IC-NZ Datasheet, PDF (15/20 Pages) IC-Haus GmbH – FAIL-SAFE LASER DIODE DRIVER

iC-NZ
FAIL-SAFE LASER DIODE DRIVER
SINGLE-FAILURE PROTECTION
Rev B2, Page 15/20
Maintaining the laser class and the single-failure pro-
tection feature are achieved by means of a second
monitor diode and by monitoring the laser current. Fur-
ther safety is provided by the layout of the laser driver
stages. The output stages consist of several identical
driver cells where each cell can discharge the integra-
tion capacitor CIx (x: L, M, H) and shut down the laser
current. So that the laser can be switched off in the
event of error iC-NZ has three completely separate cir-
cuits. The second diode monitors whether the laser
light is within a predetermined power range or not.
Calibrating the safety monitor diode
To calibrate the safety monitor diode iC-NZ is first ac-
tivated via EN (low → high) with ENCAL at low. Once
the device self-test has been successfully carried out
(NERR is high) pin ENCAL must be set to high and
the system switched to maximum optical power (pins
LENL, LENM and LENH switched to high). Resistor
RSMD at pin SMD is now set so that voltage V(SMD)
is inside the monitor window between 0.3 V and 0.7 V.
ENCAL can now be switched back to low.
In applications with a low duty cycle both the peak and
average laser power must be monitored to maintain
the laser class. To this end the resistor at RSMD is
split into RSMD1 and RSMD2 and a capacitor (CSMD)
connected in parallel to RSMD1 (see Figure 12). With
short laser light pulses capacitor CSMD behaves as if
it is a short circuit and bypasses RSMD1. In this in-
stance only RSMD2 is active (higher shutdown thresh-
old). With longer pulses resistors RSMD1 and RSMD2
are in series (lower shutdown threshold). This setup
dynamically alters the monitor window. An excess of
the maximum permissible laser power through both
too high a pulse power or too long a pulse duration
is equally detected.
VDD
VDD
C1
RSI
RSI
EN
SMD
RSMD2
CSMD
RSMD1
SMD
RGND
Current Monitor
Safety Monitor Diode
VSAH
VSAL
+
SMD(1)
−
+
SMD(0)
−
iC−NZ
SDIS
REGEN
LENL LENM LENH
NSEN
Finite State Machine
S1
NSEN
LDAOK
NTREN
VREF
+
MDL
−
Laser Driver
APC
MDOK
MDL
Monitor
VREF
+
−
VREF
+
−
GND
RVDD
LDA
CLDA
MDL
RMDM
RMDH
LDKH
CIH
CIH
LDKL
CIM
CIM
LDKL
CIL
CIL
AGND
RMDH
RMDM RMDL
LD MD
Figure 12: Dynamic adaptation of the monitor window
Demo-Board
So that the second monitor diode can be used jumper
J1 must be placed between pins 1 and 2 on the demo
board. The voltage at SMD is set using the two poten-
tiometers PSMDH (peak) and PSMDL (average).
The overcurrent monitor at LDA is, however, still ac-
tive in this mode. This ensures safe operation with so
called low-power applications – even without a second
monitor diode. Figures 13 and 14 give the circuitry for
N- and P-type laser diodes.
Operation without a second monitor diode
The monitoring function can be disabled for applica-
tions which do not require a second monitor diode. To
this end pins SMD and ENCAL must be set to high.
This no safety function (NSF) is signalled at pin NSF.
Demo-Board
So that the demo board can be operated without a sec-
ond monitor diode, jumper J1 must be placed between
pins 2 and 3.