MAX13253 Datasheet, PDF(11/14 Page) Maxim Integrated Products – 1A, Spread-Spectrum, Push-Pull, Transformer Driver for Isolated Power Supplies

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MAX13253 Datasheet, PDF (11/14 Pages) Maxim Integrated Products – 1A, Spread-Spectrum, Push-Pull, Transformer Driver for Isolated Power Supplies

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MAX13253

1A, Spread-Spectrum, Push-Pull, Transformer

Driver for Isolated Power Supplies

3.3V

1ÂµF

VDD

HICLK

SPRD

FAULT

MAX13253

CLK

GND PGND

1CT:2CT

10ÂµF

OUT

1ÂµF

SHDN

MAX8881

GND

ISO OUTPUT

4.7ÂµF

Figure 5. 3.3V to Isolated, Regulated 5V Application Circuit

Isolated USB/RS-485/RS-232 Isolated

Applications

The MAX13253 can provide isolated power for USB/

RS-485/RS-232 applications. The 1A output current capa-

bility of the MAX13253 allows multiple RS-485/RS-232

transceivers to operate simultaneously.

PCB Layout Guidelines

As with all power-supply circuits, careful PCB layout is

important to achieve low switching losses and stable

operation. Connect the exposed pad to a solid copper

ground plane for optimum thermal performance.

The traces from T1 and T2 to the transformer must be

low-resistance and low-inductance paths. Locate the

transformer as closely as possible to the MAX13253

using short, wide traces.

If possible, use a power plane for all VDD connections to

the MAX13253 and the primary-side of the transformer. If

a power plane is not available, avoid damage to the IC by

ensuring that the current flowing through the primary-side

center tap of the transformer does not flow through the

same trace that connects the supply pin of the MAX13253

to the VDD source, and connect the primary-side center tap

to the VDD supply using a very low-inductance connection.

When the internal oscillator is used, it is possible for high

frequency switching on T1 and T2 to couple into the CLK

circuitry through PCB parasitic capacitance. This capaci-

tive coupling can induce duty cycle errors in the oscillator,

resulting in a DC current through the transformer. For proper

operation, ensure that CLK has a solid ground connection.

Exposed Pad

For optimal thermal performance, ensure that the exposed

pad has a low thermal resistance connection to the

ground plane. Failure to provide a low thermal impedance

path to the ground plane results in excessive junction

temperatures when dissipating high power.

Component Selection

Transformer Selection

Transformer selection for the MAX13253 can be simpli-

fied by the use of the ET product. The ET product relates

the maximum allowable magnetic flux density in a trans-

former core to the voltage across a winding and switching

period. Inductor magnetizing current in the primary wind-

ing changes linearly with time during the switching period

of the MAX13253. Each transformer has a minimum ET

product, though not always stated on the transformer data

sheet. Ensure that the transformer selected for use with

the MAX13253 has an ET product of at least ET = VDD/

(2 x fSW) for each half of the primary winding, where fSW

is the minimum switching frequency of the T1 and T2

ouputs.

Select a transformer with sufficient ET product for each

half of the primary winding to ensure that the transformer

does not saturate during operation. Saturation of the

magnetic core results in significantly reduced inductance

of the primary, and therefore in a large increase in current

flow. This can cause the current limit to be reached even

when the load is not high.

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