LT1117/LT1117-2.85

LT1117-3.3/LT1117-5

9

1117fd

APPLICATIONS INFORMATION

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

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE*

BACKSIDE

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

The thermal resistance for each application will be

affected by thermal interactions with other components

on the board. Some experimentation will be necessary to

determine 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 100Ω to 200Ω. The size of the required adjust

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

120Hz, with R1 = 100Ω, the adjust pin capacitor should

be >13μF. 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

to 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.