iC-NZ

FAIL-SAFE LASER DIODE DRIVER

Rev B2, Page 11/20

Demo board

With an optical power of 3 mW the monitor current (IM) is 43 µA. This current must flow through the parallel

connection of RMLn and RMMn (RMMn = PMM + RMM). The following then applies:

RMLn||RMMn =

V (MDL)

=

0.5 V

43 µA

= 11.62 k Ω

RMMn =

RMLn||RMMn ∗ RMLn

RMLn − RMLn||RMMn

RMMn =

11.62 k Ω ∗ 33.34 k Ω

33.34 k Ω − 11.62 k Ω

= 17.84 k Ω

3. Calibrating the high channel

Here, pins REGEN and ENCAL must be high. This

deactivates the monitoring of the optical power, thus

enabling the calibration of the high output level. Pins

LENL and LENM must remain at high; pin LENH must

also be switched from low to high. Following the set-

tling of the control unit at CIH the required optical

power can be set at resistor RMH.

Demo board

With an optical power of 5 mW laser diode HL6339G has a typical monitor current (IM) of 80 µA. The following

value is then obtained for resistor RMHn = PMH + RMH:

RMLn||RMMn||RMHn =

V (MDL)

=

0.5 V

80 µA

= 6.25 k Ω

RMHn =

RMLn||RMMn||RMHn ∗ RMLn||RMMn

RMLn||RMMn − RMLn||RMMn||RMHn

RMHn =

6.25 k Ω ∗ 11.62 k Ω

11.62 k Ω − 6.25 k Ω

= 13.52 k Ω

Possible sources of error

• V(NERR) remains at low:

– Check the current in NERR. Is the impedance of the pull-up resistor too high?

• Too high a current is flowing at pin NERR (ERROR state):

– Is the laser diode properly connected and the control unit functioning correctly?

– Are RMDL, RMDM or RMDH of too low an impedance? (Possible shut-down due to overcurrent!)

– Are all the capacitors at CIx (x: L, M, H) large enough and have they been properly soldered?

– Is pin AGND connected to ground?

• No laser light:

– Is pin AGND connected to ground?

– Is there a short circuit between CIx and ground?

– Is the laser diode (type N or P) properly connected and the control unit functioning correctly?

– The laser diode lights up for a few seconds and then turns off:

* Is the laser diode too hot? With insufficient cooling the laser diode power consumption increases

which can trigger an overcurrent shut-down.