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MAX845 Folha de dados(PDF) 5 Page - Maxim Integrated Products |
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MAX845 Folha de dados(HTML) 5 Page - Maxim Integrated Products |
5 / 16 page _______________Detailed Description The MAX845 is a transformer driver specifically designed to provide isolated power for PCMCIA and other height- and/or space-sensitive applications. It drives a center-tapped transformer primary from a 5V or 3.3V DC power supply. The secondary can be wound to provide any isolated DC voltage needed at power levels up to 750mW. The 450kHz minimum switching frequency allows the use of very thin transformers, making the MAX845 ideal for PCMCIA and other space-limited applications. The MAX845 is designed to drive a single transformer less than 0.09 inches (2.3mm) in height, including package. Further reduction down to 0.050 inches (1.27mm) can be achieved using a transformer without a package. The MAX845 consists of an RC oscillator driving a pair of N-channel power switches. The oscillator runs at double the output frequency, driving a toggle flip-flop to ensure 50% duty cycle to each of the switches. Internal circuitry ensures break-before-make action between the two switches. A low-current shutdown mode disables all internal cir- cuitry, including the oscillator and both power switches. Drive the shutdown pin (SD) high to shut down the part; drive SD low for normal operation. The SD pin has no internal default condition and must not be allowed to float. Most MAX845 applications will operate at high frequen- cies. The frequency-select pin (FS) is pulled high or left open (FS is internally pulled up to VCC) to operate at a minimum of 450kHz. Pulling FS low selects the low-fre- quency state. Theory of Operation Figure 2 shows the MAX845 driving both a TGM-010P3 transformer with a center-tapped primary, and a sec- ondary with a voltage-doubler rectifier topology. All of the transformers driven by the MAX845 must have a center tap with VIN applied. Whenever one of the MAX845 out- puts (D1 or D2) goes low, the other goes to approximate- ly double the supply voltage. A voltage is induced in the secondary and the rectifier diodes steer the currents into the appropriate output capacitor. On alternate half cycles, each capacitor is charged. The output voltage is the sum of the voltages from each output capacitor. This topology yields the simplest and smallest transformer because the least number of secondary turns is required for a given voltage. __________Applications Information With the MAX845 transformer driver, designers have the advantages of push/pull converter topology in space-sensitive applications. The push/pull DC-DC converter topology allows isolated multiple outputs, step-up/step-down or inverted outputs, easier filtering on the input and the output, and lower overall noise. Isolated Power for PCMCIA Applications Medical instrumentation, modems, and LAN-interface cards often require isolated power supplies. One of the best switching-regulator topologies for this application is the push/pull forward-converting DC-DC power sup- ply shown in Figures 3 and 4. Because the transformer works in the forward mode (rather than the flyback mode), its core does not store energy and, therefore, can be small. Input and output capacitors can be small because of the high-frequency and continuous-current waveforms. Isolated Transformer Driver for PCMCIA Applications _______________________________________________________________________________________ 5 MAX845 D1 D2 FS GND2 GND1 VCC FREQUENCY SELECT C2 C3 C1 OUTPUT 5V @ 150mA 5V N N Q Q OSC F / F VIN SD ON / OFF 400kHz/ 700kHz T ISO GND VCC CR1 CR2 Figure 2. Detailed Block Diagram |
Nº de peça semelhante - MAX845 |
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Descrição semelhante - MAX845 |
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