7

LT1528

SHDN (Pin 4): This pin is used to put the device into

shutdown. In shutdown the output of the device is turned

off. This pin is active low. The device will be shut down if

the Shutdown pin is actively pulled low. The Shutdown pin

current with the pin pulled to ground will be 60

µA. The

Shutdown pin is internally clamped to 7V and – 0.6V (one

5V logic or by open collector logic with a pull-up resistor.

The pull-up resistor is only required to supply the leakage

current of the open collector gate, normally several micro-

amperes. Pull-up current must be limited to a maximum

of 5mA. A curve of Shutdown pin input current as a

function of voltage appears in the Typical Performance

Characteristics section. If the Shutdown pin is not used it

can be left open circuit. The device will be active output on

if the Shutdown pin is not connected.

input pin. The input pin should be bypassed to ground if

the device is more than six inches away from the main

input filter capacitor. The LT1528 is designed to withstand

reverse voltages on the input pin with respect to ground

and the Output pin. In the case of reversed input, the

LT1528 will act as if there is a diode in series with its input.

There will be no reverse current flow into the LT1528 and

no reverse voltage will appear at the load. The device will

protect both itself and the load.

Figure 1. Kelvin Sense Connection

5

4

1

2

3

+

LT1528

GND

IN

SHDN

OUT

SENSE

+

LOAD

LT1528 • F01

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

330

Ω to minimize errors in the output voltage caused by

the Sense pin current. Note that in shutdown the output is

turned off and the divider current will be zero. Curves of

Sense Pin Voltage vs Temperature and Sense Pin Current

vs Temperature appear in the Typical Performance Char-

acteristics section.

The LT1528 is a 3A low dropout regulator optimized for

microprocessor applications. Dropout voltage is only 0.6V

at 3A output current. With the Sense pin shorted to the

Output pin, the output voltage is set to 3.3V. The device

operates with a quiescent current of 400

µA. In shutdown,

the quiescent current drops to only 125

µA. The LT1528

incorporates several protection features, including protec-

tion against reverse input voltages. If the output is held at

the rated output voltage when the input is pulled to ground,

the LT1528 acts like it has a diode in series with its output

and prevents reverse current flow.

Adjustable Operation

The LT1528 can be used as an adjustable regulator with an

output voltage range of 3.3V 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 Sense pin at 3.3V. The current in R1 is

then equal to 3.3V/R1. The current in R2 is equal to the sum

of the current in R1 and the Sense pin current. The Sense

pin current, 130

µA at 25°C, flows through R2 into the

Sense pin. The output voltage can be calculated using the

+

LT1528

GND

IN

SHDN

OUT

SENSE

LT1528 • F02

OUTPUT RANGE = 3.3V TO 14V

R2

R1

)

)

R1

R2

5

4

1

2

3

Figure 2. Adjustable Operation