feature

Optocoupler, Precision Reference and Error Amplifier 2.5V Reference Voltage 50% to 100 % CTR at 1mA Current 5000V rms Isolation FOD2743A : Tolerance 0.5% FOD2743B: Tolerance 1% FOD2743C: Tolerance 2%

application

Power Regulation DC-DC Converter Description: The FOD2743 opto-isolated amplifier includes the popular ka431 precision programmable shunt reference and an optocoupler. The optocoupler is a gallium arsenide (gaas) light-emitting diode, with a silicon phototransistor. It has three grades of reference voltage tolerance = 2%, 1% and 0.5%. The current transfer ratio (CTR) is between 50% and 100%. It also has an excellent temperature coefficient of 50 ppm/°C. Mainly used as an error amplifier/reference voltage/optocoupler function for isolating AC and DC power supplies and DC/DC converters. When using the FoD2643, power supply designers can reduce component count and save space in packaging designs. Close tolerance datums eliminate the need for adjustment in many applications. The device comes in an 8-pin dipped white package.

800.png" />

The device is considered a two-terminal device: pins 1, 2, 3, and 4 are shorted together, and pins 5, 6, 7, and 8 are shorted together. The output high common mode transient immunity is the leading edge maximum allowable (positive) dvcm/dt common mode pulse signal, VCM, to ensure that the output will remain high. Common Mode Transient Output Low Immunity is the maximum allowable (negative) dvcm/dt signal on the trailing edge of the common pulse, VCM, to ensure that the output will remain low. The FOD2743 is an optically isolated error amplifier. It contains the three most common components, an isolated power supply, a voltage reference, an error amplifier and an optocoupler. It is functionally equivalent to the popular KA431 shunt voltage regulator plus CNy17F-X optocoupler. Powering the secondary side The LED pins in the FOD2743 provide power to the secondary power supply specifically to provide current to run the lead. The actual structure of the FOD2743 determines the minimum voltage that can be applied to the LED pins: the error amplifier output has a minimum reference voltage with which the LED is in series. Therefore, the minimum voltage applied to the LED pins is 2.5V+1.2V=3.7V. This voltage can be bent directly from the output of the converter, or from the output of the driven secondary winding. The secondary winding does not affect regulation, as the input to the FB pin is still available from the output winding. The LED pins need to be powered through a current limiter resistor. The resistor value sets the amount of current through the LED, so care must be taken to choose the side resistors along with the selection of the main option. The output voltage of the feedback converter is selected by a resistor divider from the regulated output to the FB pin. The FOD2743 is trying to adjust its FB pin to a reference voltage of 2.5V. The ratio of the two resistors should therefore be: The absolute value of the top resistor is set by the input offset current of 5.2 microamps. To achieve 0.5% accuracy, the resistance of RTOP should be: Compensation The compensation pin of the FOD2743 provides the designer with the opportunity to design the frequency response of the converter. The compensation network can be placed between the compressor pin and the FB pin. In a typical low bandwidth system, a 0.1µf capacitor can be used. For more stringent converter requirements, the design of the network should be based on measurements of the system loop. A good reference to this process can be found in "Practical Design", Power Supply, by Ron Lenk, IEEE Press, 1998. The secondary ground ground pin should be connected to the ground of the auxiliary converter. No Connect Pins NC pins have no internal connections. They should not have any connection to the secondary side, as shown below may break the isolation structure. Phototransistor The phototransistor is the output of the FOD2743. In a normal configuration the collector will be connected to the pull-up resistor and the transmitter to ground. No base connection required. The pull-up resistor value and current limit must be chosen carefully. The resistor powering the LED accounts for the voltage range accepted by the pwm ic, as well as for the opto-isolator itself. Example: The voltage supplying the LED pin is +12V, the supply voltage for the collector pull-up is +10V, and the pwm IC is a fairchild fan4803, which has a 5v reference. If we choose a 10KΩ resistor for the LED, the maximum current the LED can see is: (12V – 4V) / 10KΩ = 800µA. The CTR of the opto-isolator is a minimum of 50%, so the minimum collector current of the phototransistor is 400 microamps when the diode is fully energized. The collector resistor must therefore: choose 20kΩ to allow some margin.

Absolute Maximum Ratings (Ta = 25oC, unless otherwise specified) Stresses in excess of the Absolute Maximum Ratings may damage the device. Equipment may not work or be recommended for operation under recommended operating conditions and increase component stress to these levels. Additionally, prolonged exposure to stresses higher than recommended operating conditions may affect device reliability. Absolute maximum ratings are stress ratings only.