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CS5212EDR14 Datasheet(PDF) 6 Page - ON Semiconductor |
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CS5212EDR14 Datasheet(HTML) 6 Page - ON Semiconductor |
6 / 16 page CS5212 http://onsemi.com 6 THEORY OF OPERATION The CS5212 is a synchronous, programmable fixed–frequency, low–voltage buck controller using the V2 control method. It also provides overcurrent protection, undervoltage lockout, soft start and built–in adaptive non–overlap. V2 Control Method The V2 method of control uses a ramp signal 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 variations 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. Ramp Signal Error Signal Error Amplifier COMP GATE(L) GATE(H) Output Voltage Feedback PWM Comparator Figure 3. V2 Control Block Diagram Reference Voltage As illustrated in Figure 3, 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 the 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. 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 the 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 effect 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, compared to 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, because 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 regulations 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 variations, since both line and load affect the ramp signal. Constant Frequency Operation The CS5212 uses a constant frequency, trailing edge modulation architecture for generating PWM signal. During normal operation, the oscillator generates a narrow pulse at the beginning of each switching cycle to turn on the main switch. The main switch will be turned off when the ramp signal intersects with the output of the error amplifier (COMP pin voltage). Therefore, the switch duty cycle can be modified to regulate the output voltage to the desired value as line and load conditions change. The major advantage of constant frequency operation is that the component selections, especially the magnetic component design, become very easy. The oscillator frequency of CS5212 is programmable from 150 kHz to 750 kHz using an external resistor connected from the ROSC pin to ground. Startup If there are no fault conditions and the fault latch is reset, the error amplifier will start charging the COMP pin capacitor after the CS5212 is powered up. The output of the error amplifier (COMP voltage) will ramp up linearly. The COMP capacitance and the source current of the error amplifier determine the slew rate of COMP voltage. The output of the error amplifier is connected internally to the inverting input of the PWM comparator and it is compared with the VFFB pin voltage plus 0.5 V offset at the non–inverting input of the PWM comparator. Since VFFB voltage is zero before the startup, the PWM comparator output will stay high until the COMP pin voltage hits 0.5 V. There is no switching action while the PWM comparator output is high. After the COMP voltage exceeds the 0.5 V offset, the output of PWM comparator toggles and releases the PWM latch. The narrow pulse generated by the oscillator at the |
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