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5

Analog Technologies

ATLS3A212

High Voltage Constant Current 3A Laser Driver

OPERATION PRINCIPLE

The block diagram of the driver is shown in Figure 3. The

signal from pin 2, SBDN, is sent to a level detector circuit.

As shown in Figure 4, upon detecting signal level between

0V to 0.4V, the shutdown output is activated, it shuts down

the whole laser driver and drive the laser driver into

Shutdown Mode; upon detecting the level to be between

2.1V and 2.4V, the standby signal is activated, it puts the

controller into Standby Mode; when the signal is between

2.6V and above, the controller is driven to Operation Mode.

Figure 4. Input Control

At pin 9, TMO, its value comes from a temperature sensor.

The voltage at this pin reflects the internal temperature of

this driver. The relationship between the output voltage and

the temperature is shown in Figure 5.

There is a temperature protection circuit, upon detecting the

temperature to be >120

°C, it will force the laser driver into

Standby Mode. The laser driver reinitiates the power up

sequence when the junction temperature drops below 110°C.

The voltage reference circuit provides internal voltage

reference for the driver, its output is taken out after a noise

removal circuit at 2.5VR port, pin 5.

At pin 6 and pin 7, there are 2 ports for controlling the

output current: LISL and LISH. First one sets the output

current when PCN, Pulse Control, pin 3, is at low level, 0V

to 0.4V; second sets the output current when PCN pin is at

high level, 1.4V to 4V.

On PCN, pin 3, there is a 10M resistor tied to 4V. Therefore,

if leaving this pin open, it is set to 4V, a high logic level,

thus, LISH is in control. There is a 20k

Ω resistor in series with

a diode, connected between PCN pin and the 4V internal

voltage. When the PCN pin voltage is above 4V, the 20k

resistor pulls down the current on PCN pin. The circuit is

shown at Figure 3. The waveforms of LISH, LISL, PCN and

LIO are shown in Figure 6.

Both LISH and LISL pin set the output current without any

offset voltage, i.e., when LISH or LISL pin voltage is 0V, it

sets the output current to be 0A; when the LISH or LISL

voltage is 2.5V, it sets the output current is 1A. The

relationship between the voltage and the output current is:

VLISL represents the voltage on the LISH or LISL pin

respectively, in Volts.

The LIO port, pin 8, outputs an analog voltage that is

proportional to the actual output current. When the output

current is 0A, the LIO voltage is 0V; when output current is 3A,

the LIO voltage is 2.5V. The relationship is:

The waveforms of LIO vs. LISH, LISL and PCN are shown in

Figure 6.

The output stage is designed to achieve low noise, high

efficiency, and relatively high modulation speed. It has an over

current protection circuit. There is a soft start circuit which

increases the output current slowly at the start up time and

shuts down the current quickly.

The LPGD pin indicates the control loop status. When this pin

goes high, >2V, the control loop is working properly, i.e., the

when this pin goes low, <0.3V, the laser driver is not working

properly, there might be a short or open circuit at the output, or

the laser driver is protected by the over temperature protection

circuit.