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Analog Technologies

ATLS500MA106

Low Noise Constant Current Laser Controller

Turning the Controller On and Off

The controller can be turned on and off by setting the SBDN

pin high and lower respectively. It is recommended to turn

the controller on by this sequence:

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

supply voltage to the controller, turn on the controller by

releasing the SBDN pin.

To turn off: turn off the controller 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 controller by the power

supply.

In Figure 4.1, S1 is the shut down switch. The internal

equivalent input circuit of SBDN pin is a pull-up resistor of

100k being connected to VPS in parallel with a 10pF

capacitor to the ground. The switch S1 can also be an

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

with an either open drain or push/pull output. If not using a

switch (S1) to control the laser, leave the SBDN pin

unconnected. D2 is an LED, indicating when the control

loop works properly, that is: the output current equals to the

input set value. This pin has an internal pull up resistor of 5k

to the power supply pin, VPS, pin 10. The pull down

resistance is 200

Ω. This 5k resistor can drive a high

efficiency LED directly. When higher pull up current is

needed for driving such as a higher current LED, an external

resistor can be placed between the VPS and the LPGD pins.

Make sure that the resistor is not too small that the pull

down resistor will not be able to pull the pin low enough

when the controller loop is not good. When choosing not to

use an LED for indicating the working status, leave the

LPGD pin unconnected.

The LPGD pin can also be connected to a digital input pin

of a micro-controller, when software/firmware is utilized in

the system.

Setting the Output Current

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

input voltages of LIMS, pin 4. The output current will be:

× LIMS (V)/2.5V (mA).

LIMS should never be left float. Otherwise, the output current

limit may be set to too high a value that the laser might be

damaged.

The output current is set by adjusting W2, which sets input

voltages of LIS, pin 5. The output current will be:

× LIS (V)/2.5V (mA).

When no modulation is needed, it is suggested to use an RC

low-pass-filter, the R1 and C1 in Figure 4.1, to lower the AC

noise from the voltage reference source. The time constant of

this filter can be between a few to 10’s of seconds. The bigger

the time cost, the lower the output noise, but the longer time

will be needed to wait the output current to go up.

Both of LIMS and LIS, only LIS, can be configured by using a

DAC, to replace the W1 and W2 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, similar as shown in Figure 4.1.

The LIS allows modulating the output current by a signal of up

to 100kHz in bandwidth. That is, when using a sine wave

signal to modulate the LIS pin, the output current response

curve will be attenuated by 3dB, or 0.707 times the full

response magnitude in current. When using an ideal square-

wave to modulate the output current at the LIS pin, the rise and

fall time of the output current will be about 1.5

μS.

When the modulation signal is a square-wave and low output

noise is require, the low-pass-filter can still be used for

lowering the output noise. Figure 5 shows such a circuit. The

resistor R1 can be 10k to 1M, depending on the error voltage

caused by the switch leakage current. The LIMS pin can be set

by a POT as shown in Figure 5 or connect to 2.5VR.

It is recommended not to set the LIS pin to 0V, but keep

it >0.05V at all the time. The reason is that the laser diode

usually has a junction voltage of 2.5V, when setting the LIS

pin voltage to 0V, the output voltage will warble between 0V

and 2.5V, cause some oscillation slightly.