8

LT1672/LT1673/LT1674

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 LT1673 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.

APPLICATIO S I FOR ATIO

TOTAL SUPPLY VOLTAGE (V)

0

5

4

3

2

1

0

4

1672/3/4 FO1

1

2

3

5

OUTPUT HIGH

OUTPUT LOW

Figure 1. Start-Up Characteristics

16/2/3/4 F02

OUTPUT HIGH

–

+

–

+

OUTPUT LOW

–

+

820k

200k

Figure 2. Circuits for Start-Up Characteristics

Reverse Battery

The LT1672/LT1673/LT1674 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 LT1672/LT1673/LT1674 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.

Small-Signal Response

Small-Signal Response

Large-Signal Response

1672/3/4 G19

1672/3/4 G20

1672/3/4 G21