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Analog Technologies

ATLS25A219D

32V 25A Constant Current Laser Driver

loop is not working properly, there might be a short or open

circuit at the laser diode, or the laser driver is put into Standby

or Shutdown mode. The LPGD pin can also be connected to

a digital input pin of a micro-driver, when software/firmware

is utilized in the system. See Figure 6. The equivalent circuit

of this pin is a 3k

Ω resistor pulling it up to 4.5V rail and an

open drain FET, 500

Ω, 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 17 PGLDC.

Figure 9. Digitally Controlled Analog Modulation Principle

Minimizing the Output Noise

The ATLS25A219D 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 SDBN 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 SDBN pin.

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

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

on the VPS pin.

When not controlling by the SBDN 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 3 and 2. See Figure 6. The

output current will be:

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

Figure 7. 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 4.096V, the output

current is 25A.

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

measured by a multi-meter.