MAX4361/MAX4362/MAX4363
ADSL Drivers/Receivers for Customer Premise
Equipment
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Circuit). If the output current is large compared to the
quiescent current, computing the dissipation in the out-
put devices and adding it to the quiescent power dissi-
pation will give a close approximation of the total power
dissipation in the package.
For a 12.5dBm average line power on a 100Ω line, the
RMS current is 13.4mA. With a one-to-four transformer
the driver therefore supplies 53.6mA RMS. It can be
shown for a DMT signal the ratio of RMS current to the
average rectified current is 0.8. The total power con-
sumption is approximately
P
CONS
= 0.8 ✕ 53.6 x 5V = 214mW
of which 18mW is delivered as line power and 18mW is
dissipated in the back-matching resistors. Hence the
average power consumption of the IC is approximately
178mW + quiescent power (110mW), or 288mW. For
the MAX4361 in an 8-pin µMAX package, this corre-
sponds to a temperature rise of 64°C. With an ambient
temperature of +85°C this corresponds to a junction
temperature of +148°C, just below the absolute maxi-
mum of +150°C.
Please note the part is capable of over 200mA RMS,
which could cause thermal shutdown in applications
with elevated ambient temperatures and/or signals with
low crest factors. See Figure 3 for a guide to power der-
ating for each of the MAX4361/MAX4362/MAX4363
packages.
Transformer Selection
Full-rate, customer premise ADSL requires the trans-
mission of a +12.5dBm (18mW) DMT signal. The DMT
signal has a typical crest factor of 5.3, requiring the line
driver to provide peak line power of 27.5dBm (560mW).
The 27.5dBm peak line power translates into a 28.4V
peak-to-peak differential voltage on the 100Ω telephone
line. The maximum low-distortion output swing available
from the MAX4361/MAX4362/MAX4363 line driver on a
5V supply is 3.8V and, taking into account the power
lost due to the back-matching resistance, a step-up
transformer with turns ratio of 3.8 or greater is needed.
In the Typical Operating Circuit, the MAX4363 is cou-
pled to the phone line through a step-up transformer
with a 1:4 turns ratio. R1 and R2 are back-matching
resistors, each 3.1Ω (100Ω / (2 ✕ 4
2
)), where 100Ω is
the approximate phone-line impedance. The total differ-
ential load for the MAX4361/MAX4362/MAX4363,
including the termination resistors, is therefore 12.5Ω.
Even under these conditions the MAX4361/MAX4362/
MAX4363 provide low distortion signals to within 0.6V of
the power rails.
Receive Channel Considerations
A transformer used at the output of the differential line
driver to step up the differential output voltage to the line
has the inverse effect on signals received from the line.
A voltage reduction or attenuation equal to the inverse of
the turns ratio is realized in the receive channel of a typi-
cal bridge hybrid. The turns ratio of the transformer may
also be dictated by the ability of the receive circuitry to
resolve low-level signals in the noisy, twisted-pair tele-
phone plant. Higher turns-ratio transformers effectively
reduce the received signal-to-noise ratio due to the
reduction in the received signal strength.
The MAX4363 includes an amplifier with typical voltage
noise of only 8.5nV/√Hz and a low-supply current of
2mA/amplifier to be used as the receive channel.
Layout Considerations
Good layout techniques optimize performance by
decreasing the amount of stray capacitance at the
amplifier’s inputs and outputs. Excess capacitance will
produce peaking in the amplifier’s frequency response.
To decrease stray capacitance, minimize trace lengths
by placing external components as close to the amplifi-
er as possible.
Chip Information
MAX4361 TRANSISTOR COUNT: 1400
MAX4362 TRANSISTOR COUNT: 1400
MAX4363 TRANSISTOR COUNT: 1750
PROCESS: Bipolar
