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LT1721IS Datasheet(PDF) 9 Page - Linear Technology |
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LT1721IS Datasheet(HTML) 9 Page - Linear Technology |
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9 / 28 page  LT1720/LT1721 9 17201fc High Speed Design Considerations Application of high speed comparators is often plagued by oscillations. The LT1720/LT1721 have 4mV of internal hysteresis, which will prevent oscillations as long as parasitic output to input feedback is kept below 4mV. However, with the 2V/ns slew rate of the LT1720/LT1721 outputs, a 4mV step can be created at a 100Ω input source with only 0.02pF of output to input coupling. The pinouts of the LT1720/LT1721 have been arranged to minimize problems by placing the most sensitive inputs (invert- ing) away from the outputs, shielded by the power rails. The input and output traces of the circuit board should also be separated, and the requisite level of isolation is readily achieved if a topside ground plane runs between the outputs and the inputs. For multilayer boards where the ground plane is internal, a topside ground or supply trace should be run between the inputs and outputs, as illustrated in Figure 1. APPLICATIONS INFORMATION Although both VCC pins are electrically shorted internal to the LT1721, they must be shorted together externally as well in order for both to function as shields. The same is true for the two GND pins. The supply bypass should include an adjacent 10nF ce- ramic capacitor and a 2.2μF tantalum capacitor no farther than 5cm away; use more capacitance if driving more than 4mA loads. To prevent oscillations, it is helpful to balance the impedance at the inverting and noninverting inputs; source impedances should be kept low, preferably 1kΩ or less. The outputs of the LT1720/LT1721 are capable of very high slew rates. To prevent overshoot, ringing and other problems with transmission line effects, keep the output traces shorter than 10cm, or be sure to terminate the lines to maintain signal integrity. The LT1720/LT1721 can drive DC terminations of 250Ω or more, but lower characteristic impedance traces can be driven with series termination or AC termination topologies. Hysteresis The LT1720/LT1721 include internal hysteresis, which makes them easier to use than many other comparable speed comparators. The input-output transfer characteristic is illustrated in Figure 2 showing the definitions of VOS and VHYST based upon the two measurable trip points. The hysteresis band makes the LT1720/LT1721 well behaved, even with slowly moving inputs. Figure 1. Typical Topside Metal for Multilayer PCB Layouts 17201 F01 (b) (a) Figure 1a shows a typical topside layout of the LT1720 on such a multilayer board. Shown is the topside metal etch including traces, pin escape vias, and the land pads for an SO-8 LT1720 and its adjacent X7R 10nF bypass capacitor in a 1206 case. The ground trace from Pin 5 runs under the device up to the bypass capacitor, shielding the inputs from the out- puts. Note the use of a common via for the LT1720 and the bypass capacitor, which minimizes interference from high frequency energy running around the ground plane or power distribution traces. Figure 1b shows a typical topside layout of the LT1721 on a multilayer board. In this case, the power and ground traces have been extended to the bottom of the device solely to act as high frequency shields between input and output traces. Figure 2. Hysteresis I/O Characteristics VHYST (= VTRIP + – VTRIP–) VHYST/2 VOL 17201 F02 VOH VTRIP – VTRIP + VIN = VIN + – VIN– VTRIP + + VTRIP– 2 VOS = 0 |
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