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PA84S Datasheet(PDF) 4 Page - Cirrus Logic |
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PA84S Datasheet(HTML) 4 Page - Cirrus Logic |
4 / 4 page OPERATING CONSIDERATIONS PA84 • PA84A • PA84S GENERAL Please read the “General Operating Considerations” sec- tion, which covers stability, supplies, heatsinking, mounting, current limit, SOA interpretation, and specification interpreta- tion. Additional information can be found in the application notes. For information on the package outline, heatsinks, and mounting hardware, consult the “Accessory and Package Mechanical Data” section of the handbook. SAFE OPERATING AREA (SOA) The bipolar output stage of this high voltage operational amplifier has two output limitations: 1. The internal current limit which limits maximum available output current. 2. The second breakdown effect, which occurs whenever the simultaneous collector current and collector-emitter voltage exceeds specified limits. The SOA curves combine the effect of these limits. For a given application, the direction and magnitude of the output current should be calculated or measured and checked against the SOA curves. This is simple for resistive loads but more complex for reactive and EMF generating loads. However, the following guidelines may save extensive analytical efforts: 1. The following capacitive and inductive loads are safe: ±V S C(MAX) L(MAX) 150V 1.2 µF .7H 125V 6.0 µF 25H 100V 12 µF 90H 75V ALL ALL 2. Short circuits to ground are safe with dual supplies up to ±150V or single supplies up to 150V. 3. Short circuits to the supply rails are safe with total supply voltages up to 150V (i.e. ±75V). OUTPUT PROTECTION Two external diodes as shown in Figure 2, are required to protect these amplifiers against flyback (kickback) pulses exceeding the supply voltages of the amplifier when driving inductive loads. For component selection, these external diodes must be very quick, such as ultra fast recovery diodes with no more than 200 nanoseconds of reverse recovery time. STABILITY Due to its large bandwidth the PA84 is more likely to oscillate than lower bandwidth Power Operational Amplifiers such as the PA83 or PA08. To prevent oscillations, a reasonable phase margin must be maintained by: 1. Selection of the proper phase compensation capacitor and resistor. Use the values given in the table under external connections and interpolate if necessary. The phase mar- gin can be increased by using a large capacitor and a smaller resistor than the slew rate optimized values listed in the table. The compensation capacitor may be connected to common (in lieu of +V S) if the positive supply is properly bypassed to common. Because the voltage at pin 8 is only a few volts below the positive supply, this ground connec- tion requires the use of a high voltage capacitor. 2. Keeping the external sumpoint stray capacitance to ground at a minimum and the sumpoint load resistance (input and feedback resistors in parallel) below 500 Ω. Larger sumpoint load resistance can be used with increased phase compen- sation (see 1 above). 3. Connecting the amplifier case to a local AC common thus preventing it from acting as an antenna. +VS –VS FIGURE 1. PROTECTIVE, INDUCTIVE LOAD 20 30 40 50 25 35 300 200 250 150 170 SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE (V) t = 5ms t = 1ms SAFE OPERATING AREA CURVES STEADY STATE This data sheet has been carefully checked and is believed to be reliable, however, no responsibility is assumed for possible inaccuracies or omissions. All specifications are subject to change without notice. PA84U REV. L JANUARY 2000 © 2000 Apex Microtechnology Corp. Be sure the diode voltage rating is greater than the total of both supplies. The diode will turn on to divert the flyback energy into the supply rails thus protecting the output transistors from destruction due to reverse bias. A note of caution about the supply. The energy of the flyback pulse must be absorbed by the power supply. As a result, a transient will be superimposed on the supply voltage, the magnitude of the transient being a function of its transient impedance and current sinking capability. If the supply voltage plus transient exceeds the maximum supply rating or if the AC impedance of the supply is unknown, it is best to clamp the output and the supply with a zener diode to absorb the transient. |
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