10

LT1361/LT1362

tion with large, sustained differential inputs. Under

normal, closed-loop operation, an increase of power dis-

sipation 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 dissipa-

tion.

Capacitive Loading

The LT1361/LT1362 are stable with any capacitive load.

This is accomplished by sensing the load induced output

pole and adding compensation at the amplifier gain node.

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 as shown

in the typical performance curves. The photo of the small

signal response with 500pF load shows 60% peaking. The

large signal response shows the output slew rate being

limited to 5V/

µs by the short-circuit current. Coaxial cable

can be driven directly, but for best pulse fidelity a resistor

of value equal to the characteristic impedance of the cable

(i.e., 75

Ω) should be placed in series with the output. The

other end of the cable should be terminated with the same

value resistor to ground.

Circuit Operation

The LT1361/LT1362 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 a 500

Ω resistor. The input

voltage appears across the resistor generating currents

which are mirrored into the high impedance node. Comple-

mentary followers form an output stage which buffers the

gain node from the load. 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 gain node 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. The LT1361/LT1362 are tested

for slew rate in a gain of –2 so higher slew rates can be

expected in gains of 1 and –1, and lower slew rates in

higher gain configurations.

The RC network across the output stage is bootstrapped

when the amplifier is driving a light or moderate load and

has no effect under normal operation. When driving a

capacitive load (or a low value resistive load) the network

is incompletely bootstrapped and adds to the compensa-

tion at the high impedance node. The added capacitance

slows down the amplifier which improves the phase

margin by moving the unity-gain frequency away from the

pole formed by the output impedance and the capacitive

load. The zero created by the RC combination adds phase

to ensure that even for very large load capacitances, the

total phase lag can never exceed 180 degrees (zero phase

margin) and the amplifier remains stable.

Power Dissipation

The LT1361/LT1362 combine high speed and large output

drive in small packages. Because of the wide supply

voltage range, it is possible to exceed the maximum

junction temperature under certain conditions. Maximum

Worst case power dissipation occurs at the maximum

supply current and when the output voltage is at 1/2 of

either supply voltage (or the maximum swing if less than

Example: LT1362 in S16 at 70