the regulator rather

than near the load. This eliminates line drops from appearing

effectively in series with the reference and degrading regu-

lation. For example, a 5V regulator with 0.05

Ω resistance

between the regulator and load will have a load regulation

due to line resistance of 0.05

Ω xI

L. If R1 (=125

Ω) is con-

nected near the load, the effective line resistance will be

0.05

Ω (1+R2/R1) or in this case, it is 4 times worse. In

addition, the ground side of the resistor R2 can be returned

near the ground of the load to provide remote ground sens-

ing and improve load regulation.

4.0 Protection Diodes

Under normal operation, the LM1117 regulators do not need

any protection diode. With the adjustable device, the internal

resistance between the adjust and output terminals limits the

current. No diode is needed to divert the current around the

regulator even with capacitor on the adjust terminal. The

adjust pin can take a transient signal of ±25V with respect to

the output voltage without damaging the device.

When a output capacitor is connected to a regulator and the

input is shorted to ground, the output capacitor will discharge

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.

5.0 Heatsink Requirements

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 sum-

ming the individual parts consisting of a series of tempera-

ture rises from the semiconductor junction to the operating

environment. A one-dimensional steady-state model of con-

duction heat transfer is demonstrated in Figure 5. 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)

Rθ CA (Application

Variables)

Leadframe Size & Material

Mounting Pad Size,

Material, & Location

No. of Conduction Pins

Placement of Mounting

Pad

Die Size

PCB Size & Material

Die Attach Material

Traces Length & Width

Molding Compound Size

and Material

Adjacent Heat Sources

Volume of Air

Ambient Temperatue

Shape of Mounting Pad

10091918

FIGURE 2. Typical Application using Fixed Output

Regulator

10091919

FIGURE 3. Best Load Regulation using Adjustable

Output Regulator

10091915

FIGURE 4. Regulator with Protection Diode

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