3

■Block diagram

Pin

Name

Content

AMD

LD monitor input connected to anode of monitoring diode

CMP

phase compensation input of phase compensation capacitor for control feedback loop stabilization

ILM

LD current limit

input of detection of laser diode driver drive current limit

KLD

LD drive output

connected to cathode of laser diode

VCC

power output

power input of IC

GND

ground

ground of IC

Pin description

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.

DRIVER

CONTROL

KLD

VREF

ILD

VCC

CMP

AMD

GND

DRIVER

NPN TR

CONTROL

AMP

+

-

ILM

VREF

LIMIT

CURRENT

LIMIT CMP

10 -