LT1528 データシートの表示（PDF） - Linear Technology

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LT1528

3A Low Dropout Regulator for Microprocessor Applications

Linear Technology 

LT1528

PIN FUNCTIONS

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 SHDN pin is actively pulled low. The SHDN pin current

with the pin pulled to ground will be 60μA. The SHDN pin

is internally clamped to 7V and – 0.6V (one VBE). This al-

lows the SHDN 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 microamperes.

Pull-up current must be limited to a maximum of 5mA.

A curve of SHDN pin input current as a function of volt-

age appears in the Typical Performance Characteristics

section. 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.

VIN (Pin 5): 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. 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.

5

IN

1 RP

OUT

+

VIN

LT1528

+

4

2

SHDN

SENSE

GND

3

RP

LOAD

1528 F01

Figure 1. Kelvin Sense Connection

APPLICATIONS INFORMATION

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 400μA. In shutdown,

the quiescent current drops to only 125μA. 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, 130μA at 25°C, flows through R2

into the SENSE pin. The output voltage can be calculated

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

less than 330Ω 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 Characteristics section.

5

IN

1

OUT

VIN

LT1528

4 SHDN

2

SENSE

GND

3

+

R2

R1

VOUT

) VOUT = 3.3V

1

+

R2

R1

+ (ISENSE)(R2)

VSENSE = 3.3V

ISENSE = 130μA AT 25°C

OUTPUT RANGE = 3.3V TO 14V

1528 F02

Figure 2. Adjustable Operation

1528fb

7

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