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AD8155 Datasheet PDF : 36 Pages
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AD8155
frequency of maximum boost without sacrificing the amount of
boost delivered.
AD8155 POWER CONSUMPTION
There are several sections of the AD8155 that draw varying
power depending on the supply voltages, the type of I/O coupling
used, and the status of the AD8155 operation. Figure 40 shows a
block diagram of these sections. An initialization sequence is
required to enable the AD8155 in a low power mode (see the
Applications Information section).
The first section consists of the input termination resistors. The
power dissipated in the termination resistors is due to the input
differential swing and any common-mode current resulting
from dc-coupling the input.
In the next section (the receiver section), each input is powered
only when it is selected, and the disable bits are set to 0. If a
receiver is not selected, it is powered down. Thus, the total
number of active inputs affects the total power consumption.
Furthermore, the loss-of-signal detection circuits can be
disabled independent of the receiver for even greater power
savings.
The core of the device performs the multiplexer and demulti-
plexer switching functions. It draws a fixed quiescent current of
2 mA whenever the AD8155 is powered from VCC to VEE. The
switch draws an additional 4 × 4.6 mA in normal mux/demux
operation and an additional 6 × 4.6 mA with all ports in loop-
back or with bicast selected. The switch core can be disabled to
save power.
An output predriver section draws a current, IPRED, that is
related to the programmed output current, ITTO. The predriver
current always flows from VCC to VEE. It is treated separately
from the output current, which flows from VTTO and may not be
the same voltage as VCC.
The final section is the outputs section. For an individual
output, the programmed output current flows through two
separate paths. One is the on-chip termination resistor, and the
other is the transmission line and the destination termination
resistor. The nominal parallel impedance of these two paths is
25 Ω. The sum of these two currents flows through the switches
and the current source of the AD8155 output circuit and out
through VEE. The power dissipated in the transmission line and the
destination resistor is not dissipated in the AD8155 but must be
supplied from the power supply and is a factor in overall system
power. The current in the on-chip termination resistors and the
output current source dissipate power in the AD8155 itself.
Outputs
The output current is set by a combination of output level and
preemphasis settings (see Table 19). For the two logic switch
states, this current flows through an on-chip termination
resistor and a parallel path to the destination device and its
termination resistor. The power in this parallel path is not
dissipated by the AD8155. With preemphasis enabled, some
current always flows in both the P and N termination resistors.
This preemphasis current gives rise to an output common-
mode shift, which varies with ac-coupling or dc-coupling and
which is calculated for both cases in Table 19.
Perhaps the most direct method for calculating power dissi-
pated in the output is to calculate the power that would be
dissipated if all of ITOT were to flow on-die from VTTO to VEE
and to subtract from this the power dissipated off die in the
destination device termination resistors and the channel.
For this purpose, the destination device and channel can be
modeled as 50 Ω load resistors, RL, in parallel with the AD8155
termination resistors.
.
VTTI
VCC
IP_xx
IN_xx
AC-COUPLING CAPS
(OPTIONAL)
INPUT
TERMINATION
(VIN_DIFF_RMS)2
P=
100Ω
EQUALIZER
LOSS OF
SIGNAL
RECEIVER
SWITCH
DVCC
VTTO
VTT
OUTPUT
TERMINATIONS
P
=
IOUT
2
×
50Ω
50Ω
50Ω
50Ω
50Ω
IOUT
OPTIONAL COUPLING
CAPACITORS
P = (VOL) (IOUT)
VOL = VTTO – (IOUT × 25Ω)
VEE
Figure 40. AD8155 Power Distribution Block Diagram
Rev. 0 | Page 22 of 36
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