Datenblatt-Suchmaschine für elektronische Bauteile |
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ISL59837IAZ Datenblatt(PDF) 11 Page - Intersil Corporation |
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ISL59837IAZ Datenblatt(HTML) 11 Page - Intersil Corporation |
11 / 14 page 11 FN6335.1 March 5, 2007 Description of Operation and Application Information Theory Of Operation The ISL59837 is a highly practical and robust marriage of three high bandwidth, high speed, low power, rail-to-rail voltage feedback amplifiers with a charge pump to provide a negative rail without an additional power supply. Designed to operate with a single supply voltage range from 0V to 3.3V, the ISL59837 eliminates the need for a split supply with the incorporation of a charge pump capable of generating a bottom rail as much as 1.6V below ground for a 4.9V range on a single 3.3V supply. This performance is ideal for NTSC video with its negative-going sync pulses. THE AMPLIFIER The ISL59837 fabricated on a di-electrically isolated high speed 5V Bi-CMOS process with 4GHz PNPs and NPN transistor exceeding 20GHz - perfect for low distortion, low power demand and high frequency circuits. While the ISL59837 utilizes somewhat standard voltage mode feedback topologies, there are many non-standard analog features providing its outstanding bandwidth, rail-to-rail operation, and output drive capabilities. The input signal initially passes through a folded cascode, a topology providing enhanced frequency response by essentially fixing the base collector voltage at the junction of the input and gain stage. The collector of each input device looks directly into an emitter that is tied closely to ground through a resistor and biased with a very stable DC source. Since the voltage of this collector is "locked stable," the effective bandwidth limiting of the Miller capacitance is greatly reduced. The signal is then passed through a second fully- realized differential gain stage and finally through a proprietary common emitter output stage for improved rail-to-rail output performance. The result is a highly-stable, low distortion, low power, and high frequency amplifier capable of driving moderately capacitive loads with near rail-to-rail performance. INPUT OUTPUT RANGE The three amplifier channels have an input common mode voltage range from 0.15V below the bottom rail to within 100mV of the positive supply, VS+ pin (Note: bottom rail is established by the charge pump at negative one half the positive supply). As the input signal moves outside the specified range, the output signal will exhibit increasingly higher levels of harmonic distortion. And of course, as load resistance becomes lower, the current drive capability of the device will be challenged and its ability to drive close to each rail is reduced. For instance, with a load resistance of 1k Ω the output swing is within 100mV of the rails, while a load resistance of 150 Ω limits the output swing to within around 300mV of the rails. AMPLIFIER OUTPUT IMPEDANCE To achieve near rail-to-rail performance, the output stage of the ISL59837 uses transistors in the common emitter configuration, typically producing higher output impedance than the standard emitter follower output stage. The exceptionally high open loop gain of the ISL59837 and local feedback reduces output impedance to less than 2 Ω at low frequency. However, since output impedance of the device is exponentially modulated by the magnitude of the open loop gain, output impedance increases with frequency as the open loop gain decreases with frequency. This inductive-like effect of the output impedance is countered in the ISL59837 with proprietary output stage topology, keeping the output impedance low over a wide frequency range and making it possible to easily and effectively drive relatively heavy capacitive loads (see Figure 10). IN+ IN- BIAS OUT FIGURE 27. SIMPLIFIED SCHEMATIC ISL59837 |
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