ML6680CQ Micro Linear, ML6680CQ Datasheet - Page 8

• Manufacturer Part Number: ML6680CQ
• Description: Token Ring Copper-to-Fiber Converter
• Manufacturer: Micro Linear

ML6680
FUNCTIONAL DESCRIPTION
Fiber Optic LED Driver
The output stage of the transmitter is a current mode
switch which develops the output light by sinking current
from OPVCC through the LED into the OPOUT pin. Once
the current requirement for the LED is determined, the
RTSETOP resistor is selected. The following equation is
used to select the correct RTSETOP resistor:
No current is provided during the off cycles of the
Insertion, Bypass, or Echo Keys, or when the input signal
that should be routed to the Fiber Optic LED Driver does
not meet the corresponding input squelch requirements.
Fiber Optic Quantizer
The OPINP, OPINN input signal is fed into a limiting
amplifier with a gain of about 100 and input resistance of
1.3k . Maximum sensitivity is achieved through the use
of a DC restoration feedback loop and AC coupling the
input. When AC coupled, the input DC bias voltage is set
by an on-chip network at about 1.7V. These coupling
capacitors, in conjunction with the input impedance of
the amplifier, establish a high pass filter with 3dB corner
frequency, f
Since the amplifier has a differential input, two capacitors
of equal value are required. If the signal driving the input
is single ended, one of the coupling capacitors can be tied
to V
to band limit the input signal which in turn will improve
the signal to noise ratio. Although the input is AC coupled,
the offset voltage, V
at the amplifier’s output. In order to reduce this error a DC
feedback loop nulls the offset voltage, forcing V
zero. The comparator is a high-speed differential zero
crossing detector that slices and accurately digitizes the
receive signal. The output of the comparator is fed in
parallel into both the fiber optic squelch circuit and the
signal MUX. The capacitor between pin VDC and QGND
should be set to 500pF.
Fiber Optic Squelch
The ML6680 monitors the frequency and amplitude of the
input from a fiberoptic receiver. The optical squelch
circuit rejects signals whose frequencies are lower than
1MHz or whose amplitudes are lower than –32dBm.
If both requirements are met, the LED output OPINOK
goes low, and the amplitude threshold is lowered 20%.
FREQUENCY OK
8
AMPLITUDE OK
CC
. The internal amplifier has a lowpass filter built-in
L
, at
RTSETOP = (52mA/I
f
L
= 1/(2
OS
, within the amplifier will be present
1300
OUT
SET
RESET
)
C)
115
Q
OPINOK
OS
to be
Copper Pair Driver
The output stage of the twisted pair transmitter is a current
mode switch which develops the output voltage by driving
current through the terminating resistor and the output filter.
The harmonic content is controlled to simplify the filter
design. The transmitter employs a center tap 2:1 transformer
where the center tap is tied to V
transmit pair is pulled low, the other pin floats. The output
pins to the twisted pair wires, TPOUTP and TPOUTN, can
drive shielded or unshielded twisted pair cable through the
appropriate isolation transformer. The output current is set
by the value of RTSETTP. No transitions are generated at
TPOUTP and TPOUTN when the input signal that should be
routed to the Copper Pair Driver does not meet the
corresponding input squelch requirements. PECL compatible
output are obtained with an external network of 3 resistors.
In this case the current of the output stage can be reduced
by adjusting the value of RTSETTP.
Twisted Pair Line Equalizer
The receive equalizer compensates for twisted pair cable
dispersion, which otherwise would give rise to inter-symbol
interference (ISI). The amount of equalization varies with the
average amplitude of the received signal. The received
signal amplitude gives a rough value for the length of the
attached cable. The filter/equalizer characteristic is the
inverse of the cable’s dispersion characteristic. Both UTP
and STP cables approximate a low-pass filter, so the filter/
equalizer approximates an inverse square root equalizer.
Two external resistors and one external capacitors are
required between pins EQA and EQB. The output of the
equalizer is fed into the signal MUX. On a PECL application
these pins should be connected between each other.
Twisted Pair Squelch Circuit
The twisted pair line receiver internally sets the common
mode bias of the input TPINP and TPINN. Voltage offset
comparators are used to set the amplitude squelch
threshold, and analog timers are used to set the pulse width
squelch threshold. When the input signal meets amplitude
and pulse width requirements, the squelch circuit reduces
the offset voltage of the comparators, decreasing the
amplitude squelch threshold by half. This hysteresis allows
the receiver to stay on in the presence of a fading input
signal. The twisted pair squelch circuit rejects signals whose
frequencies are lower than 1MHz or whose amplitudes are
lower than 300mV
output TPINOK goes low.
Clock Oscillator
The ML6680 provides an on-chip clock oscillator by
connecting a 32.768kHz watch crystal between pins
XTAL1 and XTAL2. The part can also be driven by an
external clock applied at XTAL1 and tying XTAL2 to
ground. The frequency of the external clock should be
between 32.7kHz and 34.5kHz.
Status LED Drivers
The ML6680 has three status LED drivers. The LED driver
pins are active low. The LED’s are tied to their respective
pins through a 300 resistor to V
P-P
. If both requirements are met, the LED
CC
. While one pin of the
CC
.