9

LT1521/LT1521-3

LT1521-3.3/LT1521-5

1521335fb

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 the shutdown pin input current as a

function of voltage appears in the Typical Performance

Characteristics. 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.

IN (Pin 8): Power is supplied to the device through the

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. In general, the output impedance of

battery rises with frequency, so it is advisable to include a

bypass capacitor in battery-powered circuits. A bypass

capacitor in the range of 1

µF to 10µF is sufficient. The

LT1521 is designed to withstand reverse voltages on the

input pin with respect to ground and the output pin. In the

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

plugged in backwards, the LT1521 will act as if there is a

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

current flow into the LT1521 and no reverse voltage will

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

the load.

Figure 1. Kelvin Sense Connection

The LT1521 is a 300mA low dropout regulator with

micropower quiescent current and shutdown. The device

is capable of supplying 300mA at a dropout of 0.5V and

operates with very low quiescent current (12

µA). In shut-

down, the quiescent current drops to only 6

µA. In addition

to the low quiescent current, the LT1521 incorporates

several protection features which make it ideal for use in

battery-powered systems. The device is protected against

both reverse input voltages and reverse output voltages.

In battery backup applications where the output can be

held up by a backup battery when the input is pulled to

ground, the LT1521 acts like it has a diode in series with

its output and prevents reverse current flow.

Adjustable Operation

The adjustable version of the LT1521 has an output

voltage range of 3.75V to 20V. 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, 50nA at 25

°C, flows through R2 into the

adjust pin. The output voltage can be calculated using 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

5

4

1

2

3

+

LT1521

GND

IN

SHDN

OUT

SENSE

+

LOAD

LT1521 • F01

ADJ (Pin 2): For adjustable LT1521, the adjust pin is the

input to the error amplifier. This pin is internally clamped

which flows into the pin. See Adjust Pin Bias Current vs

Temperature in the Typical Performance Characteristics

section. The adjust pin reference voltage is 3.75V refer-

enced to ground. The output voltage range that can be

produced by this device is 3.75V to 20V.

SHDN (Pin 5): The shutdown 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 pulled low. The shutdown pin

current with the pin pulled to ground will be 1.7

µA. The

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