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LT1011MJ8 Datasheet(PDF) 7 Page - Linear Technology |
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LT1011MJ8 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  7 LT1011/LT1011A of the comparator, but require a minimum input signal slew rate to be effective. DC hysteresis works for all input slew rates, but creates a shift in offset voltage dependent on the previous condition of the input sig- nal. The circuit shown in Figure 1 is an excellent compromise between AC and DC hysteresis. APPLICATIONS INFORMATION + 3 7 1 5 6 8 LT1011 INPUTS 2 µF TANT 4 2 –15V C1 0.003 µF 0.1 µF 1011 F01 R2 15M 15V RL OUTPUT Figure 1. Comparator with Hysteresis This circuit is especially useful for general purpose comparator applications because it does not force any signals directly back onto the input signal source. Instead, it takes advantage of the unique properties of the BALANCE pins to provide extremely fast, clean output switching even with low frequency input sig- nals in the millivolt range. The 0.003 µF capacitor from Pin 6 to Pin 8 generates AC hysteresis because the voltage on the BALANCE pins shifts slightly, depend- ing on the state of the output. Both pins move about 4mV. If one pin (6) is bypassed, AC hysteresis is created. It is only a few millivolts referred to the in- puts, but is sufficient to switch the output at nearly the maximum speed of which the comparator is capable. To prevent problems from low values of input slew rate, a slight amount of DC hysteresis is also used. The sensitivity of the BALANCE pins to current is about 0.5mV input referred offset for each microampere of BALANCE pin current. The 15M resistor tied from OUTPUT to Pin 5 generates 0.5mV DC hysteresis. The combination of AC and DC hysteresis creates clean oscillation-free switching with very small input errors. Figure 2 plots input referred error versus switching frequency for the circuit as shown. Note that at low frequencies, the error is simply the DC hysteresis, while at high frequencies, an additional TIME/FREQUENCY ( µs) 1 2 3 4 5 6 10 100 1000 1011 F02 1 0 –1 –2 7 8 C8 TO C6 = 0.003 µF (50kHz) (5kHz) OUTPUT “HI” TO “LO” OUTPUT “LO” TO “HI” Figure 2. Input Offset Voltage vs Time to Last Transition error is created by the AC hysteresis. The high frequency error can be reduced by reducing CH, but lower values may not provide clean switching with very low slew rate input signals. Input Protection The inputs to the LT1011 are particularly suited to general purpose comparator applications because large differen- tial and/or common mode voltages can be tolerated with- out damage to the comparator. Either or both inputs can be raised 40V above the negative supply, independent of the positive supply voltage. Internal forward biased diodes will conduct when the inputs are taken below the negative supply. In this condition, input current must be limited to 1mA. If very large (fault) input voltages must be accom- modated, series resistors and clamp diodes should be used (see Figure 3). – + LT1011 R4* 300 Ω R3* 300 Ω 3 2 INPUTS D1 TO D4: 1N4148 MAY BE ELIMINATED FOR IFAULT ≤ 1mA SELECT ACCORDING TO ALLOWABLE FAULT CURRENT AND POWER DISSIPATION * ** R1** V + V – R2** 8 D2 4 1011 F03 D4 D1 D3 Figure 3. Limiting Fault Input Currents |
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