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LT1671 Datasheet(PDF) 8 Page - Linear Technology |
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LT1671 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LT1671 APPLICATIONS INFORMATION circuit is the lack of feedthrough from the generator to the comparator input. This prevents overshoot on the com- parator input, which would give a false fast reading on comparator response time. To adjust the circuit for exactly 5mV overdrive, V1 is adjusted so that the LT1671 output under test settles to 1.4V (in the linear region). Then V1 is changed by – 1V to set overdrive to 5mV. High Speed Design Techniques A substantial amount of design effort has made the LT1671 relatively easy to use. It is much less prone to oscillation than some slower comparators, even with slow input signals. However, as with any high speed comparator, there are a number of problems which may arise because of PC board layout and design. The most common prob- lem involves power supply bypassing. Bypassing is nec- essary to maintain low supply impedance. DC resistance and inductance in supply wires and PC traces can quickly build up to unacceptable levels. This allows the supply line to move with changing internal current levels of the connected devices. This will almost always result in improper operation. In addition, adjacent devices con- nected through an unbypassed supply can interact with each other through the finite supply impedances. Bypass capacitors furnish a simple solution to this problem by providing a local reservoir of energy at the device, keeping supply impedances low. Bypass capacitors should be as close as possible to the LT1671. A good high frequency capacitor such as a 0.1 µF ceramic is recommended, in parallel with a larger capaci- tor such as a 4.7 µF tantalum. Poor trace routes and high source impedances are also common sources of problems. Be sure to keep trace lengths as short as possible, and avoid running any output trace adjacent to an input trace to prevent unnecessary coupling. If output traces are longer than a few inches, be sure to terminate them with a resistor to eliminate any reflections that may occur. Resistor values are typically 250 Ω to 400Ω. Also, be sure to keep source impedances as low as possible, preferably 1k Ω or less. About Level Shifts The LT1671’s logic output will interface with many cir- cuits directly. Many applications, however, require some form of level shifting of the output swing. With LT1671- based circuits this is not trivial because it is desirable to maintain very low delay in the level shifting stage. When designing level shifters, keep in mind that the TTL output of the LT1671 is a sink-source pair (Figure 3) with good ability to drive capacitance (such as feedforward capaci- tors). Figure 4 shows a noninverting voltage gain stage with a 15V output. When the LT1671 switches, the base- emitter voltages at the 2N2369 reverse, causing it to switch very quickly. The 2N3866 emitter-follower gives a low impedance output and the Schottky diode aids cur- rent sink capability. – + LT1671 1671 F02 FET PROBE FET PROBE * TOTAL LEAD LENGTH INCLUDING DEVICE PIN. SOCKET AND CAPACITOR LEADS SHOULD BE LESS THAN 0.5 IN. USE GROUND PLANE ** (VOS + OVERDRIVE)/200 25 Ω 25 Ω 5V 0.01 µF* 0.01 µF 10k 50 Ω V1** 2N3866 0V –3V –5V –5V 50 Ω PULSE IN 750 Ω 400 Ω 0.1 µF 130 Ω 0V –100mV Q Q Figure 2. Response Time Test Circuit |
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Similar Description - LT1671_15 |
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