 |
Electronic Components Datasheet Search |
|
|
Electronic Components Datasheet Search |
|
FA3647 Datasheet(PDF) 8 Page - Fuji Electric |
|
||||||||||||||||||||||||||||||
FA3647 Datasheet(HTML) 8 Page - Fuji Electric |
|
|
8 / 21 page  FA3641P(N), FA3647P(N) 65 Fig. 1 Oscillator Fig. 2 Oscillator output Fig. 3 Oscillation frequency s Description of each circuit 1. Oscillator The oscillator generates a triangular waveform by charging and discharging the built-in capacitor. A desired oscillation frequency can be set by the value of the resistor connected to the RT pin (See Figure 1). The built-in capacitor voltage oscillates between about 3V and 1V, with almost the same charging and discharging gradients (Figure 2). You can set the desired oscillation frequency by changing the gradients using the resistor connected to the RT pin. (Large Rt = low frequency, small Rt = high frequency) The oscillation frequency is automatically lowered when output duty cycle is small (FB about 1.18V) in light load mode. For more information, see item 2, “Reducing oscillation frequency circuit in light-load mode.” The relationship between Rt and the fixed oscillation frequency is approximately given by: ..................................................... (1) .......................................... (2) fO: Fixed frequency [kHz] Rt: Timing resistance [k Ω] The oscillator waveform cannot be observed from the outside because a pin for this purpose is not provided. The oscillator output is connected to a PWM comparator. The RT pin is 2.5V DC in normal fixed frequency operation mode. When the frequency is lowered, the voltage also decreases linearly to about 1V. 2. Reducing oscillation frequency circuit in light-load mode To reduce the loss of the power supply in standby mode, this IC has a feature that automatically lowers the oscillation frequency when the load is light. When the load is light, with the result that the IC output pulse width narrows below about 10% and the FB pin voltage decreases below about 1.18V, the oscillation frequency begins to decreases linearly until the output pulse width becomes 0. When the output pulse width is 0, the oscillation frequency is about 46% of normal fixed frequency (Figure 3). Even while the oscillation frequency is decreasing, the built-in capacitor voltage oscillates between about 3V and 1V. The frequency reduction rate (46%) can be adjusted from the outside. (See “Design advice” for more information.) 3. PWM comparator The PWM comparator has four inputs as shown in Figure 4. Oscillator output is compared with CS pin voltage , FB pin voltage , and DT voltage . The lowest of three inputs , , and has priority and is compared with output . While the voltage is lower than the oscillator output, the comparator output is high. While the voltage is higher than the oscillator output, the PWM comparator output is low (see Figure 5). The IC OUT pin voltage is high while the PWM comparator output is low. When the IC is powered up, CS pin voltage controls soft start operation. The output pulse then begins to widen gradually. During normal operation, the output pulse width is determined within the maximum duty cycle (70%) set by DT voltage under the condition set by FB pin voltage , to stabilize the output voltage. Fig. 4 PWM comparator Fig. 5 PWM comparator timing chart Rt [k Ω] 4880 – 1.4 f0 f0 [kHz] 4880 Rt + 1.4 |
Similar Part No. - FA3647 |
|
Similar Description - FA3647 |
|
|
|
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 |
English ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
