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7

FL591FL LASER DIODE DRIVER

THEORY OF OPERATION

The FL591FL employs Wavelength’s FL500 laser diode

control chip, and can drive a single output up to 500 mA or

dual outputs up to 250 mA each.

When operating with two separate outputs, the FL591FL can

be configured to use a single setpoint to drive both outputs

to the same current, or use separate setpoint signals to

drive each current source independently.

It may be useful to remember that you do not directly set the

drive current setpoint; instead, you adjust a voltage signal

that represents the output current. The setpoint voltage is

controlled by the onboard trimpot or by an external input.

The current source continually monitors the actual output

current, compares it to the setpoint, and adjusts the current

if there is a difference between the two signals.

As current is driven through the load, there is a voltage drop

across the load because of the impedance. As the current

increases the voltage drop may increase to the point that it

reaches the Compliance Voltage limit of the current source.

Once that occurs the current source is no longer able to

increase the current driven to the load even if you increase

the setpoint.

The current sources can be operated in Constant Current

(CC) mode or in Constant Power (CP) mode. In CP mode,

the photodiode (PD) current is used in the feedback loop,

and the driver adjusts the output current in order to keep the

PD current constant.

The laser driver includes features that help protect your

laser and make the driver more versatile in a wide array of

applications:

• The current limit is set by an onboard trimpot and

protects the laser from over-current conditions.

• Slow-start delays the current ramp by 100 msec,

and then ramps the current to setpoint at a rate

of 15 mA / msec.

• The photodiode feedback control loop allows for

Constant Power operation whereby the driver adjusts

the laser forward current in order to maintain a constant

photodiode current.

SAFETY INFORMATION

SAFE OPERATING AREA — DO NOT EXCEED

INTERNAL POWER DISSIPATION LIMITS

Before attempting to operate the FL591FL, it is imperative

that you first determine that the FL500 will operate within the

Safe Operating Area (SOA). Operating the unit outside of

the SOA may damage the controller or the laser. Operating

outside of the SOA will void the warranty.

Go to the Wavelength Electronics website for the most

accurate, up-to-date, and easy to use SOA calculators:

www.teamwavelength.com/support/design-tools/soa-ld-calculator/

www.teamwavelength.com/support/design-tools/soa-ld-calculator/

SOAcharts are included in this datasheet for quick reference

(

page 15), but we recommend you use the online tools

instead.

!

ToensuresafeoperationoftheFL591FLdriver,it

is imperative that you determine if the FL500 is

going to be operating within the internal heat

dissipation Safe Operating Area (SOA).