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