8

LT1129/LT1129-3.3/LT1129-5

PI FU CTIO S

S

APPLICATI

I FOR ATIO

The LT1129 is a micropower low dropout regulator with

shutdown, capable of supplying 700mA of output current

at a dropout voltage of 0.4V. The device operates with very

low quiescent current (50

µA). In shutdown the quiescent

current drops to only 16

µA. In addition to the low quies-

cent current the LT1129 incorporates several protection

features which make it ideal for use in battery-powered

systems. The device is protected against reverse input

voltages. In battery backup applications where the output

can be held up by a backup battery when the input is pulled

to ground, the LT1129 acts like it has a diode in series with

its output and prevents reverse current flow.

Adjustable Operation

The adjustable version of the LT1129 has an output

voltage range of 3.75V to 30V. The output voltage is set by

the ratio of two external resistors as shown in Figure 2. The

device servos the output voltage to maintain the voltage at

the adjust pin at 3.75V. The current in R1 is then equal to

3.75V/R1. The current in R2 is equal to the sum of the

current in R1 and the adjust pin bias current. The adjust pin

bias current, 150nA at 25

°C, flows through R2 into the

adjust pin. The output voltage can be calculated according

to the formula in Figure 2. The value of R1 should be less

than 400k to minimize errors in the output voltage caused

by the adjust pin bias current. Note that in shutdown the

output is turned off and the divider current will be zero.

Curves of Adjust Pin Voltage vs Temperature and Adjust

Pin Bias Current vs Temperature appear in the Typical

Performance Characteristics. The reference voltage at the

adjust pin has a positive temperature coefficient of ap-

proximately 15ppm/

°C. The adjust pin bias current has a

negative temperature coefficient. These effects are small

and will tend to cancel each other.

The adjustable device is specified with the adjust pin tied

to the output pin. This sets the output voltage to 3.75V.

Specifications for output voltages greater than 3.75V will

be proportional to the ratio of the desired output voltage to

output current change of 1mA to 700mA is – 6mV typical

12

375

619

V

V

mV

mV

.

––









×

IN

LT1129

GND

SHDN

LT1129 • F02

+

ADJ

OUT

R2

R1

OUTPUT RANGE = 3.75V to 30V

R2

R1

Figure 2. Adjustable Operation

Figure 1. Kelvin Sense Connection

current is 15

µA at the nominal regulated output voltage.

Adjust Pin: For the LT1129 (adjustable version) the adjust

pin is the input to the error amplifier. This pin is internally

current of 150nA which flows into the pin. See Bias Current

curve in the Typical Performance Characteristics. The

adjust pin reference voltage is equal to 3.75V referenced

to ground.

IN

LT1129

GND

SHDN

LT1129 • F01

SENSE

OUT

+

1

µF

LOAD

+

10

µF