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LM6172AMJ-QML Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM6172AMJ-QML Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 16 page 1 ⁄2 LM6172 Simplified Schematic Application Notes LM6172 Performance Discussion The LM6172 is a dual high-speed, low power, voltage feed- back amplifier. It is unity-gain stable and offers outstanding performance with only 2.3 mA of supply current per channel. The combination of 100 MHz unity-gain bandwidth, 3000V/µs slew rate, 50 mA per channel output current and other attractive features makes it easy to implement the LM6172 in various applications. Quiescent power of the LM6172 is 138 mW operating at ±15V supply and 46 mW at ±5V supply. LM6172 Circuit Operation The class AB input stage in LM6172 is fully symmetrical and has a similar slewing characteristic to the current feedback amplifiers. In the LM6172 Simplified Schematic, Q1 through Q4 form the equivalent of the current feedback input buffer, R E the equivalent of the feedback resistor, and stage A buff- ers the inverting input. The triple-buffered output stage iso- lates the gain stage from the load to provide low output im- pedance. LM6172 Slew Rate Characteristic The slew rate of LM6172 is determined by the current avail- able to charge and discharge an internal high impedance node capacitor. This current is the differential input voltage divided by the total degeneration resistor R E. Therefore, the slew rate is proportional to the input voltage level, and the higher slew rates are achievable in the lower gain configura- tions. When a very fast large signal pulse is applied to the input of an amplifier, some overshoot or undershoot occurs. By plac- ing an external series resistor such as 1 k Ω to the input of LM6172, the slew rate is reduced to help lower the over- shoot, which reduces settling time. Reducing Settling Time The LM6172 has a very fast slew rate that causes overshoot and undershoot. To reduce settling time on LM6172,a1k Ω resistor can be placed in series with the input signal to de- crease slew rate. A feedback capacitor can also be used to reduce overshoot and undershoot. This feedback capacitor serves as a zero to increase the stability of the amplifier cir- cuit. A 2 pF feedback capacitor is recommended for initial evaluation. When the LM6172 is configured as a buffer, a feedback resistor of 1 k Ω must be added in parallel to the feedback capacitor. Another possible source of overshoot and undershoot comes from capacitive load at the output. Please see the section “Driving Capacitive Loads” for more detail. Driving Capacitive Loads Amplifiers driving capacitive loads can oscillate or have ring- ing at the output. To eliminate oscillation or reduce ringing, an isolation resistor can be placed as shown in Figure 1. The combination of the isolation resistor and the load capacitor forms a pole to increase stability by adding more phase mar- gin to the overall system. The desired performance depends on the value of the isolation resistor; the bigger the isolation resistor, the more damped (slow) the pulse response be- comes. For LM6172, a 50 Ω isolation resistor is recom- mended for initial evaluation. DS012581-55 www.national.com 11 |
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