ACS411CS datasheet(5/43 Pages) SEMTECH

5 / 43 page

ACS411CS PRE-RELEASE Issue 6.0 July 1999.

Kohms. It is important that R

RMODSET

is inserted and

adjusted to its maximum resistance value of 50

Kohms prior to applying power to the ACS9020 for

the first time and prior to following the procedure

detailed in section headed,

Laser Adjustment

Procedure.

(PMN - AVERAGE)

(BIAS + MOD) = 1/ R

RMODSET

Where R

RMODSET > 1Kohm, tolerance +/- 20%

TXMON and TxFLG

TXMON is used to monitor the current delivered to

the LED or Laser. TXMON is a current source that

proportionally mirrors the current flow through the

LED or Laser. By placing an appropriate external

resistor R

TXMON

between TXMON and GND, the

voltage developed (referenced to GND), will be

proportional to the transmit current. During the Laser

setup procedure TXMON should be monitored to

ensure that the Laser manufacturer's maximum

current specification is not exceeded.

The transmit current monitor is a current source

flowing from VDD out of pin TXMON. This current is

representative of the Laser/LED drive current.

TXMON = IBIAS/50 + IMOD/100

BIAS is the Low level bias current.

MOD is the peak Modulation level bias current. The

average modulation current is half this value.

Average drive current, I

AVG = (IBIAS + IMOD) /2

Therefore I

TXMON = IAVG/50

TXMON may also be employed during normal

operation to continuously check the Laser current.

The voltage developed across R

TXMON is compared

within an internally generated reference voltage of

1.25V. In the event that the reference voltage is

exceeded, the TXFLAG is set High, otherwise it is

set Low. In this way, the value of resistor on TXMON

can be chosen to activate TXFLAG at any desired

transmit current

e.g. If R

TXMON = 1KW,

then TXFLAG will be set if I

AVG

exceeds 62.5mA.

If desired, TXFLAG activation can be delayed by

adding a damping capacitor between TXMON and

GND.

Laser Adjustment Procedure

The output power from the Laser should be measured

with an optical power meter during the setup

procedure. In addition TXMON may be monitored to

ensure that manufacturers maximum current limits

are not exceeded during the set-up process. Select

one of the laser drive modes in accordance with the

section headed,

Optical Operational Modes.

Start be setting the current control resistors R

RMODSET

and R

RBIASSET to their highest values (at least 50Kohm

is recommended).

The bias current is then set to the desired level by

adjusting the variable resistor R

RBIASSET. Since the bias

current sets the optical low-level for the Laser, it is

essential that the Laser driver data inputs are set at

a continuous logic low level. The resistor value

(typically a 50K potentiometer) is reduced until the

desired bias current is achieved or until the desired

low-level optical output power is achieved. It should

be understood that since the bias current is fixed

(not regulated), the low level optical output power

will vary across the temperature and voltage range.

Once the bias current is set, the modulation current

maybe set by adusting the variable resistor R

RMODSET.

The automatic power regulation circuitry for the

modulation current maintains the average optical

output power and not the peak power. For this reason,

during the set-up process in the absence of the

appication data, it is recommended that the Laser

driver is stimulated with a square wave. Most

application data used in fiber optic transmission is

dc-balanced (equal number of ones and zeros), so a

square-wave is an accurate representation of the

real data.

The resistor value (typically a 50K potenmtiometer)

is reduced until the desired optical-high ouput power

is achieved. The modulation ouput power will then

be regulated such that the average ouput optical

output power (bias + modulation) is mainatined over

the recommended temperature and voltage range.

Receive Monitor RXMON and RXFLAG

The ACS9020 incorporates a power meter which

generates a current source on the RXMON pin, which

is proportional

to the received signal strength. A

voltage is generated on an internal 50Kohm resistor

which is continuously compared with an internally

generated reference of 1.25 volts.

The RXFLAG is set when the RXMON voltage

exceeds the 1.25 volt reference. The flag is used to

indicate that there is sufficient signal strength to give

a minimum differential output signal on the receiver

output pins DOUTP and DOUTN. If the voltage on

DOUTP/DOUTN exceeds 500 mV peak-to-peak then

the RXMON voltage will exceed 1.25 Volts and the

RXFLAG will be set.

Because of process tolerances on the internal resistor

and the internally generated reference voltage, the

RXFLAG should be considered only as a guide to

the receive signal strength. The receive threshold

can be adjusted by placing a 1Mohm external

potentiometer between the RXMON pin and Ground.

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