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4

Analog Technologies

ATLS100MA103

Low Noise Constant Current Laser Controller

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 LILM, pin 4. The output current will be:

LILM 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:

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 LILM 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 filter 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 large signal

of up to 1MHz 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 0.71 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 170nS (Large signal).

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 7 shows such a circuit. The

resistor R1 can be between 10k and 1M, depending on the error

voltage caused by the switch leakage current. The LILM pin

can be set by a POT as shown in Figure 4 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.

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

the LIS is modulated. The bandwidth of the LIO signal

is >10MHz, more than enough for monitoring output current

modulated by the LIS signal.

(V)

Normal

Operation

Region

3.3

5

0.35

4.6

2.6