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LT1784IS6

2.5MHz, Over-The-Top Low Power, Rail-to-Rail Input and Output Op Amp in SOT-23

Linear Technology 

LT1784

APPLICATIONS INFORMATION

Supply Voltage

The positive supply pin of the LT1784 should be bypassed

with a small capacitor (typically 0.1μF) within an inch of

the pin. When driving heavy loads, and additional 4.7μF

electrolytic capacitor should be used. When using split

supplies the same is true for the negative supply pin.

The LT1784 is protected against reverse battery voltages

up to 18V. In the event a reverse battery condition occurs

the supply current is less than 1nA.

Inputs

The LT1784 has two input stages, NPN and PNP (see the

Simplified Schematic), resulting in three distinct operating

regions as shown in the “Input Bias Current vs Common

Mode” Typical Performance Characteristic curve.

For input voltages about 1V or more below V+, the PNP

input stage is active and the input bias current is typically

–250nA. When the input common mode voltage is within

0.6V of the positive rail, the NPN stage is operating and

the input bias current is typically 500nA. Increases in

temperature will cause the voltage at which operation

switches from the PNP input stage to the NPN input stage

to move towards V+. The input offset voltage of the NPN

stage is untrimmed and is typically 3mV.

A Schottky diode in the collector of the input transistors,

along with special geometries for these NPN transistors,

allow the LT1784 to operate with either or both of its inputs

above V+. At about 0.3V above V+, the NPN input transis-

tors is fully saturated and the input bias current is typically

200μA at room temperature. The input offset voltage is

typically 3mV when operating above V+. The LT1784 will

operate with inputs 18V above V– regardless of V+.

The inputs are protected against excursions as much as

10V below V– by an internal 1k resistor in series with each

input and a diode from the input to the negative supply.

The input stage of the LT1784 incorporates phase reversal

protection to prevent the output from phase reversing for

inputs up to 9V below V–. There are no clamping diodes

between the inputs and the maximum differential input

voltage is 18V.

Output

The output of the LT1784 can swing to within 80mV of the

positive rail and within 4mV of the negative rail with no

load. When monitoring input voltages within 80mV of the

positive rail or within 4mV of the negative rail, gain should

be taken to keep the output from clipping. The LT1784

can typically sink and source over 25mA at ±5V supplies,

sourcing current is reduced to 7.5mA at 3V total supplies

as noted in the Electrical Characteristics section.

The LT1784 is internally compensated to drive at least

400pF of capacitance under any output loading condi-

tions. A 0.22μF capacitor in series with a 150Ω resistor

between the output and ground will compensate these

amplifiers for larger capacitive loads, up to 10,000pF at

all output currents.

Distortion

There are two main contributors to distortion in op amps:

output crossover distortion as the output transitions from

sourcing to sinking current, and distortion caused by non-

linear common mode rejection. If the op amp is operating

inverting, there is no common mode induced distortion.

If the op amp is operating in the PNP input stage (input

not within 1V of V+), the CMRR is very good, typically

95dB. When the LT1784 switches between input stages,

there is significant nonlinearity in the CMRR. Lower load

resistance increases the output crossover distortion but

has no effect on the input stage transition distortion. For

lowest distortion, the LT1784 should be operated single

supply, with the output always sourcing current and with

the input voltage swing between ground and (V+ – 1V). See

Typical Performance Characteristics curve, “Total Harmonic

Distortion + Noise vs Output Voltage Amplitude.”

Gain

The open-loop gain is almost independent of load when

the output is sourcing current. This optimizes performance

in single supply applications where the load is returned

to ground. The Typical Performance Characteric curve

“Open-Loop Gain” for various loads shows the details.

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