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4

Analog Technologies

ATLS500MA201D

Constant Current Laser Driver

Figure 4.1 and 4.2 shows a typical stand-alone application

circuit.

In Figure 4.1, the switch S1 is external shut down switch,

which can turn on and off the driver with the SDN pin high

and lower respectively, at the internal chip control

input: >1.4V = enable, <0.95V = shut down, normal

threshold voltage = 1.2V. The switch S1 can also be an

electronic switch, such as an I/O pin of a micro-driver, with

an either open drain or push/pull output. See Figure 5. If not

using a switch (S1) to control the laser, leave the SDN pin

unconnected.

In Figure 4.1, the LED D1 is used to indicating laser diode

status. When LDGD pin is high, >2V, the laser diode control

loop is working properly. When LDGD pin is low, <0.3V, the

laser diode is bad, or there is a short or open circuit at the laser

diode. The LDGD pin can also be connected to a digital input

pin of a micro-driver, when software/firmware is utilized in the

system. See Figure 5.

Figure 5 shows a typical micro-processor-based application

circuit.

To ADC & DAC

To ADC

To ADC

To micro-controller

To signal GND

To power GND

+5V

LIO

6

VPS 12

PGND 10

LIGD 8

LDA

9

GND

3

2.5VR

4

LIS

5

PGND

11

GND

7

LDGD

2

SDN

1

Laser Driver

To DAC

To signal GND

To micro-controller

A

K

Laser diode

D2

Figure 5. A Typical Micro-processor-based Application

In Figure 4.1, the adjustable resistor W1 is used to setting the

output current. Setting LIS from 0V to 2.5V will set the laser

current from 0A to 500mA linearly.

The laser diode D2 is connected between LDA and LDC. It is

worth mentioning that the power supply return terminal

should be connected to the pin 11 PGND and the cathode of

the laser diode should be connected to the pin 10 LDC. These

2 nodes should not be connected together externally and they

are connected together internally already by the driver.

Turning the Driver On and Off

The driver can be turned on and off by setting the SDN pin

high and lower respectively. It is recommended to turn the

driver on by this sequence:

To turn on: turn on the power by providing the power supply

voltage to the driver, turn on the driver by releasing the SDN

pin.

To turn off: turn off the driver by lowering the voltage of

SDN pin, turn off the power by stopping the voltage supply

on the VPS pin.

When not controlling by the SDN pin: leave it unconnected

and turn on and off the driver by the power supply.

Adjusting the Output Current

The output current is set by adjusting W1, which sets input

voltages of LIS, pin 5. See Figure 4.1. The output current will

be:

LIS can be configured by using a DAC, to replace the W1 in

Figure 4.1. Make sure that the DAC has output low noise, or,

if no modulation is needed, an RC low pass filtered by be

inserted between the DAC and the LIS pin. See Figure 5.

The LIO can still be used to monitor the output current when

the LIS is adjusted. 0V to 2.5V indicates the laser current of

from 0A to 500mA linearly.

Monitoring the Output Current

The output current of the driver can be monitored by

measuring the voltage on the LIO pin. This feature is very

useful for micro-driver based system where the ADC is

available and monitoring the current in real time is required.

This pin provides a very low noise voltage signal which is

proportional to the output current:

For example, when the output signal equals to 2.5V, the

output current is 500mA.

LIO can be used to drive an ADC directly, and also be

measured by a multimeter during debugging process.

Figure 6 shows the relationship between Pin VPS and LDA.

LDC