LT1351CS8 datasheet(10/16 Pages) LINER

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LT1351

APPLICATIONS INFORMATION

Shutdown

The LT1351 has a Shutdown pin for conserving power.

When this pin is open or 2V above the negative supply the

part operates normally. When pulled down to V – the

supply current will drop to about 10

µA. The current out of

the Shutdown pin is also typically 10

µA. In shutdown the

amplifier output is not isolated from the inputs so the

LT1351 cannot be used in multiplexing applications using

the shutdown feature.

A level shift application is shown in the Typical Applica-

tions section so that a ground-referenced logic signal can

control the Shutdown pin.

Circuit Operation

The LT1351 circuit topology is a true voltage feedback

amplifier that has the slewing behavior of a current

feedback amplifier. The operation of the circuit can be

understood by referring to the simplified schematic.

The inputs are buffered by complementary NPN and PNP

emitter followers which drive R1, a 1k resistor. The input

voltage appears across the resistor generating currents

which are mirrored into the high impedance node and

compensation capacitor CT. Complementary followers

form an output stage which buffers the gain node from

the load. The output devices Q19 and Q22 are connected

to form a composite PNP and composite NPN.

The bandwidth is set by the input resistor and the

capacitance on the high impedance node. The slew rate

is determined by the current available to charge the

capacitance. This current is the differential input voltage

divided by R1, so the slew rate is proportional to the

input. Highest slew rates are therefore seen in the lowest

gain configurations. For example, a 10V output step in a

gain of 10 has only a 1V input step whereas the same

output step in unity gain has a 10 times greater input step.

The curve of Slew Rate vs Input Level illustrates this

relationship.

Capacitive load compensation is provided by the RC, CC

network which is bootstrapped across the output stage.

When the amplifier is driving a light load the network has

no effect. When driving a capacitive load (or a low value

noise gain is one and a large feedback resistor is used, CF

should be greater than or equal to CIN. An example would

be an I-to-V converter as shown in the Typical Applications

section.

Capacitive Loading

The LT1351 is stable with any capacitive load. As the

capacitive load increases, both the bandwidth and phase

margin decrease so there will be peaking in the frequency

domain and in the transient response. Graphs of Fre-

quency Response vs Capacitive Load, Capacitive Load

Handling and the transient response photos clearly show

these effects.

Input Considerations

Each of the LT1351 inputs is the base of an NPN and

a PNP transistor whose base currents are of opposite

polarity and provide first-order bias current cancellation.

Because of variation in the matching of NPN and PNP

beta, the polarity of the input bias current can be positive

or negative. The offset current does not depend on

NPN/PNP beta matching and is well controlled. The use of

balanced source resistance at each input is recommended

for applications where DC accuracy must be maximized.

The inputs can withstand transient differential input volt-

ages up to 10V without damage and need no clamping or

source resistance for protection. Differential inputs, how-

ever, generate large supply currents (tens of mA) as

required for high slew rates. If the device is used with

sustained differential inputs, the average supply current

will increase, excessive power dissipation will result and

the part may be damaged.

The part should not be used as

a comparator, peak detector or other open-loop applica-

tion with large, sustained differential inputs. Under

normal, closed-loop operation, an increase of power

dissipation is only noticeable in applications with large

slewing outputs and is proportional to the magnitude of

the differential input voltage and the percent of the time

that the inputs are apart. Measure the average supply

current for the application in order to calculate the power

dissipation.

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