SP508E Datasheet, PDF (22/29 Pages) Exar Corporation – Rugged 20Mbps, 8 Channel Multi-Protocol Transceiver with Programmable DCE/DTE and Termination Resistors

This resistor is invoked when the receiver is
operating as a V.11 receiver, in modes EIA-
530, EIA-530A, RS-449/V.36, and X.21. The
same receivers also incorporate a termina-
tion network internally for V.35 applications.
For V.35, the receiver input termination is a
“Y” termination consisting of two 51Ω resis-
tors connected in series and a 124Ω resistor
connected between the two 50Ω resistors
and V35RGND output. The V35RGND is
usually grounded. The receiver itself is
identical to the V.11 receiver.
The differential receivers can be configured
to be ITU-T-V.10 single-ended receivers
by internally connecting the non-inverting
input to ground. This is internally done
by default from the decoder. The non-in-
verting input is rerouted to V10GND and
can be grounded separately. The ITU-T-
V.10 receivers can operate over 1Mbps
and are used in RS-449/V.36, E1A-530,
E1A-530A and X.21 modes as Category II
signals as indicated by their corresponding
specifications. All receivers include an en-
able/disable line for disabling the receiver
output allowing convenient half-duplex
configurations. The enable pins will either
enable or disable the output of the receivers
according to the appropriate active logic il-
lustrated on Figure 45. The receiver’s enable
lines include an internal pull-up or pull-down
device, depending on the active polarity of
the receiver, that enables the receiver upon
power up if the enable lines are left floating.
During disabled conditions, the receiver
outputs will be at a high impedance state.
If the receiver is disabled any associated
termination is also disconnected from the
inputs.
All receivers include a fail-safe feature that
outputs a logic high when the receiver inputs
are open, terminated but open, or shorted
together. For single-ended V.28 and V.10
receivers, there are internal 5kΩ pull-down
resistors on the inputs which produces a
logic high (“1”) at the receiver outputs. The
differential receivers have a proprietary cir-
cuitthat detect open or shorted inputs and
if so, will produce a logic HIGH (“1”) at the
receiver output.
CHARGE PUMP
The charge pump is a Exar-patented design
(5,306,954) and uses a unique approach
compared to older less-efficient designs.
The charge pump still requires four external
capacitors, but uses four-phase voltage
shifting technique to attain symmetrical
power supplies. The charge pump V_DD and
V_SS outputs are regulated to +5.8V and
-5.8V, respectively. There is a free-running
oscillator that controls the four phases of
the voltage shifting. A description of each
phase follows.
Phase 1
__V_SS charge storage ——During this phase
of the clock cycle, the positive side of capaci-
tors C₁ and C₂ are initially charged to V_CC. C+
isthen switched to ground and the charge
in C₁- is transferred to C₂-. Since C₂+ is con-
nected to V_CC, the voltage potential across
capacitor C₂ is now 2_XV_CC.
Phase 2
—V_SS transfer —Phase two of the clock con-
nects the negative terminal of C₂ to the V_SS
storage capacitor and the positive terminal
of C₂ to ground, and transfers the negative
generated voltage to C₃. This generated
voltage is regulated to –5.8V. Simultane-
ously, the positive side of the capacitor C₁
is switched to V_CC and the negative side is
connected to ground.
Phase 3
—V_DD charge storage —The third phase of
the clock is identical to the first phase—the
charge transferred in C₁ produces –V_CC in
the negative terminal of C₁ which is applied
to the negative side of the capacitor C₂ .
Since C₂+ is at V_CC, the voltage potential
across C₂ is 2_XV_CC.
Phase 4
—V_DD transfer —The fourth phase of the
clock connects the negative terminal of C₂
to ground, and transfers the generated5.8V
across C₂ to C₄, the V_DD storage capacitor.
This voltage is regulated to +5.8V. At the
regulated voltage, the internal oscillator is
disabled and simultaneously with this, the
positive side of capacitor C₁ is switched to
V_CC and the negative side is connected to
ground, and the cycle begins again.
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.comSP508E_100_072412
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