5

®

DRV1101

To calculate the amplifier requirements for a DSL applica-

tion:

1. Determine the average power that must be delivered to

the line. The amplifier must deliver twice this power to

account for the power dissipated in the series impedance

matching resistors. Therefore, add 3dB to the line power.

This is the average power delivered at the output of the

amplifier. For ADSL G.Lite (as of June 1998), the aver-

age line power is 10dBm. Adding 3dB results in an

average power at the amplifier output of 13dBm.

2. Next add the power crest factor needed for the line code

used. The power crest factor for ADSL is 15dB which

fier output is 28dBm (13dBm +15dB). 28dBm is 631mW.

3. The DRV1101 peak output voltage is calculated by the

impedance that the DRV1101 must drive. For ADSL

voltage out of the DRV1101 is 2 x 3.3V = 6.6V.

4. The transformer turns ratio can be changed to keep the

required output voltage and current within the range of the

The impedance that is reflected to the DRV1101 side of

transfer, the total of the impedance matching resistors

should equal the reflected impedance. Thus, for the circuit

shown in Figure 3, the reflected impedance is 100

= 8.6

Ω. With two impedance matching resistors of 4Ω

each and about 0.5

Ω transformer resistance, the total load

impedance is about (8.6

Ω + 4Ω + 4Ω + 0.5Ω) = 17Ω.

FIGURE 3. Typical Digital Subscriber Line Application.

OUTPUT PROTECTION

Figure 3 also shows overvoltage and short circuit protection

elements that are commonly included in DSL applications.

Overvoltage suppressors include diodes or MOV’s. The

outputs of the DRV1101 can be momentarily shorted to

ground or to the supply without damage. The outputs are not,

however, designed for a continuous short to ground or the

supply.

POWER DISSIPATION AND THERMAL ANALYSIS

The total internal power dissipation of the DRV1101 is the

sum of a fixed overhead power that is independent of the

load plus the power dissipated internally to deliver the

average load power. The total internal power dissipation

determines the internal temperature rise when in operation.

For DSL applications with high crest factors, such as ADSL,

the average load power delivered is much lower than the

peak power required. For practical purposes, this means that

internal temperature rise is not an issue for the DRV1101 in

high-crest factor DSL applications.

With a +5V supply, the DRV1101’s typical fixed overhead

current of 22mA (out of total no-load supply current of

29mA) creates a fixed overhead power dissipation of 110mW.

The load dependent power dissipation of the DRV1101

The internal power dissipation will reach a maximum when

As an example, compute the power and junction temperature

into a 20

Ω differential load. The total internal power dissipation

would be:

(110mW) + (5V – 2.5V) • (2.5V/20

Ω) = 423mW

In+

In–

Out+

+5V

DRV1101

GND

Out–

4

Ω

4

Ω

Protection Circuits

Impedance Matching

Resistors

1:3.3

Transformer

Line Impedance

100

Ω

Fixed

Load Related