Electronic Components Datasheet Search |
|
ML4841 Datasheet(PDF) 9 Page - Fairchild Semiconductor |
|
ML4841 Datasheet(HTML) 9 Page - Fairchild Semiconductor |
9 / 15 page PRODUCT SPECIFICATION ML4841 REV. 1.0.3 6/13/01 9 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 cross- over 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 ML4841’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 Charac- teristics. This increases 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. For more information on compensating the current and volt- age control loops, see Application Notes 33 and 34. Appli- cation Note 16 also contains valuable information for the design of this class of PFC. Oscillator (RT/CT) 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 ramp-charge time of the oscillator is derived from the following equation: at VREF = 7.5V: The discharge time 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 -5. Selecting standard com- ponents values, CT = 390pF, and RT = 24.9k Ω. RAMP 1 The ramp voltage on this pin serves as a reference to which the PFC’s current error amp output is compared in order to set the duty cycle of the PFC switch. The external ramp volt- age is derived from a 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 out- put high voltage (6V) of the current error amplifier. The tim- ing resistor value 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 cap 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 Ramp Peak = 5V to allow for component tolerances: The capacitor value should remain small to keep the dis- charge 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 the data sheet, using a 200kHz PWM and 390pF timing cap yields RT: f OSC 1 t RAMP t DISCHARGE + ------------------------------------------------------- = (2) t RAMP C T R T × In V REF 1.25 – V REF 3.75 – -------------------------------- × = (3) t RAMP C T R T × 0.51 × = t DISCHARGE 2.5V 5.1mA ------------------ C T × 490 C T × == (4) f OSC 1 t RAMP ---------------- = (5) t RAMP 0.51 R T × C T × 510 6 – × == f OSC 200kHz 1 t RAMP ---------------- == f OSC 1 t CHARGE t DISCHARGE + -------------------------------------------------------------- = (6) t CHARGE 2 f OSC ------------ = (7) t CHARGE C T R T × In V CC Ramp Valley – V CC Ramp Peak – --------------------------------------------------- × = (8) t CHARGE 0.463 R T × C T × = (9) R T 110 5 – × 0.463 () 390 10 12 – × () -------------------------------------------------------- 56.2k Ω == (10) |
Similar Part No. - ML4841 |
|
Similar Description - ML4841 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |