Isolated Transformer Driver for PCMCIA Applications VIN 0.01µF 1N4148 3.3V 5V SUPPLY C1 0.1µF 1N4148 6 5V @ 150mA MBR0520 6 ISO OUTPUT 4 VCC 1 1CT : 1.3CT MAX845 SD D1 VCC ON / OFF C2 1 D1 0.33µF MAX845 MAX845 3 8 FS D2 ISO 8 FREQUENCY GND1 GND2 GND MBR0520 D2 SELECT 2 7 GND1 GND2 2 7 SEE FIGURE 11 FOR RECTIFIER CONFIGURATIONS Figure 3. 5V to Isolated 5V Application Circuit Figure 4. 3.3V Input to Isolated Output Application Circuit The MAX845 is a versatile transformer driver, capable will be higher at 3.3V, so transformer winding resistance of driving a center-tapped transformer primary from a will be more critical and efficiencies will be lower. The 5V or 3.3V DC power supply (Figures 3 and 4). The MAX845 output current must still be limited to 200mA secondary can be wound to provide any isolated volt- (see Absolute Maximum Ratings), so the available out- age needed at power levels up to 750mW with a 5V put power will be less than with a 5V power source. supply or up to 500mW with a 3.3V supply. Figure 3 shows a typical 5V to isolated 5V application circuit that Low-Noise Power Supply delivers up to 150mA of isolated 5V power. The MAX845 topology is inherently low noise, in that either one or the other of the two power devices is on at 3.3V Supply any given time. By alternating between two identical Any of the application circuits shown may be converted states with one side on and the other off, the input cur- to 3.3V operation by changing the turns ratio of the trans- rent is nearly constant and secondary output power is former and operating the MAX845 from a boost supply, available at all times. There is an intentional break- as shown in Figure 4. In normal operation, whenever one before-make action to prevent any possibility of both of the MAX845 outputs goes low, the other goes to power switches conducting at the same time. During approximately double the supply voltage. Since the cir- this 100ns non-overlap interval, the input current goes cuit is symmetrical, the two outputs can be combined to zero. This adds a small high-frequency component with diodes, lightly filtered, then used to power the to the input current waveform. This ripple current can MAX845, and possibly other light loads as well. easily be absorbed by a small input bypass capacitor The diodes on the primary side may be any fast-switch- (0.33µF) from VCC to ground. Figure 5 shows a low- ing small-signal diodes, such as the 1N914, 1N4148, or noise bias supply using the MAX845 transformer driver. CMPD2838. The value of the primary filter capacitor is When using the two-diode push-pull (Figure 11a) not critical and can be very small, since it only needs to rectifier or the four-diode bridge (Figure 11b), the out- supply current to the MAX845 during the break-before- put voltage tends to be more constant than in most make interval. alternative topologies. As described above, the circuit The transformer could be any of the same ones used for alternates between two identical states that both pro- 5V operation, but for optimum performance it should vide power to the load. The only part of the cycle that have fewer primary turns, as the ET product required is produces output ripple is the 100ns non-overlap inter- now only 3.3V-µs. For a given power level, the currents val, which can easily be filtered by a small ceramic output capacitor (0.33µF). 6_______________________________________________________________________________________
