8

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 LT1495/LT1496 are protected against reverse battery

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

tion occurs, the supply current is typically less than 100nA

(inputs grounded and outputs open). For typical single sup-

ply 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 LT1495/LT1496 will function normally with its

inputs taken above the positive supply, the common mode

range does not extend beyond approximately 300mV be-

low 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 current

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 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 com-

mon emitter stage that provides the current gain.

SUPPLY VOLTAGE (V)

0

5

4

3

2

1

5

1495 F01

01234

OUTPUT HIGH

OUTPUT LOW

Figure 1. Start-Up Characteristics

1495 F02

OUTPUT HIGH

–

+

–

+

OUTPUT LOW

–

+

Figure 2. Circuits for Start-Up Characteristics