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LT1123CST

Low Dropout Regulator Driver

Linear Technology 

LT1123

TYPICAL PERFOR A CE CHARACTERISTICS

Feedback Pin Bias Current

vs Temperature

400

VFB = 5V

300

200

100

0

25

50

75 100 125

TEMPERATURE (°C)

LT1123 G01

Feedback Pin Bias Current

vs Feedback Pin Voltage

500

400

300

200

100

0

0

TJ = 125°C

TJ = 25°C

TJ = 0°C

1

2

3

4

5

FEEDBACK PIN VOLTAGE (V)

LT1123 G04

Minimum Drive Pin Current

vs Temperature

600

VDRIVE = 3V

500

400

300

200

100

0

0

25

50

75 100 125

TEMPERATURE (°C)

LT1123 G02

Drive Pin Saturation Voltage

vs Drive Current

2.5

VFB = 4.5V

2.0

TJ = 0°C

1.5

TJ = 125°C

1.0

TJ = 25°C

0.5

0

0 20 40 60 80 100 120 140

DRIVE CURRENT (mA)

LT1123 G05

Drive Current

vs Feedback Pin Voltage

200

VDRIVE = 3V

TJ = 125°C

150

TJ = 25°C

100

TJ = –50°C

50

0

01

2

3

4

5

6

FEEDBACK PIN VOLTAGE (V)

LT1123 G03

Output Voltage vs Temperature

5.03

5.02

5.01

5.00

4.99

4.98

4.97

–50 –25

0 25 50 75

TEMPERATURE (°C)

100 125

LT1123 G06

PI FU CTIO S

Drive Pin: The drive pin serves two functions. It provides

current to the LT1123 for its internal circuitry including

start-up, bias, current limit, thermal limit and a portion of

the base drive current for the output Darlington. The sum

total of these currents (450µA typical) is equal to the

minimum drive current. This current is listed in the speci-

fications as Drive Current with VFB = 5.2V. This is the

minimum current required by the drive pin of the LT1123.

The second function of the drive pin is to sink the base

drive current of the external PNP pass transistor. The

available drive current is specified for two conditions.

Drive current with VFB = 4.80V gives the range of current

available under nominal operating conditions, when the

device is regulating. Drive current with VFB = 0.5V gives the

range of drive current available with the feedback pin

pulled low as it would be during start-up or during a short-

circuit fault. The drive current available when the feedback

pin is pulled low is less than the drive current available

when the device is regulating (VFB = 5V). This can be seen

in the curve of Drive Current vs VFB Voltage in the Typical

Performance Characteristics curves. This can provide

some foldback in the current limit of the regulator circuit.

1123fb

3

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