Isolated Transformer Driverfor PCMCIA ApplicationsMAX845 VIN 5V C1 OUTPUT CR1 V 5V @ 150mA V CC D1 CC F / F MAX845 Q N C2 FS T OSC D2 FREQUENCY SELECT Q C3 N 400kHz/ CR2 700kHz SD GND2 GND1 ISO ON / OFF GND Figure 2. Detailed Block Diagram _______________Detailed DescriptionTheory of Operation Figure 2 shows the MAX845 driving both a TGM-010P3 The MAX845 is a transformer driver specifically transformer with a center-tapped primary, and a sec- designed to provide isolated power for PCMCIA and ondary with a voltage-doubler rectifier topology. All of the other height- and/or space-sensitive applications. It transformers driven by the MAX845 must have a center drives a center-tapped transformer primary from a 5V tap with V or 3.3V DC power supply. The secondary can be IN applied. Whenever one of the MAX845 out- puts (D1 or D2) goes low, the other goes to approximate- wound to provide any isolated DC voltage needed at ly double the supply voltage. A voltage is induced in the power levels up to 750mW. secondary and the rectifier diodes steer the currents into The 450kHz minimum switching frequency allows the the appropriate output capacitor. On alternate half use of very thin transformers, making the MAX845 ideal cycles, each capacitor is charged. The output voltage is for PCMCIA and other space-limited applications. The the sum of the voltages from each output capacitor. This MAX845 is designed to drive a single transformer less topology yields the simplest and smallest transformer than 0.09 inches (2.3mm) in height, including package. because the least number of secondary turns is required Further reduction down to 0.050 inches (1.27mm) can for a given voltage. be achieved using a transformer without a package. __________Applications Information The MAX845 consists of an RC oscillator driving a pair of N-channel power switches. The oscillator runs at With the MAX845 transformer driver, designers have double the output frequency, driving a toggle flip-flop the advantages of push/pull converter topology in to ensure 50% duty cycle to each of the switches. space-sensitive applications. The push/pull DC-DC Internal circuitry ensures break-before-make action converter topology allows isolated multiple outputs, between the two switches. step-up/step-down or inverted outputs, easier filtering on the input and the output, and lower overall noise. A low-current shutdown mode disables all internal cir- cuitry, including the oscillator and both power switches. Isolated Power for PCMCIA Applications Drive the shutdown pin (SD) high to shut down the part; Medical instrumentation, modems, and LAN-interface drive SD low for normal operation. The SD pin has no cards often require isolated power supplies. One of the internal default condition and must not be allowed to best switching-regulator topologies for this application float. is the push/pull forward-converting DC-DC power sup- Most MAX845 applications will operate at high frequen- ply shown in Figures 3 and 4. Because the transformer cies. The frequency-select pin (FS) is pulled high or left works in the forward mode (rather than the flyback open (FS is internally pulled up to VCC) to operate at a mode), its core does not store energy and, therefore, minimum of 450kHz. Pulling FS low selects the low-fre- can be small. Input and output capacitors can be small quency state. because of the high-frequency and continuous-current waveforms. _______________________________________________________________________________________5
