10

LT1494/LT1495/LT1496

Start-Up Characteristics

Micropower op amps are sometimes not micropower

during start-up, wreaking havoc on low current supplies.

In the worst case, there may not be enough supply current

available to take the system up to nominal voltages. Figure

1 is a graph of LT1495 supply current vs supply voltage

for the three limit cases of input offset that could occur

during start-up. The circuits are shown in Figure 2. One

circuit creates a positive offset, forcing the output to come

up saturated high. Another circuit creates a negative

offset, forcing the output to come up saturated low, while

the last brings up the output at half supply. In all cases, the

supply current is well behaved. Supply current is highest

with the output forced high, so if one amplifier is unused,

it is best to force the output low or at half supply.

Reverse Battery

The LT1494/LT1495/LT1496 are protected against reverse

battery voltages up to 18V. In the event a reverse battery

condition occurs, the supply current is typically less than

100nA (inputs grounded and outputs open). For typical

single supply applications with ground referred loads and

feedback networks, no other precautions are required. If

the reverse battery condition results in a negative voltage

at either the input pins or output pin, the current into the

pin should be limited by an external resistor to less than

10mA.

Inputs

While the LT1494/LT1495/LT1496 will function normally

with its inputs taken above the positive supply, the com-

mon mode range does not extend beyond approximately

300mV below the negative supply at room temperature.

The device will not be damaged if the inputs are taken lower

than 300mV below the negative supply as long as the cur-

rent out of the pin is limited to less than 10mA. However,

the output phase is not guaranteed and the supply current

will increase.

Output

The graph, Capacitive Load Handling, shows amplifier sta-

bility with the output biased at half supply. If the output is

to be operated within about 100mV of the positive rail, the

allowable load capacitance is less. With this output volt-

the load capacitance should be less than 500pF with a 5V

supply and less than 100pF with a 30V supply.

Rail-to-Rail Operation

The simplified schematic, Figure 3, details the circuit

design approach of the LT1494/LT1495/LT1496. The

amplifier topology is a three-stage design consisting of a

rail-to-rail input stage, that continues to operate with the

inputs above the positive rail, a folded cascode second

stage that develops most of the voltage gain, and a rail-to-

rail common emitter stage that provides the current gain.

Figure 1. Start-Up Characteristics

1495 F02

OUTPUT HIGH

–

+

–

+

OUTPUT LOW

–

+

Figure 2. Circuits for Start-Up Characteristics

SUPPLY VOLTAGE (V)

0

5

4

3

2

1

5

1495 F01

01234

OUTPUT HIGH

OUTPUT LOW