10
LT1494/LT1495/LT1496
APPLICATIONS INFORMATION
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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-
age, the worst case occurs at A
V
= 1 and light loads, where
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
V
S
–
+
OUTPUT AT V
S
/2
V
S
–
+
OUTPUT LOW
V
S
–
+
V
S
/2
Figure 2. Circuits for Start-Up Characteristics
SUPPLY VOLTAGE (V)
0
5
4
3
2
1
SUPPLY CURRENT PER AMPLIFIER (µA)
5
1495 F01
01234
OUTPUT HIGH
OUTPUT LOW
OUTPUT V
S
/2
