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LMC7101AIM5 Datasheet(PDF) 9 Page - Micrel Semiconductor |
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LMC7101AIM5 Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 12 page September 1999 9 LMC7101 LMC7101 Micrel Application Information Input Common-Mode Voltage Some amplifiers exhibit undesirable or unpredictable perfor- mance when the inputs are driven beyond the common-mode voltage range, for example, phase inversion of the output signal. The LMC7101 tolerates input overdrive by at least 200mV beyond either rail without producing phase inversion. If the absolute maximum input voltage (700mV beyond either rail) is exceeded, the input current should be limited to ±5mA maximum to prevent reducing reliability. A 10k Ω series input resistor, used as a current limiter, will protect the input structure from voltages as large as 50V above the supply or below ground. See Figure 1. VIN VOUT 10k Ω RIN Figure 1. Input Current-Limit Protection Output Voltage Swing Sink and source output resistances of the LMC7101 are equal. Maximum output voltage swing is determined by the load and the approximate output resistance. The output resistance is: R V I OUT DROP LOAD = V DROP is the voltage dropped within the amplifier output stage. V DROP and ILOAD can be determined from the VO (output swing) portion of the appropriate Electrical Character- istics table. I LOAD is equal to the typical output high voltage minus V+/2 and divided by R LOAD. For example, using the Electrical Characteristics DC (5V) table, the typical output high voltage using a 2k Ω load (connected to V+/2) is 4.989V, which produces an I LOAD of 1 245 4 989 2 5 2 1 245 . .. . mA V – V k mA Ω = . Voltage drop in the amplifier output stage is: V DROP = 5.0V – 4.989V V DROP = 0.011V Because of output stage symmetry, the corresponding typical output low voltage (0.011V) also equals V DROP. Then: R V A 9 OUT == ≈ Ω 0 011 0 001245 88 . . . Driving Capacitive Loads Driving a capacitive load introduces phase-lag into the output signal, and this in turn reduces op-amp system phase margin. The application that is least forgiving of reduced phase margin is a unity gain amplifier. The LMC7101 can typically drive a 100pF capacitive load connected directly to the output when configured as a unity-gain amplifier. Using Large-Value Feedback Resistors A large-value feedback resistor (> 500k Ω) can reduce the phase margin of a system. This occurs when the feedback resistor acts in conjunction with input capacitance to create phase lag in the fedback signal. Input capacitance is usually a combination of input circuit components and other parasitic capacitance, such as amplifier input capacitance and stray printed circuit board capacitance. Figure 2 illustrates a method of compensating phase lag caused by using a large-value feedback resistor. Feedback capacitor C FB introduces sufficient phase lead to overcome the phase lag caused by feedback resistor R FB and input capacitance C IN. The value of CFB is determined by first estimating C IN and then applying the following formula: R C R C IN IN FB FB ×≤ × VIN CFB RFB VOUT CIN RIN Figure 2. Cancelling Feedback Phase Lag Since a significant percentage of C IN may be caused by board layout, it is important to note that the correct value of C FB may change when changing from a breadboard to the final circuit layout. |
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