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CS51311GDR14 Datasheet(PDF) 8 Page - ON Semiconductor |
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CS51311GDR14 Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 21 page CS51311 http://onsemi.com 8 APPLICATIONS INFORMATION THEORY OF OPERATION V2 Control Method The V2 method of control uses a ramp signal that is generated by the ESR of the output capacitors. This ramp is proportional to the AC current through the main inductor and is offset by the value of the DC output voltage. This control scheme inherently compensates for variation in either line or load conditions, since the ramp signal is generated from the output voltage itself. This control scheme differs from traditional techniques such as voltage mode, which generates an artificial ramp, and current mode, which generates a ramp from inductor current. The V2 control method is illustrated in Figure 8. The output voltage is used to generate both the error signal and the ramp signal. Since the ramp signal is simply the output voltage, it is affected by any change in the output regardless of the origin of that change. The ramp signal also contains the DC portion of the output voltage, which allows the control circuit to drive the main switch to 0% or 100% duty cycle as required. Figure 8. V2 Control Diagram − + − + C E PWM Comparator Ramp Signal COMP Error Amplifier Error Signal Reference Voltage Output Voltage Feedback VFB GATE(H) GATE(L) A change in line voltage changes the current ramp in the inductor, affecting the ramp signal, which causes the V2 control scheme to compensate the duty cycle. Since the change in inductor current modifies the ramp signal, as in current mode control, the V2 control scheme has the same advantages in line transient response. A change in load current will have an affect on the output voltage, altering the ramp signal. A load step immediately changes the state of the comparator output, which controls the main switch. Load transient response is determined only by the comparator response time and the transition speed of the main switch. The reaction time to an output load step has no relation to the crossover frequency of the error signal loop, as in traditional control methods. The error signal loop can have a low crossover frequency, since transient response is handled by the ramp signal loop. The main purpose of this “slow” feedback loop is to provide DC accuracy. Noise immunity is significantly improved, since the error amplifier bandwidth can be rolled off at a low frequency. Enhanced noise immunity improves remote sensing of the output voltage, since the noise associated with long feedback traces can be effectively filtered. Line and load regulation are drastically improved because there are two independent voltage loops. A voltage mode controller relies on a change in the error signal to compensate for a deviation in either line or load voltage. This change in the error signal causes the output voltage to change corresponding to the gain of the error amplifier, which is normally specified as line and load regulation. A current mode controller maintains fixed error signal under deviation in the line voltage, since the slope of the ramp signal changes, but still relies on a change in the error signal for a deviation in load. The V2 method of control maintains a fixed error signal for both line and load variation, since the ramp signal is affected by both line and load. Constant Off−Time To minimize transient response, the CS51311 uses a Constant Off−Time method to control the rate of output pulses. During normal operation, the Off−Time of the high side switch is terminated after a fixed period, set by the COFF capacitor. Every time the VFB pin exceeds the COMP pin voltage an Off−Time is initiated. To maintain regulation, the V2 Control Loop varies switch On−Time. The PWM comparator monitors the output voltage ramp, and terminates the switch On−Time. Constant Off−Time provides a number of advantages. Switch Duty Cycle can be adjusted from 0 to 100% on a pulse−by pulse basis when responding to transient conditions. Both 0% and 100% Duty Cycle operation can be maintained for extended periods of time in response to Load or Line transients. Programmable Output The CS51311 is designed to provide two methods for programming the output voltage of the power supply. A five bit on board digital to analog converter (DAC) is used to program the output voltage within two different ranges. The first range is 2.125 V to 3.525 V in 100 mV steps, the second is 1.325 V to 2.075 V in 50 mV steps, depending on the digital input code. If all five bits are left open, the CS51311 enters adjust mode. In adjust mode, the designer can choose any output voltage by using resistor divider feedback to the VFB pin, as in traditional controllers. The CS51311 is specifically designed to meet or exceed Intel’s Pentium II specifications. Error Amplifier An inherent benefit of the V2 control topology is that there is no large bandwidth requirement on the error amplifier design. The reaction time to an output load step has no |
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