Datasheet

8
®
DRV1100
loss through the series impedance matching resistors shown
in Figure 3. Twice the required line power must be
delivered by the DRV1100 through the frequency band
of interest with the distortion required by the system.
• Calculate the RMS voltage required at the output of the
DRV1100 with this 2X line power requirement. Vrms =
(2 • P
LINE
• R
L
)
1/2
, where R
L
is the total load impedance
that the DRV1100 must drive. Multiply this Vrms by 2 •
crest factor to get the total required differential peak-to-
peak voltage at the output. The DRV1100 must be able
to drive the peak-to-peak differential voltage into the
load impedance.
Where possible, the transformer turns ratio may be adjusted
to keep within the DRV1100 output voltage and current
constraints for a given R
LINE
and desired power onto the
line.
Using the example of Figure 3, assume the average power
desired on a 135Ω line is 14dBm (HDSL). Twice this power
(17dBm) is required into the matching resistors on the
primary side of the transformer. This 135Ω load is reflected
through the 1:4 transformer as a (135/(4
2
)) = 8.4Ω load. The
two series 4.1Ω resistors, along with the 0.2Ω differential
output impedance of the DRV1100, will provide impedance
matching into this 8.4Ω load. The DRV1100 will see ap-
proximately a 16.5Ω load under these conditions. The re-
quired 17dBm (50mW) into this load will need an output
Vrms = (50mW • 16.5)
1/2
= 0.91Vrms. Assuming a crest
factor of 3, the differential peak-to-peak output voltage = 6
• 0.91 = 5.45Vp-p. The Typical Performance Curves show
that, at 100kHz, the DRV1100 can deliver this voltage swing
with less than –62dB THD.
OUTPUT PROTECTION
Figure 3 also shows overvoltage and short circuit protection
elements that are commonly included in xDSL applications.
Overvoltage suppressors include diodes or MOV’s. The
outputs of the DRV1100 can be momentarily shorted to
In+
In–
Out+
+5V
DRV1100
GND
Out–
4Ω
4Ω
Protection Circuits
1:4
Transformer
Line Impedance
135Ω
FIGURE 3. Typical Digital Subscriber Line Application.
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 DRV1100 is the
sum of a quiescent term and the power dissipated internally
to deliver the load power. The Typical Performance Curves
show the quiescent current over temperature. At +5V sup-
ply, the typical no load supply current of 11mA will dissi-
pate 55mW quiescent power. The rms power dissipated in
the output circuit to deliver a Vrms to a load R
L
is:
Prms = (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 16Ω differential load (peak output current for a
sinusoid would be 222mA). The total internal power dissi-
pation would be:
(5V • 11mA) + (5V – 2.5V) • (2.5V/16Ω) = 446mW
To compute the internal junctions temperature, this power is
multiplied by the junction to ambient thermal impedance (to
get the temperature rise above ambient) then added to the
ambient temperature. Using the specified maximum ambient
temperature of +85°C, the junction temperature for the
DRV1100 in an SO-8 package under these worst case
conditions will be:
T
J
= 85°C + 0.446W • 125°C/W = 141°C