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LT1715 Datasheet(PDF) 10 Page - Linear Technology |
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LT1715 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LT1715 LT1715 is the significant reduction in these effects, which is important whenever an LT1715 is used to detect a threshold crossing in one direction only. In such a case, the relevant trip point will be all that matters, and a stable offset voltage with an unpredictable level of hysteresis, as seen in competing comparators, is useless. The LT1715 is many times better than prior generation comparators in these regards. In fact, the CMRR and PSRR tests are performed by checking for changes in either trip point to the limits indicated in the specifications table. Because the offset voltage is the average of the trip points, the CMRR and PSRR of the offset voltage is therefore guaranteed to be at least as good as those limits. This more stringent test also puts a limit on the common mode and power supply dependence of the hysteresis voltage. Additional hysteresis may be added externally. The rail-to- rail outputs of the LT1715 make this more predictable than with TTL output comparators due to the LT1715’s small variability of VOH (output high voltage). To add additional hysteresis, set up positive feedback by adding additional external resistor R3 as shown in Fig- ure 6. Resistor R3 adds a portion of the output to the threshold set by the resistor string. The LT1715 pulls the outputs to + VS and ground to within 200mV of the rails with light loads, and to within 400mV with heavy loads. For the load of most circuits, a good model for the voltage on the right side of R3 is 300mV or +VS – 300mV, for a total voltage swing of (+VS–300mV)–(300mV)=+VS–600mV. At frequencies well beyond 100MHz, the toggling of one channel may be impaired by toggling on the other. This is a rather complex interaction of supply bypassing and bond inductance, and it cannot be entirely prevented. However, good bypassing and board layout techniques will effec- tively minimize it. Power Supply Sequencing The LT1715 is designed to tolerate any power supply sequencing at system turn-on and power down. In any of the previously shown power supply configurations, the various supplies can activate in any order without exces- sive current drain by the LT1715. As always, the Absolute Maximum Ratings must not be exceeded, either on the power supply terminals or the input terminals. Power supply sequencing problems can occur when input signals are powered from supplies that are independent of the LT1715’s supplies. No problems should occur if the input signals are powered from the same VCC and VEE supplies as the LT1715. Unused Comparators If a comparator is unused, its output should be left floating to minimize load current. The unused inputs can be tied off to the rails and power consumption can be further mini- mized if the inputs are connected to the power rails to induce an output low. Connecting the inverting input to VCC and the noninverting input to VEE will likely be the easiest method. Hysteresis The LT1715 includes internal hysteresis, which makes it easier to use than many other similar speed comparators. The input-output transfer characteristic is illustrated in Figure 5 showing the definitions of VOS and VHYST based upon the two measurable trip points. The hysteresis band makes the LT1715 well behaved, even with slowly moving inputs. The exact amount of hysteresis will vary from part to part as indicated in the specifications table. The hysteresis level will also vary slightly with changes in supply voltage and common mode voltage. A key advantage of the APPLICATIO S I FOR ATIO Figure 5. Hysteresis I/O Characteristics VHYST (= VTRIP + – V TRIP –) VHYST/2 VOL 1715 F05 VOH VTRIP – VTRIP + ∆VIN = VIN+ – VIN– VTRIP + + V TRIP – 2 VOS = 0 |
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