LT1123CST データシートの表示（PDF） - Linear Technology

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LT1123CST

Low Dropout Regulator Driver

Linear Technology 

LT1123

APPLICATIO S I FOR ATIO

PNP. If the output of the regulator circuit can be shorted,

heat sinking must be adequate to limit the rate of tempera-

ture rise of the power device to approximately 50°C/

minute. This can be accomplished with a fairly small heat

sink, on the order of 3 to 4 square inches of surface area.

Design Example

Given the following operating requirements:

5.5V < VIN < 7V

IOUTMAX = 1.5A

Max ambient temperature = 70°C

VOUT = 5V

1. The first step is to determine the required drive current.

This can be found from the Maximum Dropout Voltage

curve. 50mA of drive current will guarantee 0.4V drop-

out at an output current of 2A. This satisfies our

requirements.

IDRIVE = 50mA

2. The next step is to determine the value of RD. Based on

50mA of drive current and a minimum input voltage of

5.5V, we can select RD from the graph of Figure 4. From

the graph the value of RD is equal to 50Ω, so we should

use the next lowest 5% value which is 47Ω.

RD = 47Ω

3. We can now look at the thermal requirements of the

circuit.

Worst-case power in the LT1123 will be equal to:

( ) VIN(MAX) – VBE 2

4RD

Given: VIN(MAX) = 7V, VBE = 0.6V, RD = 47Ω

Then: PMAX (LT1123) = 0.22W.

Assuming a thermal resistance of 150°C/W, the maximum

junction temperature rise above ambient will be equal to

(PMAX)(150°C/W) = 33°C. The maximum operating junc-

tion temperature will be equal to the maximum ambient

temperature plus the junction temperature rise above

ambient. In this case we have (maximum ambient = 70°C)

plus (junction temperature rise = 33°C) is equal to 103°C.

This is well below the maximum operating junction tem-

perature of 125°C for the LT1123.

The power rating for RD can be found from the plot of

Figure 8 using VIN = 7V and RD = 47Ω. From the plot, RD

should be sized to dissipate a minimum of 1/2W.

The worst-case power dissipation, for normal operation,

in the MJE1123 will be equal to:

(VINMAX – VOUT)(IOUTMAX) = (7V – 5V)(1.5A) = 3W

The maximum operating junction temperature of the

MJE1123 is 150°C. The difference between the maximum

operating junction temperature of 150°C and the maxi-

mum ambient temperature of 70°C is 80°C. The device

must be mounted to a heat sink which is sized such that the

thermal resistance from the junction of the MJE1123 to

ambient is less than 80°C/3W = 26.7°C/W.

It is recommended that the LT1123 be thermally coupled

to the MJE1123 so that the thermal limit circuit of the

LT1123 can protect both devices. In this case the ambient

temperature for the LT1123 will be equal to the tempera-

ture of the heat sink. The heat sink temperature, under

normal operating conditions, will have to be limited such

that the maximum operating junction temperature of the

LT1123 is not exceeded.

Refer to Linear Technology’s list of Suggested Manufac-

turers of Specialized Components for information on

where to find the required heat sinks, resistors and capaci-

tors. This listing is available through Linear Technology’s

marketing department.

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