Micrel, Inc. MIC7111 Then, Application Information Input Common Mode Voltage The MIC7111 tolerates input overdrive by at least 300mV beyond either rail without producing phase inversion. R If the absolute maximum input voltage is exceeded, the input current should be limited to ±5mA maximum to prevent reducing reliability. A 10kΩ 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. OUT 1.243mA = 12.1 = 12Ω Eq. 3 Figure 2 illustrates a method of compensating phase lag caused by using a large-value feedback resistor. Feedback capacitor CFB introduces sufficient phase lead to overcome the phase lag caused by feedback resistor RFB and input capacitance CIN. The value of CFB is determined by first estimating CIN and then applying the following formula: V DROP 15mV Using Large-Value Feedback Resistors A large-value feedback resistor (> 500kΩ) 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 feedback 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. Output Voltage Swing Sink and source output resistances of the MIC7111 are equal. Maximum output voltage swing is determined by the load and the approximate output resistance. The output resistance is presented in Equation 1: = = 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 MIC7111 can typically drive a 500pF capacitive load connected directly to the output when configured as a unity-gain amplifier. Figure 1. Input Current-Limit Protection R OUT Eq. 1 I RIN × CIN ≤ RFB × CFB LOAD Eq. 4 VDROP is the voltage dropped within the amplifier output stage. VDROP and ILOAD can be determined from the VO (output swing) portion of the appropriate electrical characteristics table. ILOAD is equal to the typical output high voltage minus V+/2 and divided by RLOAD. For example, using the DC Electrical Characteristics (5V) table, the typical output voltage drop using a 2kΩ load (connected to V+/2) is 0.015V, which produces an ILOAD of: 2.5V − 0.015V 2kΩ February 11, 2013 = 1.243mA Figure 2. Cancelling Feedback Phase Lag Eq. 2 9 Revision 2.0