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71128 数据表(PDF) 4 Page - Vishay Siliconix |
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71128 数据表(HTML) 4 Page - Vishay Siliconix |
4 / 7 page AN731 Vishay Siliconix www.vishay.com S FaxBack 408-970-5600 4 Document Number: 71128 28-Jan-00 DYNAMIC RESPONSE PERFORMANCE Figures 7 and 8 show the dynamic response of the output voltage with different dynamic control voltages. The VO response and settling time depend on the following factors: 1. the bandwidth of the converter control loop: the larger the bandwidth, the faster VO response time 2. the large signal response slew rate of the converter: the faster the slew rate, the faster the response 3. the slew rate of the converter control loop error amplifier 4. the bandwidth of the difference amplifier. The bandwidth of the converter control loop is the most important parameter. The maximum attainable bandwidth of a converter control loop is 1/(2*pi) times the switching frequency. For a converter switching at 100 kHz, the control bandwidth is limited to 16 kHz. Vishay Siliconix high-frequency switching regulators (including the Si9165, Si9169, and Si9170) offer switching frequencies up to 2 MHz and have a theoretical bandwidth limit of 318 kHz. Thus they would be good candidates for this kind of application. Figure 5 shows a complete example of a dc-to-dc converter using the Si9165 controller IC and the op-amp circuit. The output voltage can be programmed from 0.4 V to 3.4 V with a control voltage from 0.2 V to 2.7 V. Figure 6 shows a similar example using the Si9166 controller. The signal response slew rate of the converter dictates how fast the converter can slew its output voltage up or down given an infinitely large control loop bandwidth. The converter slew rate depends on the converter output averaging LC filter. The smaller the LC, the faster the slew rate. With respect to the slew rate of the converter control loop error amplifier, most of the converter control loop is figured as an integrator. The error amplifier often drives a fairly large integrator capacitor. The slew rate of the op-amp should be sized adequately. The bandwidth of the difference amplifier requires the difference amplifier to have its unity gain bandwidth a decade or more larger than the control loop bandwidth. CONCLUSION A simple op-amp circuit is used to program the output voltage of a typical dc-to-dc converter. The circuit offers flexible voltage scaling using a linear control voltage. The control voltage can be hard-wired for fixed-voltage operation. When used together with a wide bandwidth dc-to-dc converter, the circuit offers a simple solution for controlling the output voltage dynamically. |
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