REG1117

REG1117A

SBVS001D − OCTOBER 1992 − REVISED JULY 2004

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9

A simple experiment will determine whether the maximum

recommended junction temperature is exceeded in an

actual circuit board and mounting configuration: Increase

the ambient temperature above that expected in normal

operation until the device’s thermal shutdown is activated.

If this occurs at more than 40

°C above the maximum

125

°C during normal operation.

The internal protection circuitry of the REG1117 was

designed to protect against overload conditions. It was not

intended to replace proper heat sinking. Continuously

running the REG1117 into thermal shutdown will degrade

reliability.

LAYOUT CONSIDERATIONS

The DDPAK (REG1117F-3.3 and REG1117FA) is a

surface-mount power package that has excellent thermal

characteristics. For best thermal performance, the

mounting tab should be soldered directly to a circuit board

copper area, as shown in Figure 3. Increasing the copper

area improves heat dissipation. Figure 4 shows typical

thermal resistance from junction-to-ambient as a function

of the copper area.

NOTE: (1) For improved thermal performance increase

footprint area. See Figure 4 (Thermal Resistance vs

Circuit Board Copper Area).

All measurements

in inches.

0.51

0.10

0.155

0.05

Figure 3. DDPAK Footprint

THERMAL RESISTANCE vs

CIRCUIT BOARD COPPER AREA

60

50

40

30

20

10

01

2

3

45

REG1117F

DDPAK Surface Mount Package

1oz copper

Circuit Board Copper Area

REG1117F

DDPAK Surface−Mount Package

Figure 4. DDPAK Thermal Resistance versus Circuit Board Copper Area