7

LT1117/LT1117-2.85

LT1117-3.3/LT1117-5

Thermal Considerations

LT1117 series regulators have internal thermal limiting

circuitry designed to protect the device during overload

conditions. For continuous normal load conditions how-

ever, the maximum junction temperature rating of 125

°C

must not be exceeded.

It is important to give careful consideration to all sources

of thermal resistance from junction to ambient. For the

SOT-223 package, which is designed to be surface

mounted, additional heat sources mounted near the de-

vice must also be considered. Heat sinking is accom-

plished using the heat spreading capability of the PC board

and its copper traces. The thermal resistance of the

LT1117 is 15

°C/W from the junction to the tab. Thermal

resistances from tab to ambient can be as low as 30

°C/W.

The total thermal resistance from junction to ambient can

be as low as 45

°C/W. This requires a reasonable sized PC

board with at least one layer of copper to spread the heat

across the board and couple it into the surrounding air.

Experiments have shown that the heat spreading copper

layer does not need to be electrically connected to the tab

of the device. The PC material can be very effective at

transmitting heat between the pad area, attached to the tab

of the device, and a ground plane layer either inside or on

the opposite side of the board. Although the actual thermal

resistance of the PC material is high, the Length/Area ratio

of the thermal resistor between layers is small. The data in

Table 1 was taken using 1/16" FR-4 board with 1oz. copper

foil. It can be used as a rough guideline in estimating

thermal resistance.

Table 1.

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

2500 Sq. mm

2500 Sq. mm

2500 Sq. mm

45

°C/W

1000 Sq. mm

2500 Sq. mm

2500 Sq. mm

45

°C/W

225 Sq. mm

2500 Sq. mm

2500 Sq. mm

53

°C/W

100 Sq. mm

2500 Sq. mm

2500 Sq. mm

59

°C/W

1000 Sq. mm

1000 Sq. mm

1000 Sq. mm

52

°C/W

1000 Sq. mm

0

1000 Sq. mm

55

°C/W

* Tab of device attached to topside copper

APPLICATI

HI TS

The thermal resistance for each application will be affected

by thermal interactions with other components on the

board. Some experimentation will be necessary to deter-

mine the actual value.

The power dissipation of the LT1117 is equal to:

Maximum junction temperature will be equal to:

ambient))

Maximum junction temperature must not exceed 125

°C.

Ripple Rejection

The curves for Ripple Rejection were generated using an

adjustable device with the adjust pin bypassed. These

curves will hold true for all values of output voltage. For

proper bypassing, and ripple rejection approaching the

values shown, the impedance of the adjust pin capacitor,

at the ripple frequency, should be < R1. R1 is normally in

the range of 100

Ω-200Ω. The size of the required adjust

pin capacitor is a function of the input ripple frequency. At

120Hz, with R1=100

Ω, the adjust pin capacitor should be

> 13

µF. At 10kHz only 0.16µF is needed.

For fixed voltage devices, and adjustable devices without

an adjust pin capacitor, the output ripple will increase as

the ratio of the output voltage to the reference voltage

5V, the output ripple will be increased by the ratio of 5V/

1.25V. It will increase by a factor of four. Ripple rejection

will be degraded by 12dB from the value shown

on the curve.