ACS406CS Semtech Corporation, ACS406CS Datasheet - Page 4

• Manufacturer Part Number: ACS406CS
• Description: Two-chip Fiber Optic Modem Ics / ACS4060 100-pin Tqfp, ACS9010 44-pin TQFP
• Manufacturer: Semtech Corporation

During the transmit cycle, the LED drive current is
directly proportional to the programmed reference
current. To minimise switching delay, a permanent
bias current is maintained through the LED. The data
modulation produces bursts of high light and low
light to represent logic high and low levels
respectively. The low light level is set by a current
equal to 10% of the drive current generated for the
high light level. Since LEDs have a linear relationship
between current and optical output power, this results
in a optical high power of 10 times the optical low
power.
The optical power is controlled by a variable resistor
(Rtset) connected between TSET and Ground (GND).
The lower the value of Rtset the greater the current,
for this reason the lower limit for Rtset should be
1K , which equates to a nominal peak transmit
current of 100mA. A practical maximum for Rtset is
50K . Acapella, therefore recommends that Rtset
should comprise a fixed resistor of 1K
with a logarithmic potentiometer of 50K , thus,
affording overload protection for the LED together
with precise control of current.
* where Rtset > 1K , tolerance = +/- 20%.
I = Amps
During the transmit cycle, the Laser drive current is
controlled so as to maintain a constant optical output
power from the Laser. The monitor-pin resident in
the Laser converts the incident light power (from the
Laser itself) to a monitor-current, which is directly
compared to a preset programmed current (the
current flowing through TSET). The Laser drive
current is automatically adjusted to maintain the
original preset light level over the recommended
temperature and voltage range. The designer should
be aware that whilst the control loop maintains the
current generated by the monitor-pin within a
tolerance of 2%, there is additional uncertainty
attributed to the monitor-pin's temperature coefficient
of responsivity. Data relating to the Laser
characteristics should be acquired from the Laser
supplier.
The monitor-pin current is set by a variable resistor
(Rtset), connected between TSET and Ground
(GND). Acapella recommends that Rtset should
comprise a logarithmic potentiometer of value 50K .
It is important to ensure that the Rtset resistor is
inserted and adjusted to its maximum resistance
value of 50K prior to applying power to the ACS9010
for the first time and prior to following the procedure
detailed in section headed, Laser Adjustment
Procedure.
Control of LED current
Control of Laser Current
I
I
*
*
LED (High)
LED (Low)
= 10 / Rtset
= 100 / Rtset
in series
4
* Tolerance = +/- 20%.
I = Amps
TxMN is used to monitor the current delivered to the
LED or Laser. TxMN is a current source that
proportionally mirrors the current flow through the
LED or Laser. By placing an appropriate resistor
(Rtxmn) between TxMN and GND, the voltage
developed (referenced to GND), will be proportional
to the transmit current. During the Laser setup
procedure TxMN should be monitored to ensure that
the Laser manufacturer's maximum current
specification is not exceeded (see section headed
Laser Adjustment Procedure ).
TxMN may also be employed during normal operation
to continuously check the Laser current. The voltage
developed across Rtxmn is compared within an
internally generated reference voltage of 1.25V. In
the event that the reference voltage is exceeded, the
TxFLG is set high, otherwise it is set low.
The relationship between the transmit current and
the TxMN current is :
* Tolerance = +/- 10%.
I = Amps
If Rtxmn = 1K , then TxFLG = 1 when the Laser
current exceeds 125mA.
The output power from the Laser should be measured
with a optical power meter during the setup
procedure.
Select one of the Laser drive modes in accordance
with section headed, Operational Modes . Start with
the highest resistance for Rtset (at least 50K
recommended). This defines the monitor-pin
reference current. Select the Laser setup mode on
the ACS4060 by asserting SETB low, this forces the
Laser into a continuous transmit mode and applies
dc-balanced data (equal number of ones and zeros)
to the Txdat input of the ACS9010 device.
As Rtset is reduced in value, the current to the Laser
will increase. The current to the Laser should be
monitored at the TxMN in accordance with section
headed, TxMN and TxFLAG .
The Laser current will be stable when the Laser starts
to lase, and produces enough optical power for the
PIN feedback current to equal the current flowing
through Rtset.
The Rtset impedance should be decreased until the
desired Laser output high-power is achieved. Once
the setup procedure has been completed, the SETB
TxMN and TxFLG
Laser Adjustment Procedure
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I
*
*
PIN (High)
LASER (High)
= 2.0 / Rtset
= 100 * TxMN
ACS406CS Issue 1.6 January 1999.
is