ACS411CS datasheet(4/43 Pages) SEMTECH

4 / 43 page

ACS411CS PRE-RELEASE Issue 6.0 July 1999.

Wave Division

Multiplexing Device

Laser

PIN Diode

LAP

LAN

PINP/VP

PINN/VN

PMN

Single

Fiber

WDM device containing 3-pin Laser and PIN.

If the device does not have an integrated TIA then

the PIN Diodes's Cathode/Anode is connected to

PINP/PINN respectively with PINRX set High.

Control of LED Current

To minimise the switching delay, a permenent bias

current is maintained through the LED. A second

current source called the modulation current varies

the intensity of the output light power such that:

Optical Low current = Bias current.

Optical high current = Bias current + Modulation current

Unlike Lasers, LED's have a linear relationship

between current and output light power. Also, the

output power of LEDs does not vary significantly with

temperature. Therefore, LEDs are driven with a

predetermined biased current and modulation current

fixed by the resistors between RBIASET and GND

and RMODSET and GND respectively. In order to

fix the modulation current the signal MODFIX should

be set = High.

The bias current is determined by a resistor

connected between pin RBIASSET and Ground.The

bias current can be calculated from the formula

below:

(LAN) (BIAS) = 50/ RRBIASSET

Where R

RBIASSET

> 1Kohm, tolerance +/- 20%

I = Amps

The modulation current is determined by a resistor

connected between pin RMODSET and Ground.The

modulation current can be calculated from the formula

below:

(LAN) (MOD) = 100/ RRMODSET

Where R

RMODSET

> 1Kohm, tolerance +/- 20%

I = Amps

When setting the bias current and the modulation

current it is important to ensure that the sum of the

component currents do not exceed 100 mA.

(LAN)

(BIAS)

(MOD)

<= 100 mA

The bias current and modulation currents should be

set to give the appropraite extinction ratio. The

extinction ratio is the ratio of the optical high power

compared to the optical low power.

Eg. An extinction ratio of 13db, is where the optical

high power is 20 times the optical low power.

Control of LASER Current

To minimise switching the delay, a permenent bias

current is maintained through the LASER. A second

current source called the modulation current varies

the intensity of the output light power such that:

Optical Low current = Bias current.

Optical high current = Bias current + Modulation current

For Lasers, there is a non-linear relationship between

the output power and the applied current. In addition,

Laser output power will vary significantly with

temperarure for a constant current. For these reason

Laser drive current must be 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 (typically leaked from the

rear facet of the laser itself) to a monitor current,

which is directly compared to a preset programmed

current (the current flowing through RMODSET). The

Laser drive current is automatically adjusted to

maintain the original preset light level over the

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 bias current is set in the same way as it is for

the LED driver. The bias current is determined by a

resistor connected between pin RBIASSET and GND.

The bias current can be calculated from the formula

below:

(LAN) (BIAS) = 50/ RRBIASSET

Where R

RBIASSET

> 1Kohm, tolerance +/- 20%

I = Amps

Whilst the bias current flowing through the Laser is

fixed, the modulated component is automatically

regulated to maintain a near constant output light

power. In order to activate the automatic regulation

of the modulation current it is important that the pin

MODFIX is set Low.

The monitor-pin current is set by a variable resistor

RMODSET) connected

between pin RMODSET and

Ground. Acapella recommends that R

RMODSET should

comprise a logarithmic potentiometer of value 50

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