LT1123CST Просмотр технического описания (PDF) - Linear Technology

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LT1123CST

Low Dropout Regulator Driver

Linear Technology 

LT1123

PI FU CTIO S

All internal circuitry connected to the drive pin is designed

to operate at the saturation voltage of the Darlington

output driver (1.4 to 2V). This allows a resistor to be

inserted between the base of the external PNP device and

the drive pin. This resistor is used to limit the base drive to

the external PNP below the value set internally by the

LT1123, and also to help limit power dissipation in the

LT1123. The operating voltage range of this pin is from

0V to 30V. Pulling this pin below ground by more than one

VBE will forward bias the substrate diode of the device.

This condition can only occur if the power supply leads are

reversed and will not damage the device if the current is

limited to less than 200mA.

Feedback Pin (VFB): The feedback pin also serves two

functions. It provides a path for the bias current of the

reference and error amplifier and contributes a portion of

the drive current for the Darlington output driver. The sum

total of these currents is the Feedback Pin Bias Current

(300µA typical). The second function of this pin is to

provide the voltage feedback to the error amplifier.

SI PLIFIED BLOCK DIAGRA

DRIVE

CURRENT

LIMIT

THERMAL

LIMIT

GROUND

FB

5V

LT1123 SBD01

FU CTIO AL DESCRIPTIO

The LT1123 is a 3-pin device designed to be used in

conjunction with a discrete PNP transistor to form an

inexpensive ultralow dropout regulator. The device incor-

porates a trimmed 5V bandgap reference, error amplifier,

a current-limited Darlington driver and an internal thermal

limit circuit. The internal circuitry connected to the drive

pin is designed to function at the saturation voltage of the

Darlington driver. This allows a resistor to be inserted in

series with the drive pin. This resistor is used to limit the

base drive to the PNP and also to limit the power dissipa-

tion in the LT1123. The value of this resistor will be defined

by the operating requirements of the regulator circuit. The

LT1123 is designed to sink a minimum of 125mA of base

current. This is sufficient base drive to form a regulator

circuit which can supply output currents up to 4A at a

dropout voltage of less than 0.75V.

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