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LT1721IGN Datasheet(PDF) 13 Page - Linear Technology |
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LT1721IGN Datasheet(HTML) 13 Page - Linear Technology |
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13 / 28 page  LT1720/LT1721 13 17201fc Circuit Description The block diagram of one comparator in the LT1720/LT1721 is shown in Figure 6. There are differential inputs (+IN/–IN), an output (OUT), a single positive supply (VCC) and ground (GND). All comparators are completely independent, shar- ing only the power and ground pins. The circuit topology consists of a differential input stage, a gain stage with hysteresis and a complementary common-emitter output stage. All of the internal signal paths utilize low voltage swings for high speed at low power. The input stage topology maximizes the input dynamic range available without requiring the power, complex- ity and die area of two complete input stages such as are found in rail-to-rail input comparators. With a 2.7V supply, the LT1720/LT1721 still have a respectable 1.6V of input common mode range. The differential input volt- age range is rail-to-rail, without the large input currents found in competing devices. The input stage also features phase reversal protection to prevent false outputs when the inputs are driven below the –100mV common mode voltage limit. The internal hysteresis is implemented by positive, nonlin- ear feedback around a second gain stage. Until this point, the signal path has been entirely differential. The signal path is then split into two drive signals for the upper and lower output transistors. The output transistors are con- nected common emitter for rail-to-rail output operation. The Schottky clamps limit the output voltages at about 300mV from the rail, not quite the 50mV or 15mV of Linear APPLICATIONS INFORMATION Technology’s rail-to-rail amplifiers and other products. But the output of a comparator is digital, and this output stage can drive TTL or CMOS directly. It can also drive ECL, as described earlier, or analog loads as demonstrated in the applications to follow. The bias conditions and signal swings in the output stages are designed to turn their respective output transistors off faster than on. This nearly eliminates the surge of current from VCC to ground that occurs at transitions, keeping the power consumption low even with high output-toggle frequencies. The low surge current is what keeps the power consump- tion low at high output-toggle frequencies. The frequency dependence of the supply current is shown in the Typical Performance Characteristics. Just 20pF of capacitive load on the output more than triples the frequency dependent rise. The slope of the no-load curve is just 32μA/MHz. With a 5V supply, this current is the equivalent of charging and discharging just 6.5pF. The slope of the 20pF load curve is 133μA/MHz, an addition of 101μA/MHz, or 20μA/MHz-V, units that are equivalent to picoFarads. The LT1720/LT1721 dynamic current can be estimated by adding the external capacitive loading to an internal equivalent capacitance of 5pF to 15pF, multiplied by the toggle frequency and the supply voltage. Because the capacitance of routing traces can easily approach these values, the dynamic current is dominated by the load in most circuits. Figure 6. LT1720/LT1721 Block Diagram – + – + – + – + +IN –IN AV1 AV2 NONLINEAR STAGE OUT GND 17201 F06 VCC + + |
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