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LT1123CST

Low Dropout Regulator Driver

Linear Technology 

LT1123

APPLICATIO S I FOR ATIO

The curve in Figure 6 can be used to determine the range

of current limit of an LT1123 regulator circuit using an

MJE1123 as a pass transistor. The curve was generated

using the Beta versus IC curve of the MJE1123. The

minimum and maximum value curves are extrapolated

from the minimum and maximum Beta specifications.

Thermal Conditions

The thermal characteristics of three components need to

be considered; the LT1123, the pass transistor and RD.

Power dissipation should be calculated based on the

worst-case conditions seen by each component during

normal operation.

The worst-case power dissipation in the LT1123 is a

function of drive current, supply voltage and the value of

RD. Worst-case dissipation for the LT1123 occurs when

the drive current is equal to approximately one half of its

maximum value. Figure 7 plots the worst-case power

dissipation in the LT1123 versus RD and VIN. The graph

was generated using the following formula:

PD

=

(VIN – VBE)2

4RD

;RD

>

10Ω

where:

VBE = the emitter/base voltage of the PNP pass

transistor (assumed to be 0.6V)

1k

0.1W

0.2W

100

0.3W

0.4W

0.5W

0.7W

10

5 6 7 8 9 10 11 12 13 14 15

VIN (V)

LT1123 F07

Figure 7. Power in LT1123

For some operating conditions RD may be replaced with a

short. This is possible in applications where the operating

requirements (input voltage and drive current) are at the

low end and the output will not be shorted. For RD = 0 the

following formula may be used to calculate the maximum

power dissipation in the LT1123.

PD = (VIN – VBE)(IDRIVE)

where:

VIN = maximum input voltage

VBE = emitter/base voltage of PNP

IDRIVE = required maximum drive current

The maximum junction temperature rise above ambient

for the LT1123 will be equal to the worst-case power

dissipation multiplied by the thermal resistance of the

device. The thermal resistance of the device will depend

upon how the device is mounted, and whether a heat sink

is used. Measurements show that one of the most effective

ways of heat sinking the TO-92 package is by utilizing the

PC board traces attached to the leads of the package. The

table below lists several methods of mounting and the

measured value of thermal resistance for each method. All

measurements were done in still air.

THERMAL

RESISTANCE

Package alone ............................................................................. 220°C/W

Package soldered into PC board with plated through

holes only ................................................................................ 175°C/W

Package soldered into PC board with 1/4 sq. in. of copper trace

per lead .................................................................................... 145°C/W

Package soldered into PC board with plated through holes in

board, no extra copper trace, and a clip-on type heat sink:

Thermalloy type 2224B .................................................... 160°C/W

Aavid type 5754 ................................................................ 135°C/W

The maximum operating junction temperature of the

LT1123 is 125°C. The maximum operating ambient tem-

perature will be equal to 125°C minus the maximum

junction temperature rise above ambient.

The worst-case power dissipation in RD needs to be

calculated so that the power rating of the resistor can be

determined. The worst-case power in the resistor will

occur when the drive current is at a maximum. Figure 8

plots the required power rating of RD versus supply

1123fb

8

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