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6

Analog Technologies

ATLS25A219GUI

32V 25A Constant Current Laser Driver

OPERATION PRINCIPLE

The block diagram of the driver is shown in Figure 6. The

signal from pin 14, SBDN, is sent to a level detector circuit.

As shown in Figure 7, upon detecting signal level between

0V to 2V, the shutdown output is activated; it shuts down the

whole laser driver and drives the laser driver into Shutdown

Mode; upon detecting the level to be between 2.2V and 2.4V,

the standby signal is activated, it put the controller into

Standby Mode; when the signal is between 2.7V and above,

the controller is driven to Operation Mode.

Figure 7. Input Control

At pin 14, TMPO, 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 8.

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 4VR port, pin 9.

At pin 12 and pin 11, there are 2 ports for controlling the

output current: LISL and LISH. The former sets the output

current when PCN, Pulse Control, pin 13, is at low level, 0V

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

high level, 1.4V to 15V.

On PCN, pin 13, there is a 10M resistor tied to 5V. Therefore, if

leaving this pin open, it is set to 5V, 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 5V internal voltage. When

the PCN pin voltage is above 5V, the 20k

Ω resistor pulls down

the current on PCN pin. The circuit is shown at Figure 6. The

waveforms of LISH, LISL, PCN and LIO are shown in Figure 9.

Both LISH and LISL pin set the output current without any

offset voltage. The relationship between the voltage and the

output current is:

respectively, in volt.

The LIO port, pin 15, outputs an analog voltage that is

proportional to the actual output current. When the output

current is 0A, the LIO voltage is 0.1V; when output current is

15A, the LIO voltage is 2.5V. The relationship is:

The waveform of LIO vs. LISH, LISL and PCN is shown in

Figure 12.

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

output current equals to the desired value, LISH or LISL = LIO

voltage; 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.