ACS405CS Semtech Corporation, ACS405CS Datasheet - Page 4

• Manufacturer Part Number: ACS405CS
• Description: Acapella Optical Modem ic
• 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.
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
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
* where Rtset > 1K , tolerance = +/- 20%.
I = Amps
*
*
LED (High)
LED (Low)
= 10 / Rtset
= 100 / Rtset
prior to applying power to the
in series
4
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 :
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 ACS4050 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 pin should then be set high to
TxMN and TxFLG
Laser Adjustment Procedure
I
* Tolerance = +/- 20%.
I = Amps
I
* Tolerance = +/- 10%.
I = Amps
*
*
PIN (High)
LASER (High)
= 2.0 / Rtset
= 100 * TxMN
ACS405CS Issue 4.3 June 1999.
is