13

ML4664/ML4669

OPTICAL LINK DETECT CIRCUIT AND LOW LIGHT

The link detect circuit monitors the input signal and

determines when the input falls below a preset voltage

level. When the input falls below a preset voltage, the

ML4664/ML4669 goes into the Low Light state. In the

Low Light state the TP transmitter is disabled, but

continues sending link pulses, the receiver is disabled,

and the LMON LED pin goes to high shutting off the

LMON LED. To return to the Link Pass state, the optical

receiver power must be 20% higher than the shut-off

state. This built-in hysteresis adds stability to the Link

Monitor circuit. Once the receiver power threshold is

exceeded, the ML4664/ML4669 waits 250ms to 750ms,

then checks to see that no data is being received before

re-enabling the transmitter, and receiver, and lighting up

the LMON LED.

When the ML4664 is in the low light state, both the link

pulses at the twisted pair transmitter and idle at the

optical transmitter continue to be sent. However, when

the ML4669 is in the low light state, link pulses are not

sent at the twisted pair transmitter. Data is not sent at

either transmitter. An idle signal may be sent at the

optical transmitter, depending on the twisted pair inputs.

See Table 1.

receiver. The ML4664/ML4669 is capable of exceeding

the 10BASE-FL specifications for sensitivity. The sensitivity

is dependent on the layout of the PC board. A good low

noise layout will exceed the 10BASE-FL specifications,

while a poor layout will fail to meet the sensitivity and

BER spec.

The threshold generator shifts the reference voltage at

coefficient matching that of the limiting amplifier. The

peak input threshold) is:

VV

THADJ

TH

= 408

(4)

errors at a receive power level of –32.5dBm average. One

procedure to determine the sensitivity of a receiver is to

start at the lowest optical power level and gradually

increase the optical power until the BER is met. In this

case the Link Detect circuit must not disable the receiver

above the power level that meets the BER specification.

This way the receiver will shut-off before the BER is

exceeded.

However if greater sensitivity is required the circuit in

with these two resistors available. This will allow

potential future adjustments without board revisions.

The response time of the Link Detect circuit is set by the

switched on if the input exceeds the set threshold for a

time given by:

T =

A

700

m

(5)

To switch the link from on to off, the above time will be

doubled. A value of 0.05µF will meet to 10BASE-FL

specifications.

LED DRIVERS

The ML4664/ML4669 has five LED drivers. The LED driver

pins are active low, and the LEDs are normally off (except

for LTF and LMON). The LEDs are tied to their respective

pins through a 300

W resistor to 5 Volts.

The OPLED and TPLED pins have pulse stretchers on

them which enables the LEDs to be visible. If another

event occurs before the timer expires, the LED timer will

reset and restart the timing. Therefore rapid events will

leave the LEDs continuously on. The LMON, LTF, and

POLDIS LEDs do not have a pulse stretcher on since its

condition occurs long enough for the eye to see.

LOW LIGHT CONDITION

The LMON LED output is used to indicate a low light

condition. LMON is activated low when both the receive

power exceeds the Link Monitor threshold and there are

transitions on OPINP, OPINN less than 3µs apart. If either

one of these conditions do not exist, LMON will go high.