The MAX845 is a versatile transformer driver, capable

of driving a center-tapped transformer primary from a

5V or 3.3V DC power supply (Figures 3 and 4). The

secondary can be wound to provide any isolated volt-

age needed at power levels up to 750mW with a 5V

supply or up to 500mW with a 3.3V supply. Figure 3

shows a typical 5V to isolated 5V application circuit that

delivers up to 150mA of isolated 5V power.

3.3V Supply

Any of the application circuits shown may be converted

to 3.3V operation by changing the turns ratio of the trans-

former and operating the MAX845 from a boost supply,

as shown in Figure 4. In normal operation, whenever one

of the MAX845 outputs goes low, the other goes to

approximately double the supply voltage. Since the cir-

cuit is symmetrical, the two outputs can be combined

with diodes, lightly filtered, then used to power the

MAX845, and possibly other light loads as well.

The diodes on the primary side may be any fast-switch-

ing small-signal diodes, such as the 1N914, 1N4148, or

CMPD2838. The value of the primary filter capacitor is

not critical and can be very small, since it only needs to

supply current to the MAX845 during the break-before-

make interval.

The transformer could be any of the same ones used for

5V operation, but for optimum performance it should

have fewer primary turns, as the ET product required is

now only 3.3V-µs. For a given power level, the currents

will be higher at 3.3V, so transformer winding resistance

will be more critical and efficiencies will be lower. The

MAX845 output current must still be limited to 200mA

(see

Absolute Maximum Ratings), so the available out-

put power will be less than with a 5V power source.

Low-Noise Power Supply

The MAX845 topology is inherently low noise, in that

either one or the other of the two power devices is on at

any given time. By alternating between two identical

states with one side on and the other off, the input cur-

rent is nearly constant and secondary output power is

available at all times. There is an intentional break-

before-make action to prevent any possibility of both

power switches conducting at the same time. During

this 100ns non-overlap interval, the input current goes

to zero. This adds a small high-frequency component

to the input current waveform. This ripple current can

easily be absorbed by a small input bypass capacitor

noise bias supply using the MAX845 transformer driver.

When using the two-diode push-pull (Figure 11a)

rectifier or the four-diode bridge (Figure 11b), the out-

put voltage tends to be more constant than in most

alternative topologies. As described above, the circuit

alternates between two identical states that both pro-

vide power to the load. The only part of the cycle that

produces output ripple is the 100ns non-overlap inter-

val, which can easily be filtered by a small ceramic

output capacitor (0.33µF).

Isolated Transformer Driver

for PCMCIA Applications

6

_______________________________________________________________________________________

C2

0.33µF

C1

0.1µF

5V @ 150mA

ISO OUTPUT

5V

MAX845

D1

D2

FS

GND1 GND2

1

8

6

2

7

3

FREQUENCY

SELECT

SD

4

ISO

GND

MBR0520

1CT:1.3CT

MBR0520

ON / OFF

Figure 3. 5V to Isolated 5V Application Circuit

MAX845

GND1

GND2

1

8

6

2

7

1N4148

1N4148

D1

D2

3.3V

SUPPLY

SEE FIGURE 11

FOR RECTIFIER

CONFIGURATIONS

0.01µF

Figure 4. 3.3V Input to Isolated Output Application Circuit