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:
V(MDL) 0.5V
RMLn||RMMn =
RMMn =
=
= 11.62kΩ
IM
43µA
RMLn||RMMn ∗RMLn
RMLn −RMLn||RMMn
11.62kΩ ∗33.34kΩ
RMMn =
= 17.84kΩ
33.34kΩ −11.62kΩ
3. Calibrating the high channel
LENL and LENM must remain at high; pin LENH must
Here, pins REGEN and ENCAL must be high. This also be switched from low to high. Following the set-
deactivates the monitoring of the optical power, thus tling of the control unit at CIH the required optical
enabling the calibration of the high output level. Pins 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:
V(MDL) 0.5V
RMLn||RMMn||RMHn =
=
= 6.25kΩ
IM
80µA
RMLn||RMMn||RMHn ∗RMLn||RMMn
RMLn||RMMn −RMLn||RMMn||RMHn
RMHn =
6.25kΩ ∗11.62kΩ
RMHn =
= 13.52kΩ
11.62kΩ −6.25kΩ
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.
ICHAUS [ IC-HAUS GMBH ]
