iC-WKM

M-TYPE CW LASER DIODE DRIVER

Rev C2, Page 10/12

APPLICATION NOTES

Laser diode types

In principle, all three laser diode types can be operated

by iC-WKM.

As iC-WKM has been optimised for use with M-type

laser diodes, it is with this type of diode that the best

functionality is achieved. This type of setup also en-

ables the laser diode package to be connected to GND.

With P-type laser diodes it is also possible to connect

the laser diode package to GND. Here, however, the

current mirror at MDK introduces a certain error to the

automatic power control (c.f. Electrical Characteristics

Nos. 108, 110).

Even N-type laser diodes can be operated by iC-WKM;

iC-WKN, however, which has been optimised for use

with these diodes, may be a better choice.

Layout

To prevent instability of the regulator an additional ca-

pacitor (ca. 100 pF) must be positioned directly at the

iC’s pin CI. Depending on the laser diode, capacitor

CM with typ. 2 nF in parallel to RM may be required for

stability.

Pin AGND acts solely as a ground reference for the

power regulator (CI and RM) and should not be short-cir-

cuited with GND externally. This could have a negative

effect on the control behavior and monitor functions.

Cooling

Laser diodes should be sufficiently cooled, particularly

with the power-regulated operation of setups which use

iC-WKM, for example. The power dissipation in the

laser diode otherwise causes the diode to heat up and

the level of efficiency to drop which the power control

unit then compensates for by increasing the laser cur-

rent. This in turn brings about a rise in power dissipation

in the laser diode and thus also in temperature. In this

instance the laser diode could be damaged (resulting in

spottiness and an increase in operating current) or even

destroyed before the overcurrent shutdown threshold

in iC-WKM is reached. This applies in particular to blue

laser diodes whose power dissipation is considerably

higher than that of red laser diodes, for example, due

to the higher forward voltage.

Figure 3: Operation of an M-type laser diode

Figure 4: Operation of a P-type laser diode

Figure 5: Operation of an N-type laser diode