ML6680

8

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

Ω. 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

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 band limit the input signal which in turn will improve

the signal to noise ratio. Although the input is AC coupled,

at the amplifier’s output. In order to reduce this error a DC

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

AMPLITUDE OK

SET

RESET

Q

OPINOK

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

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

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