Chapter 4 Ld2000R Overview; Slow Start Circuit; Limit Current Circuit; Constant Power Feedback Loop - THORLABS EK2000 User Manual

OEM Laser Diode Driver Evaluation Kit

Chapter 4 LD2000R Overview

The LD2000R is composed of three independent circuits: slow start circuit, limit
current circuit, and output control circuit. Each is described below.
4.1.

Slow Start Circuit

The slow start circuit is used to monitor the supply voltage and keep the laser
output off until the power supply stabilizes. The slow start circuit uses a voltage
reference and a comparator to monitor the supply voltage. An internal 2.5 V
reference is compared to the voltage at the ON/OFF pin (pin 17). When this
voltage exceeds 2.5 V, the laser is enabled. The comparator input (pin 17,
ON/OFF) has an input impedance of 20 kΩ. This resistance is used with an
external resistor to form a voltage divider that sets the LD2000R dropout voltage.
For most applications a 15 kΩ resistor tied from the 12 V power supply to the
ON/OFF pin which disables the laser when the power supply drops below 4.5 V
is adequate.
Note, the ON/OFF pin can also be used to disable the laser by pulling this pin low
to 0 V.
The slow start circuit uses an internal time constant formed by a 1 MΩ and a
1 µF capacitor to yield a 50 ms turn on delay. This can be extended by adding an
external capacitor to the SLOW_START pin.
4.2.

Limit Current Circuit

The limit current circuit is a constant current source which can be set by the on-
board trim pot or an external control voltage. This determines the maximum drive
current that can be supplied to the laser. The transfer function for this control is
40 mA/V. The current limit also determines the laser current when operating in
the constant current mode.
4.3.

Constant Power Feedback Loop

The constant power feedback loop circuit uses the laser monitor photodiode
current (which is proportional to the laser output power) to regulate the laser
output power. An internal transimpedance amplifier converts the photodiode
current to a voltage used by the feedback circuit. The feedback loop varies the
drive current to the laser such that the voltage derived from the photodiode
monitor current matches an adjustable setpoint voltage (described below). The
laser output can be adjusted by varying the setpoint voltage.
When the current limit is set higher than the laser current needed by the
feedback loop the laser is operating in a constant power mode. If the current
needed by the feedback loop is higher than the current limit, the laser drive
current will be clipped to the current limit and the laser will then be operating in
the constant current mode.
2468-D01
Chapter 4: LD2000R Overview
Page 10