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ML4827IP-1 Datasheet(PDF) 9 Page - Micro Linear Corporation |
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ML4827IP-1 Datasheet(HTML) 9 Page - Micro Linear Corporation |
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9 / 16 page  ML4827 9 FUNCTIONAL DESCRIPTION (Continued) Error Amplifier Compensation The PWM loading of the PFC can be modeled as a negative resistor; an increase in input voltage to the PWM causes a decrease in the input current. This response dictates the proper compensation of the two transconductance error amplifiers. Figure 2 shows the types of compensation networks most commonly used for the voltage and current error amplifiers, along with their respective return points. The current loop compensation is returned to VREF to produce a soft-start characteristic on the PFC: as the reference voltage comes up from zero volts, it creates a differentiated voltage on IEAO which prevents the PFC from immediately demanding a full duty cycle on its boost converter. There are two major concerns when compensating the voltage loop error amplifier; stability and transient response. Optimizing interaction between transient response and stability requires that the error amplifier’s open-loop crossover frequency should be 1/2 that of the line frequency, or 23Hz for a 47Hz line (lowest anticipated international power frequency). The gain vs. input voltage of the ML4827’s voltage error amplifier has a specially shaped nonlinearity such that under steady- state operating conditions the transconductance of the error amplifier is at a local minimum. Rapid perturbations in line or load conditions will cause the input to the voltage error amplifier (VFB) to deviate from its 2.5V (nominal) value. If this happens, the transconductance of the voltage error amplifier will increase significantly, as shown in the Typical Performance Characteristics. This raises the gain-bandwidth product of the voltage loop, resulting in a much more rapid voltage loop response to such perturbations than would occur with a conventional linear gain characteristic. The current amplifier compensation is similar to that of the voltage error amplifier with the exception of the choice of crossover frequency. The crossover frequency of the current amplifier should be at least 10 times that of the voltage amplifier, to prevent interaction with the voltage loop. It should also be limited to less than 1/6th that of the switching frequency, e.g. 16.7kHz for a 100kHz switching frequency. There is a modest degree of gain contouring applied to the transfer characteristic of the current error amplifier, to increase its speed of response to current-loop perturbations. However, the boost inductor will usually be the dominant factor in overall current loop response. Therefore, this contouring is significantly less marked than that of the voltage error amplifier. For more information on compensating the current and voltage control loops, see Application Notes 33 and 34. Application Note 16 also contains valuable information for the design of this class of PFC. Oscillator (RAMP 1) The oscillator frequency is determined by the values of RT and CT, which determine the ramp and off-time of the oscillator output clock: f tt OSC RAMP DEADTIME = + 1 (2) The deadtime of the oscillator is derived from the following equation: tC R In V V RAMP T T REF REF =´ ´ - - F HG I KJ 125 375 . . (3) at VREF = 7.5V: tC R RAMP T T =´ ´ 051 . The deadtime of the oscillator may be determined using: t V mA CC DEADTIME T T =´ = ´ 25 51 490 . . (4) The deadtime is so small (tRAMP >> tDEADTIME) that the operating frequency can typically be approximated by: f t OSC RAMP = 1 (5) EXAMPLE: For the application circuit shown in the data sheet, with the oscillator running at: fkHz t OSC RAMP == 100 1 tC R RAMP T T =´ ´ = ´ - 0 51110 5 . Solving for RT x CT yields 2 x 10-4. Selecting standard components values, CT = 470pF, and RT = 41.2kΩ. The deadtime of the oscillator adds to the Maximum PWM Duty Cycle (it is an input to the Duty Cycle Limiter). With zero oscillator deadtime, the Maximum PWM Duty Cycle is typically 45% for the ML4827-1. In many applications of the ML4827-1, care should be taken that CT not be made so large as to extend the Maximum Duty Cycle beyond 50%. This can be accomplished by using a stable 470pF capacitor for CT. |
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Similar Description - ML4827IP-1 |
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