This smooth transition between the lower voltage ampli-

fier and the higher voltage amplifier guarantees no

glitch on the output signal, and ensures high linearity for

high output power, while at the same time consuming

minimum power. The relation between the input voltage

and the output current of this pseudo-class-G buffer is

illustrated in Figure 8.

Thermal Protection and Power Dissipation

The MAX9480/MAX9481/MAX9482 are available in the

EP version of the TSSOP. The EP facilitates heat trans-

fer out of the package if the pad is soldered to a heat

sink made from an area of circuit board copper.

DMT-modulated signal with a crest factor greater than

or equal to 5.3, the power dissipation of the

MAX9480/MAX9481/MAX9482 should not exceed

700mW; the 20-pin TSSOP-EP package with its EP

floating allows 714mW peak power at +85°C. Hence,

heat sinking is not essential, but is desirable for attain-

ing optimal electrical performance. Note that the part is

capable of 500mA peak output current, which could

cause thermal shutdown in applications with elevated

ambient temperatures and/or signals with low crest fac-

tors. The thermal shutdown feature prevents the die

temperature from exceeding +150°C. See Figure 9 for

a guide to power derating for each of the packages.

Transformer Selection

Full-rate, central-office ADSL requires the transmission

of a +20.4dBm (110mW) DMT signal. The DMT signal

has a typical crest factor of 5.3, requiring the line driver

to provide peak line power of 35.4dBm (3.4W). The

35.4dBm peak line power translates to a 36V peak-to-

peak differential voltage on a 100

Ω line. The output

swing available from the MAX9480 family of line drivers

transformer with turns ratio of 1:2.5 is needed. In the

Typical Operating Circuit, the MAX9480 is coupled to

the phone line through just such a transformer. The

total differential load for the MAX9480 is therefore 16

Ω.

Active termination is included on all devices in the

MAX9480 family (as explained above in the Output-

Impedance Synthesis section).

Receiver Channel Considerations

A step-up transformer at the output of the differential

line driver has a step-down effect on signals received

from the line. A voltage attenuation equal to the inverse

of the turns ratio is realized in the receive channel. This

is an addition to the attenuation due to the hybrid cir-

cuitry itself (see the Hybrid Considerations section).

Low-Power, Low-Distortion, Central-Office

ADSL Drivers and Integrated Drivers/Receivers

12

______________________________________________________________________________________

LOWER

VOLTAGE

AMPLIFIER

HIGHER

VOLTAGE

AMPLIFIER

LOAD

-2.5V

+5V

-5V

+2.5V

Figure 7. Output Stage of Pseudo-Class-G Amplifier

TEMPERATURE (

°C)

60

35

10

-15

1000

2000

3000

4000

5000

0

-40

85

20-TSSOP, PAD FLOATING

28-TSSOP, PAD FLOATING

QFN

Figure 9. Maximum Power Dissipation vs. Temperature

I

Figure 8. Amplifier Output Characteristics