LT1521 datasheet(9/18 Pages) AD

9 / 18 page

LT1521/LT1521-3

LT1521-3.3/LT1521-5

Rev. C

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PIN FUNCTIONS

OUT (Pin 1): The output pin supplies power to the load.

A minimum output capacitor of 1.5µF is required to pre-

vent oscillations, but larger values of output capacitor

will be necessary to deal with larger load transients. See

the Applications Information section for more on output

capacitance and reverse output characteristics.

SENSE (Pin 2): For fixed voltage versions of the LT1521

(LT1521-3, LT1521-3.3, LT1521-5), the sense pin is the

input to the error amplifier. Optimum regulation will be

obtained at the point where the sense pin is connected

to the output pin of the regulator. In critical applications

small voltage drops caused by the resistance (RP) of PC

traces between the regulator and the load, which would

normally degrade regulation, may be eliminated by con-

necting the sense pin to the output at the load as shown in

Figure 1 (Kelvin Sense Connection). Note that the voltage

drop across the external PC traces will add to the dropout

voltage of the regulator. The sense pin bias current is

5µA at the nominal regulated output voltage. This pin is

internally clamped to –0.6V (one VBE).

Temperature in the Typical Performance Characteristics

section.Theadjustpinreferencevoltageis3.75Vreferenced

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 1.7µA. The

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

VBE). This allows the shutdown pin to be driven directly by

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 1µF 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.

LT1521

GND

SHDN

OUT

SENSE

VIN

LOAD

LT1521 • F01

Figure 1. Kelvin Sense Connection

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

input to the error amplifier. This pin is internally clamped

to 6V and –0.6V (one VBE). It has a bias current of 50nA

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

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