LT1123(Rev0) データシートの表示（PDF） - Linear Technology

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LT1123
(Rev.:Rev0)

Low Dropout Regulator Driver

Linear Technology 

LT1123

APPLICATI S I FOR ATIO

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

temperature 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

voltage and resistor value. Power dissipation can be

calculated using the following formula:

( ) PRD =

VIN – VBE – VDRIVE 2

R

1k

0.25W

0.5W

1W

100

2W

10

5 6 7 8 9 10 11 12 13 14 15

VIN (V)

LT1123 F08

Figure 8. Power in RD

where VBE = emitter/base voltage of the PNP pass

transistor

VDRIVE = voltage at the drive pin of the LT1123

= VSAT of the drive pin in the worst case

The worst case power dissipation in the PNP pass transis-

tor is simply equal to:

PMAX = (VIN – VOUT)(IOUT)

where VIN = Maximum VIN

IOUT = Maximum IOUT

The thermal resistance of the MJE1123 is equal to:

70°C/W Junction to Ambient (no heat sink)

1.67°C/W Junction to Case

The PNP will normally be attached to either a chassis or a

heat sink so the actual thermal resistance from junction to

ambient will be the sum of the PNP's junction to case

thermal resistance and the thermal resistance of the heat

sink or chassis. For non-standard heat sinks the user will

need to determine the thermal resistance by experiment.

The maximum junction temperature rise above ambient

for the PNP pass transistor will be equal to the maximum

power dissipation times the thermal resistance, junction

to ambient, of the PNP. The maximum operating junction

temperature of the MJE1123 is 150°C. The maximum

operating ambient temperature for the MJE1123 will be

equal to 150°C minus the maximum junction temperature

rise.

8

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