Datasheet LT1494, LT1495, LT1496 (Analog Devices) - 10

LT1494/LT1495/LT1496
U U W U APPLICATIONS INFORMATION Start-Up Characteristics Reverse Battery
Micropower op amps are sometimes not micropower The LT1494/LT1495/LT1496 are protected against reverse during start-up, wreaking havoc on low current supplies. battery voltages up to 18V. In the event a reverse battery In the worst case, there may not be enough supply current condition occurs, the supply current is typically less than available to take the system up to nominal voltages. Figure 100nA (inputs grounded and outputs open). For typical 1 is a graph of LT1495 supply current vs supply voltage single supply applications with ground referred loads and for the three limit cases of input offset that could occur feedback networks, no other precautions are required. If during start-up. The circuits are shown in Figure 2. One the reverse battery condition results in a negative voltage circuit creates a positive offset, forcing the output to come at either the input pins or output pin, the current into the up saturated high. Another circuit creates a negative pin should be limited by an external resistor to less than offset, forcing the output to come up saturated low, while 10mA. the last brings up the output at half supply. In all cases, the supply current is well behaved. Supply current is highest
Inputs
with the output forced high, so if one amplifier is unused, While the LT1494/LT1495/LT1496 will function normally it is best to force the output low or at half supply. with its inputs taken above the positive supply, the com- mon mode range does not extend beyond approximately 5 300mV below the negative supply at room temperature. µA) The device will not be damaged if the inputs are taken lower 4 than 300mV below the negative supply as long as the cur- OUTPUT HIGH rent out of the pin is limited to less than 10mA. However, 3 the output phase is not guaranteed and the supply current will increase. 2 OUTPUT LOW
Output
1 OUTPUT VS/2 SUPPLY CURRENT PER AMPLIFIER ( The graph, Capacitive Load Handling, shows amplifier sta- 0 bility with the output biased at half supply. If the output is 0 1 2 3 4 5 SUPPLY VOLTAGE (V) to be operated within about 100mV of the positive rail, the 1495 F01 allowable load capacitance is less. With this output volt-
Figure 1. Start-Up Characteristics
age, the worst case occurs at AV = 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. VS V V S S
Rail-to-Rail Operation
+ + VS/2 + 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 1495 F02 rail-to-rail input stage, that continues to operate with the inputs above the positive rail, a folded cascode second OUTPUT HIGH OUTPUT LOW OUTPUT AT VS/2 stage that develops most of the voltage gain, and a rail-to-
Figure 2. Circuits for Start-Up Characteristics
rail common emitter stage that provides the current gain. 10