_______________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

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

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

FREQUENCY

SELECT

C2

C3

C1

OUTPUT

5V @ 150mA

5V

N

N

Q

Q

OSC

F / F

SD

ON / OFF

400kHz/

700kHz

T

ISO

GND

CR1

CR2

Figure 2. Detailed Block Diagram