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LMC7225IMX Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LMC7225IMX Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 11 page Typical Performance Characteristics T A= 25˚C unless otherwise specified (Continued) Application Information RESPONSE TIME Depending upon the amount of overdrive, the delay will typi- cally be between 10 µs to 200 µs. The curve showing delay vs overdrive in the “Typical Characteristics” section shows the delay time when the input is preset with 100 mV across the inputs and then is driven the other way by 1 mV to 500 mV. The transition from high to low or low to high is fast. Typically 1 µs rise and 400 ns fall. With a small signal input, the comparators will provide a square wave output from sine wave inputs at frequencies as high as 25 kHz. Figure 1 shows a worst case example where a ±5 mV sine wave is applied to the input. Note that the out- put is delayed by almost 180˚. NOISE Most comparators have rather low gain. This allows the out- put to spend time between high and low when the input sig- nal changes slowly. The result is the output may oscillate be- tween high and low when the differential input is near zero. The exceptionally high gain of these comparators, 10,000V/ mV, eliminates this problem. Less then 1 µV of change on the input will drive the output from one rail to the other rail. If the input signal is noisy, the output cannot ignore the noise unless some hysteresis is provided by positive feedback. INPUT VOLTAGE RANGE The LMC7215/25 have input voltage ranges that are larger than the supply voltage guarantees that signals from other parts of the system cannot overdrive the inputs. This allows sensing supply current by connecting one input directly to the V + line and the other to the other side of a current sense resistor. The same is true if the sense resistor is in the ground return line. Sensing supply voltage is also easy by connecting one input directly to the supply. The inputs of these comparators are protected by diodes to both supplies. This protects the inputs from both ESD as well as signals that greatly exceed the supply voltages. As a re- sult, current will flow through these forward biased diodes whenever the input voltage is more than a few hundred mil- livolts larger than the supplies. Until this occurs, there is es- sentially no input current. As a result, placing a large resistor in series with any input that may be exposed to large volt- ages, will limit the input current but have no other noticeable effect. If the input current is limited to less than 5 mA by a series re- sistor, (see Figure 2), a threshold or zero crossing detector, that works with inputs from as low as a few millivolts to as high as 5,000V, is made with only one resistor and the com- parator. INPUTS As mentioned above, these comparators have near zero in- put current. This allows very high resistance circuits to be used without any concern for matching input resistances. Output Leakage Current vs Output Voltage LMC7225 DS012853-20 DS012853-4 FIGURE 1. DS012853-5 FIGURE 2. www.national.com 6 |
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