ADSL Drivers/Receivers for Customer Premise

Equipment

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9

Applications Information

Power Supply and Decoupling

The MAX4361/MAX4362/MAX4363 should be powered

from a well-regulated, low-noise, 4.5V to 5.5V supply in

order to optimize the ADSL upstream drive capability to

+12.5dBm and maintain the best SFDR.

High-quality capacitors with low equivalent series resis-

tance (ESR) such as multilayer ceramic capacitors

(MLCCs) should be used to minimize supply voltage

ripple and power dissipation. A larger capacitor located

in proximity to the MAX4361/MAX4362/MAX4363

improves decoupling for lower frequency signals.

In addition, 0.1µF MLCC decoupling capacitors should

be located as close as possible to each of the power-

supply pins, no more than 1/8 inch away. An additional

large (4.7µF to 10µF) tantalum capacitor should be

placed on the board near the supply terminals to sup-

ply current for fast, large-signal changes at the

MAX4361/MAX4362/MAX4363 outputs.

MAX4361/MAX4362

The MAX4361/MAX4362 require a single 0.1µF bypass

from V+ to ground located as close as possible to the

IC leads.

MAX4363

The MAX4363 features separate supply and ground

pins for the receiver and driver amplifiers. Bypass the

V+ (RX) supply to the GND (RX) pin with a 0.1µF capaci-

tor. Bypass the V+ (TX) supply to the GND (TX) pin with

a separate 0.1µF capacitor. Both capacitors should be

placed as close as possible to their respective IC leads.

USB Applications

The 5V supplied at the universal serial bus (USB) port

may be poorly regulated or unable to supply the peak

currents required by an ADSL modem. Improving the

quality of the supply will optimize the performance of

the MAX4361/MAX4362/MAX4363 in a USB-supplied

CPE ADSL modem. This can be accomplished through

the use of a step-up DC-to-DC converter or switching

power supply followed by a low-dropout (LDO) regula-

tor. Careful attention must be paid to decoupling the

power supply at the output of the DC-to-DC converter,

the output of the LDO regulator and the supply pins of

the MAX4361/MAX4362/MAX4363.

Driving a Capacitive Load

The MAX4361/MAX4362/MAX4363 are capable of dri-

ving capacitive loads up to 2nF. Most hybrid circuits

are well under this limit. For additional capacitive-drive

capability use isolation resistors between the output

and the load to reduce ringing on the output signal. In a

typical hybrid the back-matching resistors provide suffi-

cient isolation for most any capacitive-loading condition

(see Figure 1).

Method for Generating a Midsupply

Voltage

To operate an amplifier on a single-voltage supply, a

voltage midway between the supply and ground must be

generated to properly bias the inputs and the outputs.

A voltage divider can be created with two equal-value

resistors (Figure 2). There is a trade-off between the

power consumed by the divider and the voltage drop

across these resistors due to the positive input bias

currents. Selecting 2.7k

Ω for R1 and R2 will create a

voltage divider that draws less than 1mA from a 5V

supply. Use a decoupling capacitor (0.1µF) at the node

where VREF is generated.

Power Dissipation

It is important to consider the total power dissipation of

the MAX4361/MAX4362/MAX4363 in order to properly

size the heat sink area of an application. With some

simplifying assumptions we can estimate the total

power dissipated in the driver (see Typical Operating

Figure 2. Voltage-Divider Reference

Figure 1. Driving Capacitive Load

1k

Ω

500

Ω

3.1

Ω

CLOAD

OUTPUT

INPUT

MAX436 _

R1

2.7k

Ω

R2

2.7k

Ω

0.1

µF

VREF

5V