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ML4802IS データシート(PDF) 10 Page - Fairchild Semiconductor |
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ML4802IS データシート(HTML) 10 Page - Fairchild Semiconductor |
10 / 15 page ML4802 10 REV. 1.0.1 12/12/2000 The major concern when compensating the ML4802's voltage loop error amplifier is that the current amplifier compensation is chosen to optimize frequency response while maintaining good stability. This leads to the following rules of thumb: 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. For more information on compensating the current and voltage control loops, see Application Notes 33, 34, and 55. Application Note 16 also contains valuable information for the design of this class of PFC. Oscillator The oscillator frequency is determined by the values of RT and CT, which determine the ramp and off-time of the oscillator output clock: The deadtime of the oscillator is derived from the following equation: at VREF = 7.5V: The ramp of the oscillator may be determined using: The deadtime is so small (tRAMP >> tDEADTIME) that the operating frequency can typically be approximated by: EXAMPLE: For the application circuit shown in the data sheet, with the oscillator running at: Solving for RT x CT yields 1 x 10-4. Selecting standard components values, CT = 100pF, and RT = 100k W. RAMP 1 The ramp voltage on this pin serves as a reference to which the PFC control signal is compared in order to set the duty cycle of the PFC switch. The external ramp voltage is derived from an RC network similar to the oscillator's. The PWM's oscillator sends a synchronous pulse every other cycle to reset this ramp. The ramp voltage should be limited to no more than the output high voltage (6V) of the current error amplifier. The timing resistor values should be selected such that the capacitor will not charge past this point before being reset. In order to ensure the linearity of the PFC loop's transfer function and improve noise immunity, the charging resistor should be connected to the 13.5V VCC rather than the 7.5V reference. This will keep the charging voltage across the timing capacitor in the "linear" region of the charging curve. The component value selection is similar to oscillator RC component selection. The charge time of RAMP 1 is derived from the following equations: At VCC = 13.5V and assuming RampPeak = 5V to allow for component tolerances: The capacitor value should remain small to keep the discharge energy and the resulting discharge current through the part small. A good value to use is the same value used in the pwm timing circuit (CT). For the application circuit shown in Figure 7, using a 200kHz PWM and a 100pF timing capacitor yeilds RT: FUNCTIONAL DESCRIPTION (Continued) fOSC = 1 tRAMP + DEADTIME tRAMP = CT RT IN VREF - 1.25 VREF - 3.75 tRAMP = CT RT 0.51 tDEADTIME = 2.5V 5.5mA CT = 455 CT fOSC = 1 tRAMP fOSC = 200kHz = 1 tRAMP tRAMP = 0.51 RT CT = 1 10 -5 Rt 110 0.463 100 10 5 12 = - - 27 Rt 215k =W tCHARGE = .463 R C 0 tCHARGE = C R 1n VCC - RampValley VCC - RampPeak tCHARGE = 2 fOSC fOSC = 1 tCHARGETIME + tDISCHARGETIME |
同様の部品番号 - ML4802IS |
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同様の説明 - ML4802IS |
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