ELM185xB Laser Diode Driver
■Block diagram
VCC
CMP
KLD
ILM
DRIVER
CONTROL
DRIVER
NPN TR
CONTROL
CURRENT
LIMIT CMP
AMP
-
AMD
GND
+
+
-
VREF
ILD
VREF
LIMIT
Pin description
Pin
AMD
CMP
ILM
Name
Content
LD monitor input connected to anode of monitoring diode
phase compensation input of phase compensation capacitor for control feedback loop stabilization
LD current limit input of detection of laser diode driver drive current limit
LD drive output connected to cathode of laser diode
KLD
VCC
GND
power output
ground
power input of IC
ground of IC
Description :
The internal APC circuit of ELM185xB controls the output of laser oscillation at constant value with the
feedback of monitor photo diode light output current. Monitor photo diode light current will be changed to
voltage by resistance and becomes the input of AMD; this voltage is set to be 0.3V, which is the same as the
reference voltage inside IC. It is possible to acquire constant laser output by the laser diode drive current which
is controlled by this voltage; hence, laser light output can be adjusted by the resistor connected to AMD.
Laser diode drive current can be changed to voltage by resistance connected to ILM, and the increase of this
voltage is limited under current limit voltage (Typ.0.15V). ILM is also capable of avoiding the damage of
overmuch drive current of laser diode which is caused by control loop abnormality.
For stability of control return loop, a phase compensation capacitor is required to be connected to CMP. Please
set to small value at least above 3nF when making ON/OFF control with fast speed.
Laser diode is driven by the open collector output of NPN transistor inside ELM185xB; therefore, drive current
(Max.18V) higher than power voltage of IC can be added as drive voltage of laser diode. ELM recommends
inserting resistance between laser diode module (LDM) and KLD if the drive voltage is high so as to disperse
power loss.
By monitor current detection function of ELM185xB, AMD can be used as ON/OFF control pin. It is also
possible to control switching of laser diode when the input source is from external control signal; the ON/OFF
control will become 0.3V threshold voltage.
By 7V voltage, the Zener diode between power and ground of ELM185xB is able to absorb power surge;
meanwhile, the Zener diode is also capable of decreasing the damage of surge by clipping the surge voltage of
power line.
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ELM-TECH [ ELM Technology Corporation ]
