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LT3088

3088fb

For more information www.linear.com/LT3088

APPLICATIONS INFORMATION

Introduction

The LT3088 regulator is easy to use and has all the pro-

tection features expected in high performance regulators.

Includedareshort-circuitprotection,reverse-inputprotec-

tion and safe operating area protection, as well as thermal

shutdown with hysteresis. Safe operating area (SOA) for

the LT3088 is extended, allowing for use in harsh indus-

trial and automotive environments where sudden spikes

in input voltage lead to high power dissipation.

The LT3088 fits well in applications needing multiple rails.

This new architecture adjusts down to zero with a single

resistor, handling modern low voltage digital ICs as well

as allowing easy parallel operation and thermal manage-

ment without heat sinks. Adjusting to zero output allows

shutting off the powered circuitry.

Aprecision“0”TC50µAreferencecurrentsourceconnects

to the noninverting input of a power operational amplifier.

Thepoweroperationalamplifierprovidesalowimpedance

buffered output to the voltage on the noninverting input.

A single resistor from the noninverting input to ground

sets the output voltage. If this resistor is set to 0Ω, zero

output voltage results. Therefore, any output voltage can

be obtained between zero and the maximum defined by

the input power supply is obtainable.

The benefit of using a true internal current source as the

reference, as opposed to a bootstrapped reference in older

regulators, is not so obvious in this architecture. A true

reference current source allows the regulator to have gain

and frequency response independent of the impedance on

the positive input. On older adjustable regulators, such as

the LT1086 loop gain changes with output voltage and

bandwidth changes if the adjustment pin is bypassed to

ground. For the LT3088, the loop gain is unchanged with

output voltage changes or bypassing. Output regulation

is not a fixed percentage of output voltage, but is a fixed

fraction of millivolts. Use of a true current source allows

all of the gain in the buffer amplifier to provide regulation,

and none of that gain is needed to amplify up the reference

to a higher output voltage.

Programming Linear Regulator Output Voltage

The LT3088 generates a 50µA reference current that flows

out of the SET pin. Connecting a resistor from SET to

ground generates a voltage that becomes the reference

point for the error amplifier (see Figure 1). The reference

voltage equals 50µA multiplied by the value of the SET

pin resistor (Ohm’s Law). Any voltage can be generated

and there is no minimum output voltage for the regulator.

Figure 1. Basic Adjustable Regulator

3088 F01

SET

OUT

–

LT3088

50µA

RLOAD

CSET

RSET

CIN

VOUT = 50µA • RSET

COUT

Table 1 lists many common output voltages and the clos-

est standard 1% resistor values used to generate that

output voltage.

Regulationoftheoutputvoltagerequiresaminimumload

current of 2mA. For true zero voltage output operation,

return this 2mA load current to a negative output voltage.

Table 1. 1% Resistors for Common Output Voltages

VOUT (V)

RSET (kΩ)

1.2

24.3

1.5

30.1

1.8

35.7

2.5

49.9

3.3

66.5

100

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