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MAX253

Transformer Driver for Isolated RS-485 Interface

Maxim Integrated 

MAX253

Transformer Driver for

Isolated RS-485 Interface

age. In addition, include in the calculations an is a good choice for through-hole applications, and the

allowance for worst-case losses in the rectifiers. Since NIEC* SB05W05C dual in an SOT-23 package is rec-

the turns ratio determined in this manner will ordinarily ommended for surface-mount applications. Use the

produce a much higher voltage at the secondary under higher frequency setting to reduce ripple.

conditions of high input voltage and/or light loading, be

careful to prevent an overvoltage condition from occur-

ring (see Output Voltage vs. Load Current in the Typical

Operating Characteristics).

Transformers used with the MAX253 will ordinarily be

wound on high-permeability magnetic material. To min-

imize radiated noise, use common closed-magnetic-

path physical shapes (e.g., pot cores, toroids, E/I/U

cores). A typical core is the Philips 213CT050-3B7,

which is a toroid 0.190” in diameter and 0.05” thick.

For operation with this core at 5.5V maximum supply

voltage, the primary should have approximately 22

turns on each side of the center tap, or 44 turns total.

This will result in a nominal primary inductance of

approximately 832µH. The secondary can be scaled to

Output Filter Capacitor

In applications sensitive to output-ripple noise, the out-

put filter capacitor C2 should have a low effective

series resistance (ESR), and its capacitance should

remain fairly constant over temperature. Sprague 595D

surface-mount solid tantalum capacitors and Sanyo

OS-CON through-hole capacitors are recommended

due to their extremely low ESR. Capacitor ESR usually

rises at low temperatures, but OS-CON capacitors pro-

vide very low ESR below 0°C.

In applications where output ripple is not critical, a

0.1µF chip or ceramic capacitor is sufficient. Refer to

Table 4 for suggested capacitor suppliers. Use the

higher frequency setting to reduce ripple.

produce the required DC output.

Input Bypass Capacitor

Diode Selection

The MAX253’s high switching frequency demands

high-speed rectifiers. Schottky diodes are recom-

mended. Ensure that the Schottky diode average cur-

rent rating exceeds the load-current level. The 1N5817

The input bypass capacitor C1 is not critical. Unlike

switching regulators, the MAX253’s supply current is

fairly constant, and is therefore less dependent on the

input bypass capacitor. A low-cost 0.1µF chip or

ceramic capacitor is normally sufficient for input

bypassing.

Table 4. Suggested Capacitor Suppliers

PRODUCTION METHOD

Surface Mount

CAPACITORS

Matsuo

267 series (low ESR)

USA Phone: (714) 969-2491, FAX: (714) 960-6492

Sprague Electric Co.

595D/293D series (very low ESR)

USA Phone: (603) 224-1961, FAX: (603) 224-1430

Murata Erie

Ceramic

USA Phone: (800) 831-9172, FAX: (404) 436-3030

High-Performance

Through Hole

Sanyo

OS-CON series (very low ESR)

USA Phone: (619) 661-6835, FAX: (619) 661-1055

Japan Phone: 81-7-2070-1005, FAX: 81-7-2070-1174

Through Hole

Nichicon

PL series (low ESR)

USA Phone: (708) 843-7500, FAX: (708) 843-2798

Japan Phone: 81-7-5231-8461, FAX: 81-7-5256-4158

* Nihon Inter Electronics Corp.

USA Phone: (805) 867-2555

FAX: (805) 867-2556

Japan Phone: 81-3-3494-7411

FAX: 81-3-3494-7414

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