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Analog Technologies

Low Noise Constant Current Laser Controller

ATLS500MA103

To Avoid Output Current Distortions

The laser diode’s forward voltage and current has a non-linear

relationship shown in Figure 9.1 and 9.2. It can be seen that

when the current is low, the voltage is uncertain, it can be

between 0V to 1V or more. Thus, when setting the output

current to zero, the output voltage will oscillate between 0V

to about 1V or 1.5V, depending on the wavelength of the

laser diode. If we set the lowest output current to a non-zero

value, such as 1/10 of the laser’s operating current but lower

than the laser’s threshold current, the laser’s optical beam can

still be cut

−off, but the output voltage will not oscillate, thus

the output current will not have distortions. The status of the

LIO is similar as the Figure 10 shown without output current

distortions.

t

0

t

0

The laser’s threshold current is shown in Figure 11. It can be

seen that when the laser’s current fall below a certain value,

there is no output optical power. For example, the operating

current and threshold current of a red laser diode of 650nm

are 30mA and 20mA respectively and the optical output

power is 4mW. It will have no optical output power if the

output current of this laser diode is lower than 20mA which is

its threshold current. Figure 12 and Figure 13 will describe

t

0

Monitoring the Output Current

The output current of the controller can be monitored by

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

useful for micro-controller 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:

× 5(V).