LT1495 Linear Technology, LT1495 Datasheet - Page 8

• Manufacturer Part Number: LT1495
• Description: 1.5uA Max/ Dual and Quad Precision Rail-to-Rail Input and Output Op Amps
• Manufacturer: Linear Technology

LT1495/LT1496
APPLICATIONS
8
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.
+
–
OUTPUT HIGH
V
S
Figure 2. Circuits for Start-Up Characteristics
5
4
3
2
1
0
0
Figure 1. Start-Up Characteristics
1
U
SUPPLY VOLTAGE (V)
+
–
OUTPUT LOW
INFORMATION
2
V
U
S
OUTPUT HIGH
3
OUTPUT LOW
OUTPUT V
W
V
4
S
S
/2
/2
1495 F01
OUTPUT AT V
–
+
5
V
S
U
S
/2
1495 F02
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-
age, the worst case occurs at A
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.
V
= 1 and light loads, where