8

LT1529

LT1529-3.3/LT1529-5

PI FU CTIO S

function of voltage appears in the Typical Performance

Characteristics. If the SHDN pin is not used it can be left

open circuit. The device will be active, output on, if the

SHDN pin is not connected.

ground if the device is more than six inches away from the

main input filter capacitor. In general, the output imped-

ance of a battery rises with frequency so it is advisable to

S

APPLICATI

I FOR ATIO

The LT1529 is a 3A low dropout regulator with mi-

cropower quiescent current and shutdown capable of

supplying 3A of output current at a dropout voltage of

0.6V. 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 quiescent current the LT1529

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 LT1529 acts

like it has a diode in series with its output and prevents

reverse current flow.

Adjustable Operation

The adjustable version of the LT1529 has an output

voltage range of 3.75V to 14V. 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 ADJ 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 ADJ pin bias current. The ADJ

pin bias current, 150nA at 25

°C, flows through R2 into the

ADJ 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 ADJ pin bias current. Note that in shutdown the

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

Curves of ADJ Pin Voltage vs Temperature and ADJ Pin

include a bypass capacitor in battery-powered circuits. A

bypass capacitor in the range of 1

µF to 10µF is sufficient.

The LT1529 is designed to withstand reverse voltages on

case of a reversed input, which can happen if a battery is

plugged in backwards, the LT1529 will act as if there is a

diode in series with its input. There will be no reverse

current flow into the LT1529 and no reverse voltage will

appear at the load. The device will protect both itself and

the load.

+

R2

R1

1

2

3

LT1529 • F02

5

4

OUTPUT

SENSE

LT1529

SHDN

GND

()

1 + R2

R1

OUTPUT RANGE = 3.3V TO 14V

Figure 2. Adjustable Operation

Bias Current vs Temperature appear in the Typical Perfor-

mance Characteristics. The reference voltage at the ADJ

pin has a positive temperature coefficient of approxi-

mately 15ppm/

°C.TheADJpinbiascurrenthasanegative

temperature coefficient. These effects will tend to cancel

each other.

The adjustable device is specified with the ADJ pin tied to

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

Specifications for output voltage greater than 3.75V will be

proportional to the ratio of the desired output voltage to

output current change of 1mA to 3A is – 0.5mV typical at

12

375

05

16

V

V

mV

mV

.

–.

– .







=

()