 |
Electronic Components Datasheet Search |
|
|
Electronic Components Datasheet Search |
|
FOD2742CR2 Datasheet(PDF) 11 Page - Fairchild Semiconductor |
|
||||||||||||||||||||||||||||||
FOD2742CR2 Datasheet(HTML) 11 Page - Fairchild Semiconductor |
|
|
11 / 16 page  ©2003 Fairchild Semiconductor Corporation www.fairchildsemi.com FOD2742A, FOD2742B, FOD2742C Rev. 1.0.1 11 The FOD2742 The FOD2742 is an optically isolated error amplifier. It incorporates three of the most common elements neces- sary to make an isolated power supply, a reference volt- age, an error amplifier, and an optocoupler. It is functionally equivalent to the popular KA431 shunt volt- age regulator plus the CNY17F-X optocoupler. Powering the Secondary Side The LED pin in the FOD2742 powers the secondary side, and in particular provides the current to run the LED. The actual structure of the FOD2742 dictates the minimum voltage that can be applied to the LED pin: The error amplifier output has a minimum of the reference voltage, and the LED is in series with that. Minimum volt- age applied to the LED pin is thus 2.5V + 1.5V = 4.0V. This voltage can be generated either directly from the output of the converter, or else from a slaved secondary winding. The secondary winding will not affect regula- tion, as the input to the FB pin may still be taken from the output winding. The LED pin needs to be fed through a current limiting resistor. The value of the resistor sets the amount of cur- rent through the LED, and thus must be carefully selected in conjunction with the selection of the primary side resistor. Feedback Output voltage of a converter is determined by selecting a resistor divider from the regulated output to the FB pin. The FOD2742 attempts to regulate its FB pin to the reference voltage, 2.5V. The ratio of the two resistors should thus be: The absolute value of the top resistor is set by the input offset current of 5.2µA. To achieve 0.5% accuracy, the resistance of RTOP should be: Compensation The compensation pin of the FOD2742 provides the opportunity for the designer to design the frequency response of the converter. A compensation network may be placed between the COMP pin and the FB pin. In typical low-bandwidth systems, a 0.1µF capacitor may be used. For converters with more stringent requirements, a network should be designed based on measurements of the system’s loop. An excellent reference for this process may be found in “Practical Design of Power Supplies” by Ron Lenk, IEEE Press, 1998. Secondary Ground The GND pin should be connected to the secondary ground of the converter. No Connect Pins The NC pins have no internal connection. They should not have any connection to the secondary side, as this may compromise the isolation structure. Photo-Transistor The Photo-transistor is the output of the FOD2742. In a normal configuration the collector will be attached to a pull-up resistor and the emitter grounded. There is no base connection necessary. The value of the pull-up resistor, and the current limiting resistor feeding the LED, must be carefully selected to account for voltage range accepted by the PWM IC, and for the variation in current transfer ratio (CTR) of the opto-isolator itself. Example: The voltage feeding the LED pins is +12V, the voltage feeding the collector pull-up is +10V, and the PWM IC is the Fairchild KA1H0680, which has a 5V reference. If we select a 10KV resistor for the LED, the maximum current the LED can see is: (12V-4V) /10K Ω = 800µA. The CTR of the opto-isolator is a minimum of 100%, so the minimum collector current of the photo-transistor when the diode is full on is also 800µA. The collector resistor must thus be such that: select 12K Ω to allow some margin. R TOP R BOTTOM -------------------------- V OUT V REF -------------- 1 – = V OUT 2.5 – R TOP ----------------------------- 1040 µA > 10V 5V – R COLLECTOR ----------------------------------- 800 µA or R COLLECTOR 6.25K Ω; > < |
Similar Part No. - FOD2742CR2 |
|
Similar Description - FOD2742CR2 |
|
|
|
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
