LT6232 datasheet(16/28 Pages) AD

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LTC6226/LTC6227

Rev 0

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APPLICATIONS INFORMATION

Output

The LTC6226 family has excellent output drive capabil-

ity. The amplifiers can typically deliver more than 50mA

of output drive current at a total supply of 10V, and can

typicallyswingtowithin600mVoftherailforloadcurrents

as high as 25mA. As the supply voltage to the amplifier

decreases, the output current capability also decreases.

Attention must be paid to keep the junction temperature

of the IC below 150°C (refer to power dissipation section)

when the output is in continuous short-circuit. The output

of the amplifier has reverse-biased diodes connected to

each supply. If the output is forced beyond either supply,

extremely high currents will flow through those diodes

whichcanresultindamagetothedevice.Forcingtheoutput

to even 1V beyond either supply could result in several

hundred milliamps of current through either diode. Thus

forcing the output beyond the supplies should be avoided.

Input Protection

The LTC6226/LTC6227 has a pair of back to back diodes

(D5 and D7) to prevent the emitter base breakdown of the

inputtransistorsandlimitthedifferentialinputto±700mV.

Unlike many other high performance amplifiers, the bases

of the input pair transistors Q1 and Q2 are not connected

to the pins using internal resistors to limit input current,

since that would cause the noise to increase. For instance,

a 100Ω resistor in series with each input would generate

1.8nV/√Hz of noise, and the total amplifier noise voltage

would rise from 1nV/√Hz to 2.06nV/√Hz. Once the input

differential voltage exceeds ±0.7V, current conducted

though the protection diodes should be limited to ±10mA.

This implies 25Ω of protection resistance per quarter volt

(250mV) of overdrive beyond ±0.7V. In addition, the input

and shutdown pins have reverse biased diodes connected

tothesupplies.Thecurrentinthesediodesmustbelimited

to less than 10mA. The amplifiers should not be used as

comparators or in other open loop applications.

ESD

The LTC6226 family has reverse biased ESD protection

diodes on all inputs as shown in Figure 1. There is an

additional clamp between the positive and negative sup-

plies that further protects the device during ESD strikes.

Hot plugging of the device into a powered socket must be

avoided since this can trigger the clamp resulting in larger

currents flowing between the supply pins.

Capacitive Loads

The LTC6226/LTC6227 are optimized for high bandwidth

applications, and have not been designed to directly drive

capacitive loads. Hence any trace capacitance at the

output should be made as small as possible. Increased

capacitance at the output creates an additional pole in

the open loop frequency response, worsening the phase

margin. When driving capacitive loads, a resistor of 10Ω

to 100Ω should be connected between the amplifier

output and the capacitive load to avoid ringing or oscil-

lation. The feedback should be taken directly from the

amplifier output. Higher voltage gain configurations tend

to have better capacitive drive capability than lower gain

configurations due to lower closed loop bandwidth and

hence higher phase margin. The graphs titled Overshoot

vs Capacitive Load demonstrate the transient response of

the amplifier when driving capacitive loads with various

series resistors.

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