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MAX366MJA Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX366MJA Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 12 page Signal-Line Circuit Protectors _______________________________________________________________________________________ 7 The current through each protector should never exceed 30mA. Always calculate the power dissipated by all the protectors in worst-case conditions (maximum voltage and current through each protector) to ensure the pack- age dissipation limit is not reached. With single-supply operation, grounded loads will have zero voltage (and current) whenever the input voltage is below approximately 2V. In effect, both the IN and OUT pins are in fault condition. A special case arises when power is off: The part is in a perpetual fault condition but no fault current flows because all the internal FETs are off. Single-Supply Output Operation Single-supply operation is a special case. Signals can- not go to ground, since from 0V to approximately +2V is a fault condition. Extremely Low-Current Operation Figure 3 shows the typical high-impedance transfer characteristics with a 100M Ω load. Compared to the transfer characteristic at 1M Ω (see Typical Operating Characteristics), the two knees are closer to the supply voltages and the slopes of the flat portions of the curve (fault conditions) are steeper. As the load resistance is increased even further, the positive and negative knees increase, and the slopes in fault conditions increase even more. Eventually, at some extremely high output resistance (e.g., Tera ohms), the output voltage can exceed the supply voltage during fault conditions. This is due to extremely low leakage currents from the input to output. When the protector’s output side is connected to very high-resistance, very low-current loads (such as op- amp inputs), a small leakage current flows from the input to the output during fault conditions. This current is typically below a nano-ampere (<10-9A) but, if the output resistance is high enough, it can cause the out- put voltage to exceed the supply voltages during fault conditions. This condition can be self-correcting, however, if the high-resistance load has protection diodes to the sup- ply rails (either external or internal to the op amp). These diodes conduct the leakage current to the supply rails and safely limit the output voltage. An alternative is to add a high-value resistor to ground in parallel with the load. This resistor may be as low as 1000M Ω; its value must be determined experimentally at the highest anticipated operational temperature. The fault protectors will not normally be used with high- impedance FET-input amplifiers that lack input protection diodes. Such amplifiers are fragile and are normally reserved for use when ultra-low leakage (pA) is needed. The MAX366/MAX367 have nano-amperes of leakage, which would negate the low leakage of the unprotected amplifier. Low-Voltage Operation The MAX366/MAX367 “operate” with supply voltages all the way down to 0V, but what they do to the signal is not obvious. With a total supply voltage of 3.5V, the protector is in a fault condition with nearly any input that is not close to 2.0V. Below 3.5V (including power off), the protector is perpetually in a fault condition (i.e., high impedance). When the supply voltage(s) ramps up (and/or down) from zero, the signal path is initially in a fault condition (open), until the supply voltage passes the input volt- age. The output starts at zero and is delayed from reaching the input voltage as the part comes out of the fault condition. If the supply voltage exceeds about 3.5V, but never exceeds the input voltage, the output will follow the supply, always remaining about 1.3V below the positive supply voltage or 2V above the neg- ative supply voltage. If the input voltage subsequently comes out of the fault condition, the output returns to the input value. This set of conditions is exactly reversed when power ramps down to zero. Since the input and output pins are identical and inter- changeable, predicting whether or not the part is in a fault condition is easy: If either IN or OUT exceeds V+ or V-, a fault condition exists and the current that flows will be just enough to cause the other signal pin (OUT or IN) to approach the appropriate supply rail. 0 1 2 3 4 5 -1 -2 -3 -4 -30 30 INPUT VOLTAGE (V) 0 -20 -10 20 10 V+ = +5V V- = -5V ROUT = 100MΩ Figure 3. High-Impedance Transfer Characteristic |
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