ML4664/ML4669
SYSTEM DESCRIPTION
OPTICAL TRANSMISSION
The optical transmit function consists of detecting the
presence of data from the TP inputs TPINP and TPINN and
driving that data onto the fiber optic LED transmitter. A
positive signal on the TPINP lead relative to the TPINN
lead will result in no current, hence the fiber optic LED is
in a low light condition. When TPINP is more negative
than TPINN, the ML4664/ML4669 will sink current into
the chip and the fiber optic LED will light up.
Before data will be transmitted onto the fiber optic cable
it must exceed the squelch requirements. The TP inputs,
squelch circuit serves the function of preventing any noise
from being transmitted onto the fiber.
2. All continuous sinusoidal signals of amplitude less
than 6.2V
P–P
and frequency less than 2MHz.
3. All single sinusoidal cycles of amplitude less than
6.2V
P–P
and either polarity, where the frequency is
between 2MHz and 15MHz. For a period of 4 BT
before and after this single cycle, the signal will
conform to (1) above.
4. All sinusoidal cycles gated by a 100ns pulse gate of
amplitude less than 6.2V
P–P
and either polarity, where
the sinusoidal frequency is between 2MHz and
30MHz. The off time of the pulse gate on the
sinusoidal signal shall be at least 400ns.
The first three receive squelch criteria are required to
conform to the 10BASE-T standard. The fourth receive
squelch criteria exceeds the 10BASE-T requirements and
enhances performance. The fourth squelch criteria
prevents a false unsquelch caused by cross talk or noise
typically found coupling from the phone lines onto the
receive twisted pair.
After the TP inputs are unsquelched, the detection
threshold is lowered to 225mV. Upon passing the TP
squelch requirements the receive data passes to the LED
Driver. The addition of jitter through the TP to OP path is
no more than ±1.5ns.
While in the unsquelch state, the TP squelch circuit looks
for the start of idle signal at the end of the packet. When
start of idle is detected, TP squelch is turned on again. The
proper start of idle occurs when the input signal remains
above 300mV for 160ns.
FIBER OPTIC LED DRIVER
The output stage of the transmitter is a current mode
switch which develops the output light by sinking current
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:
RTSETOP =
52mA
´
115
W
I
OUT
(1)
The ML4664/ML4669 optical transmitter (in the PLCC
package only) is capable of driving up to 100mA
maximum which requires resistor RSTETOP to equal 60W.
(This option is only available with the PLCC package) The
transmitter enters the idle state when it detects start of
idle on TPINP and TPINN input pins. After detecting the
start of idle the transmitter switches to a 1MHz output
idle signal.
The output current is switched through the OPOUT pin
during the on cycle and the OPVCC pin during the off
cycle. Since the sum of the current in these two pins is
constant, OPVCC should be connected as close as
possible to the V
CC
connection for the LED.
LINK TEST FUNCTION
Transmission — Whenever data is not being delivered to
the twisted pair link, the idle signal is applied. The idle
signal is a sequence of Link Pulses separated by a 16ms
period of silence. The idle signal starts with a period of
silence after a packet transmission ends. The link test
pulse is a single high pulse with the same amplitude
requirements as the data signal.
Reception — The transceiver monitors the receive twisted
pair input for packet and link pulse activity. If neither a
packet nor a link test pulse is received for 50 to 150ms,
the transceiver enters the Link Test Fail state and inhibits
transmission and reception. Link pulses received with the
wrong polarity will be ignored and cause the chip to go
into link test fail.
When a packet, or five consecutive link test pulses is
received from the twisted pair input, the transceiver will
exit the Link Test Fail state upon transmit and receive data
being idle, and re-enable transmission and reception.
TP SQUELCH
The twisted pair receive data is transformer coupled and
low pass filtered before it is fed into the input pins TPINP
and TPINN. The input is differential with the common
mode voltage internally set. At the start of packet
reception from the twisted pair link, no more than 5 bits
are received from the twisted pair cable and not
transmitted. The first bit sent at the optical transmitter may
contain phase violations or invalid data, but all
subsequent bits are valid.
The TP squelch will reject the following signals on the
TPINP and TPINN inputs:
1. All signals that produce a peak magnitude less
than 300mV (450mV typical).
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
