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ADDC02803SCTV Datenblatt(PDF) 8 Page - Analog Devices |
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ADDC02803SCTV Datenblatt(HTML) 8 Page - Analog Devices |
8 / 18 page ADDC02803SC/ADDC02805SA REV. A –8– one of the output pins of the converter, or remotely at the load. A remote connection at the load can adjust for voltage drops of as much as 0.25 V dc between the converter and the load. Long remote sense leads can affect converter stability, although this condition is rare. The impedance of the long power leads between the converter and the remote sense point could affect the converter’s unity gain crossover frequency and phase margin. Consult factory if long remote sense leads are to be used. Pin 3 (ADJUST) An adjustment pin is provided so that the user can change the nominal output voltage during the prototype stage. Since very low temperature coefficient resistors are used to set the output voltage and maintain tight regulation over temperature, using standard external resistors to adjust the output voltage will loosen output regulation over temperature. Furthermore, since the status trip point is not changed when the output voltage is adjusted using external resistors, the status line will no longer trip at the standard levels of the newly adjusted output voltage. If necessary, modified standard units can be ordered with the necessary changes made inside the package at the factory. The ADJUST function is sensitive to noise, and care should be taken in the routing of connections. To make the output voltage higher, place a resistor from ADJUST (Pin 3) to –SENSE (Pin 1). To make the output voltage lower, place a resistor from ADJUST (Pin 3) to +SENSE (Pin 2). Figures 24 and 25 show resistor values for a ±5% change in output voltage: OUTPUT VOLTAGE – % 8 7 1 99 95 98 97 96 5 4 3 2 6 Figure 24. External Resistor Value for Reducing Output Voltage OUTPUT VOLTAGE – % 5 4 0 101 105 102 103 104 3 2 1 Figure 25. External Resistor for Increasing Output Voltage BASIC OPERATION The ADDC02803SC and ADDC02805SA converters use a flyback topology with dual interleaved power trains operating 180 ° out of phase. Each power train switches at a fixed fre- quency of 500 kHz, resulting in a 1 MHz fixed switching fre- quency as seen at the input and output of the converter. In a flyback topology, energy is stored in the inductor during one half portion of the switching cycle and is then transferred to the output filter during the next half portion. With two interleaved power trains, energy is transferred to the output filter during both halves of the switching cycle, resulting in smaller filters to meet the required ripple. A five pole differential input EMI filter, along with a common- mode EMI capacitor and careful attention to layout parasitics, is designed to meet all applicable requirements in MIL-STD-461D when installed in a typical system setup. A more detailed dis- cussion of CE102 and other EMI issues is included in the sec- tion entitled EMI Considerations. The converters use current mode control and employ a high performance opto-isolator in its feedback path to maintain isolation between input and output. The control circuit is designed to give a nearly constant output current as the output voltage drops from VO nom to VSC during a short circuit condition. It does not let the current fold back below the maximum rated output current. The output overvoltage protection circuitry, which is independent of the normal feedback loop, protects the load against a break in the remote sense leads. Remote sense connections, which can be made at the load, can adjust for voltage drops of as much as 0.25 V dc between the converter and the load, thereby maintaining an accurate voltage level at the load. An input overvoltage protection feature shuts down the con- verter when the input voltage exceeds (nominally) 52.5 V dc. An internal temperature sensor shuts down the unit and pre- vents it from becoming too hot if the heat removal system fails. The temperature sensed is the case temperature and is factory set to trip at a nominal case temperature of 110 °C to 115°C. The shutdown temperature setting can be raised externally or disabled by the user. Each unit has an INHIBIT pin that can be used to turn off the converter. This feature can be used to sequence the turn-on of multiple converters and to reduce input power draw during extended time in a no load condition. A SYNC pin, referenced to the input return line (Pin 10), is available to synchronize multiple units to one switching frequency. This feature is particularly useful in eliminating beat frequencies which may cause increased output ripple on paralleled units. A current share pin (ISHARE) is available that permits paralleled units to share current typically within 5% at full load. A low level dc auxiliary voltage supply referenced to the input return line is provided for miscellaneous system use. PIN CONNECTIONS Pins 1 and 2 ( SENSE) Pins 1 and 2 must always be connected for proper operation, although failure to make these connections will not be catastrophic to the converter under normal operating conditions. Pin 1 must always be connected to the output return and Pin 2 must always be connected to +VOUT. These connections can be made at any |
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