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SI9140CQ-T1 Datasheet(PDF) 11 Page - Vishay Siliconix |
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SI9140CQ-T1 Datasheet(HTML) 11 Page - Vishay Siliconix |
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11 / 19 page  Si9140 Vishay Siliconix Document Number: 70026 S-40699—Rev. H, 19-Apr-04 www.vishay.com 11 The functions of each circuit are explained in detail below. Design equations are provided to optimize each application circuit. PWM Controller There are generally two types of controllers, voltage mode or current mode. In voltage mode control, an error voltage is generated by comparing the output voltage to the reference voltage. The error voltage is then compared to an artificial ramp, and the result is the duty cycle necessary to regulate the output voltage. In current mode, an actual inductor current is used, in place of the artificial ramp, to sense the voltage across the current sense resistor. The logic and timing sequence for voltage mode control is shown in Figure 5. The Si9140 offers voltage mode control, which is better suited for applications requiring both fast transient response and high output current. Current mode control requires a current sense resistor to monitor the inductor current. A 10-mW sense resistor in a 10-A design will dissipate 1 W, decreasing efficiency by 3.5%. Such a design would require a 2-W resistor to satisfy derating criteria, besides requiring additional board space. Voltage mode control is a second-order LC system and has a faster natural transient response compared to current mode control (first-order RC system). Current mode has the advantage of providing an inherently good line regulation. But the situations where line voltage is fixed, as in the point-of-use conversion for microprocessors, this feature is wasted. Current mode control also provides automatic pulse-to-pulse current limiting. This feature requires a current sense resistor as stated above. These characteristics make voltage mode control ideal for high-end microprocessor power supplies. FIGURE 5. Voltage Mode Logic and Timing Diagram OSC COMP DS DR The error amplifier of the PWM controller plays a major role in determining the output voltage, stability, and the transient response of the power supply. In the Si9140, the non-inverting input of the error amplifier is available for use with an external precision reference for tighter tolerance regulation. With a two-pair lead-lag compensation network, it is easy to create a stable 100-kHz closed loop converter with the Si9140 error amplifier. The Si9140 achieves the 5-mS transient response by generating a 100-kHz closed-loop bandwidth. This is possible only by switching above 400 kHz and utilizing an error amplifier with at least a 10-MHz bandwidth. The Si9140 controller has a 25-MHz unity gain bandwidth error amplifier. The switching frequency must be at least four times greater than the desired closed-loop bandwidth to prevent oscillation. To respond to the stimuli, the error amplifier bandwidth needs to be at least 10 times larger than the desired bandwidth. FIGURE 6. 100-kHz BW Synchronous Buck Converter Gain Phase Frequency (Hz) The Si9140 solution requires only three 330-mF OS-CON capacitors on the output of power supply to meet the 10-A transient requirement. Other converter solutions on the market with 20- to 50-kHz closed loop bandwidths typically require two to five times the output capacitance specified above to match the Si9140’s performance. The theoretical issues and analytical steps involved in compensating a feedback network are beyond the scope of this application note. However, to ease the converter design for today’s high-performance microprocessors, typical component values for the feedback network are provided in Table 1 for various combinations of output capacitance. Figure 6 shows the Bode plot (frequency domain) of the 2.9-V converter shown schematically in Figure 1. |
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