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EL2021 Datasheet(PDF) 6 Page - Intersil Corporation |
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EL2021 Datasheet(HTML) 6 Page - Intersil Corporation |
6 / 8 page ![]() 6 This circuit allows the external transistors to run from B+ and B- supplies that are of less voltage than V+ and V- to conserve power. Reducing B± supplies also reduces dissipations in the output devices themselves. B+ is typically made K volts more than VCH and B- made K volts more negative than VCL. Ideally K is made as small as possible to minimize output transistor dissipation, but two factors limit how small K can be. These factors are both related to the fact that transistors have two collector resistance numbers: “hard” and “soft” saturation resistance. As a transistor begins to saturate at high collector currents and small collector- emitter voltages, minority carriers begin to be generated from the base-collector junction. These carriers act as more collector dopant and actually reduce effective series collector resistance. At conditions of heavy saturation, the collector is flooded with minority carriers and exhibits minimum collector resistance. In this way, small geometry transistors like the 2N2222 and 2N2907 devices have excellent collector-emitter voltage drops at high currents, but are actually still in heavy saturation for 1V-2V drops. This “soft” saturation shows up as reduced beta at high currents and moderate VCE's as well as very poor AC performance. A transistor may exhibit an ft of only 2MHz in soft saturation when, like the 2N2222, it gives 300MHz in non-saturated mode. The EL2021 requires the output transistors to have an ft of at least 200MHz to prevent degradation in overshoot, slew rate into heavy loads, and tolerance of heavy output capacitance. With a K of 3.2V and 1 Ω collector resistors, almost all 2N2222 and 2N2907 devices perform well, but we have obtained devices from some vendors where the beta does indeed fall prematurely at reduced VCE and high currents. It is important to characterize the external devices for the service that the EL2021 will be expected to provide. The output stage of the EL2021 does not ring appreciably into a capacitive load in quiescent conditions, but it does ring while it slews. This is an unusual characteristic, but the output slews monotonically and the slew “ripple” does not cause problems in use. The slew ripple does cause a similar “ripple” in the overshoot-vs-VSR characteristic: the overshoot may decrease for slightly increasing VSR, then increase again for larger VSR's again. The overshoot-vs- VSR graphs presented in this data sheet thus reflect the range of overshoot rather than one particular device's wavy curve. The typical 2N2222 and 2N2907 will deliver 750mA into a short-circuit. This puts four watts of dissipation into the 2N2222 for VCH = 5V. The npn can dissipate this power for a few tenths of a second as long as a metal-base TO-39 package is used. The small or non-metal-based packages have short thermal time constants and high thermal resistances, so they should withstand shorts for only a few milliseconds. The Sense Out signal should be used to control OE or reduce VCH and VCL to relieve the output devices from overcurrent conditions. Transistors such as the MJE200 and MJE210 have very much improved collector resistances and high-current beta compared to the 2N2222 and 2N2907. Their fT's are almost as good and sustain at higher currents, and high-current output accuracy will improve. They allow a K of 2V to reduce dissipations further, but short-circuit currents will be as much as two amperes! The geometries of these transistors are larger, and the added transistor capacitances will slow the maximum Slew Rates that the EL2021 can provide. If transistors with ft's less than 200MHz are used, the EL2021 will need to be overcompensated. This is accomplished by connecting equal capacitors from the Drive pins to ground. These capacitors will range from 10pF to 50pF. The overcompensation will slow the maximum slew rate, but it will improve the overshoot and reactive load driving capability, and can be considered a useful technique. Figure 2 shows the equivalent output stage schematic when the circuit is in high-impedance mode (OE = H). The external transistors have their base-emitter junctions each reverse- biased by a Schottky diode drop. A buffer amplifier copies the output voltage to give a bootstrapped bias for the Schottky stack. This scheme guarantees that the external transistors will be off for any output level, and the output leakage current is simply the bias current of the buffer. The circuit works properly for AC signals up to 500V/µs. Above this slew rate, the buffer cannot keep up and the external transistors may turn on transiently. Because of the bootstrap action, the output capacitance is less than 10pF up to 10MHz of small-signal bandwidth and 300V/µs slew rate, increasing beyond these values. Adding overcompensation capacitors will degrade the slew rate that the output can withstand before current is drawn. It is sometime necessary to provide a “snubber” network-a series R and C- to provide a local R.F. impedance for the buffer to look into. 330 Ω and 56pF should serve. Also, it is well to provide some DC path to ground (47k for instance) to bias the output stage when no actual circuit is connected to the EL2021 in high-impedance mode. FIGURE 1. SIMPLIFIED OUTPUT STAGE (NORMAL MODE) EL2021 |
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