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LT1123CST

Low Dropout Regulator Driver

Linear Technology 

LT1123

APPLICATIO S I FOR ATIO

The LT1123 is designed to be used in conjunction with an

external PNP transistor. The overall specifications of a

regulator circuit using the LT1123 and an external PNP will

be heavily dependent on the specifications of the external

PNP. While there are a wide variety of PNP transistors

available that can be used with the LT1123, the specifica-

tions given in typical transistor data sheets are of little use

in determining overall circuit performance.

Linear Technology has solved this problem by cooperating

with Motorola to design and specify the MJE1123. This

transistor is specifically designed to work with the LT1123

as the pass element in a low dropout regulator. The

specifications of the MJE1123 reflect the capability of the

LT1123. For example, the dropout voltage of the MJE1123

is specified up to 4A collector current with base drive

currents that the LT1123 is capable of generating (20mA

to 120mA). Output currents up to 4A with dropout voltages

less than 0.75V can be guaranteed.

The following sections describe how specifications can be

determined for the basic regulator. The charts and graphs

are based on the combined characteristics of the LT1123

and the MJE1123. Formulas are included that will enable

the user to substitute other transistors that have been

characterized. A chart is supplied that lists suggested

resistor values for the most popular range of input volt-

ages and output current.

Basic Regulator Circuit

The basic regulator circuit is shown in Figure 1. The

LT1123 senses the voltage at its feedback pin and drives

the base of the PNP (MJE1123) in order to maintain the

output at 5V. The drive pin of the LT1123 can only sink

current; RB is required to provide pull up on the base of the

PNP. RB must be sized so that the voltage drop caused by

the minimum drive pin current is less than the emitter/

base voltage of the external PNP at light loads. The

recommended value for RB is 620Ω. For circuits that are

required to run at junction temperatures in excess of

100°C the recommended value of RB is 300Ω.

RB

VIN

620Ω

RD

DRIVE

FB

LT1123

GND

MJE1123

VOUT = 5V

+

10µF ALUM

LT1123 F01

Figure 1. Basic Regulator Circuit

RD is used to limit the drive current available to the PNP

and to limit the power dissipation in the LT1123. Limiting

the drive current to the PNP will limit the output current of

the regulator which will minimize the stress on the regu-

lator circuit under overload conditions. RD is chosen

based on the operating requirements of the circuit, prima-

rily dropout voltage and output current.

Dropout Voltage

The dropout voltage of an LT1123-based regulator circuit

is determined by the VCE saturation voltage of the discrete

PNP when it is driven with a base current equal to the

available drive current of the LT1123. The LT1123 can sink

up to 150mA of base current (150mA typ, 125mA min)

when output voltage is up near the regulating point (5V).

The available drive current of the LT1123 can be reduced

by adding a resistor (RD) in series with the drive pin (see

the section below on current limit). The MJE1123 is

specified for dropout voltage (VCE sat.) at several values of

output current and up to 120mA of base drive current. The

chart below lists the operating points that can be guaran-

teed by the combined data sheets of the LT1123 and

MJE1123. Figure 2 illustrates the chart in graphic form.

Although these numbers are only guaranteed by the data

sheet at 25°C, Dropout Voltage vs Temperature (Figure 3)

clearly shows that the dropout voltage is nearly constant

over a wide temperature range.

1123fb

5

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