DRV1101 Datasheet, PDF (5/8 Pages) Burr-Brown (TI) – HIGH POWER DIFFERENTIAL LINE DRIVER

+5V
DRV1101
Out+
4Ω
In+
4Ω
In–
Out–
GND
Impedance Matching
Resistors
Protection Circuits
1:3.3
Transformer
Line Impedance
100Ω
FIGURE 3. Typical Digital Subscriber Line Application.
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
means that the peak power (P_PEAK) needed at the ampli-
fier output is 28dBm (13dBm +15dB). 28dBm is 631mW.
3. The DRV1101 peak output voltage is calculated by the
formula: V_PEAK = (P_PEAK • R_L)^1/2 where R_L is the load
impedance that the DRV1101 must drive. For ADSL
Lite, using the circuit shown in Figure 3, V_PEAK = (P_PEAK
• R_L)^1/2 = (.631W x 17Ω)^1/2 = 3.3V. The peak-to-peak
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
DRV1101. The line impedance (R_LINE) is 100Ω for ADSL.
The impedance that is reflected to the DRV1101 side of
the transformer is R_LINE/(turns ratio)². For best power
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 100Ω/(3.4)²
= 8.6Ω. With two impedance matching resistors of 4Ω
each and about 0.5Ω transformer resistance, the total load
impedance is about (8.6Ω + 4Ω + 4Ω + 0.5Ω) = 17Ω.
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
when delivering an output voltage Vrms to a load R_L is:
P = (V_DD – Vrms) • (Vrms/R_L)
The internal power dissipation will reach a maximum when
Vrms is equal to V_DD/2. For a sinusoidal output, this
corresponds to an output Vp-p = 1.41 • V_DD.
As an example, compute the power and junction temperature
under a worst case condition with V_DD = +5V and Vrms = 2.5V
into a 20Ω differential load. The total internal power dissipation
would be:
(110mW) + (5V – 2.5V) • (2.5V/20Ω) = 423mW
Fixed
Load Related
®
5
DRV1101