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Analog Technologies

ATLS4A217

High Voltage Constant Current 4A Laser Driver

pulling it down to the ground. The pull-up current can be

increased by connecting an external pull-up resistor between

LPGD and VPS. Tie this added additional pull up resistor to

a 3.3V or 5V power supply if they are available. Make sure

that the pull up current is not too high, otherwise, the internal

open drain FET cannot pull-down the LPGD pin low enough

to turn off the LED.

The laser diodes are connected between LDA and LDC pins. It

is worth mentioning that the power supply return terminal

should be connected to the pin 12 PGND.

Figure 8. Digitally Controlled Analog Modulation Principle

Minimizing the Output Noise

The ATLS4A217 has extremely low output noise; care must

be taken in using this laser driver to achieve this lower noise

performance. One of the noise sources is the beating

frequency interference from other power mode electronics,

such as another laser driver, TEC (Thermo-Electric Cooler)

controller, or power supply. There are 2 ways to avoid the

frequency beating: set the switching frequencies of other

electronics to be far away from the switching frequency of

the laser driver, 700kHz, or synchronize the other

electronics switching with this laser driver. For this purpose,

there is a SYNO pin, which can be used by other electronics

as the switching trigger signal.

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

high and low 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 on VPS pin, 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 LISL and LISH, pin 6 and 7. See Figure 6. The

output current will be:

LIS can also be set by using a DAC to replace the W1 in

Figure 6. Make sure that the DAC has low output noise.

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 and is

proportional to the output current:

For example, when the output signal is 2.5V, the output

current is 4A.

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

measured by a multi-meter.

Driver Power Consumption

The best way for arranging the heat sinking for the driver is

as follows: transferring the heat by sandwiching a piece of

thermal conductive pad between the top metal surface of the

laser driver and the internal metal surface of the final

product as shown in Figure 9.1 and 9.2 below. The

recommended thickness of the thermal conductive pad in

Figure 9.1 is 1~4mm, and in Figure 9.2 is 0.5mm. ATI also

provides a series of thermal conductive pads, click here for

more information.