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© Copyrights 2000 – 2006, Analog Technologies, Inc. All Rights Reserved. Updated on 1/23/2006
2
Analog Technologies
ATLS1A103
Low Noise Constant Current Laser Driver
APPLICATIONS
Figure 3 shows a typical application circuit. W1 and W2 set
the output current limit and output current respectively.
Resistor R1 and capacitor C1 form a low pass filters, to
lower the noise from the voltage reference.
Laser diode D1 is connected between LDA and PGND. It is
worth mentioning that the power supply return terminal
should be connected to the pin 11 PGND and the cathode of
the laser diode should be connected to the pin 12 PGND.
These 2 nodes should not be connected together externally
and they are connected together internally already by the
controller.
power supply 0V
D2
LED
2
W1
20K
(Clock-wise)
Power Supply 5V
S1
SPST
D1
Laser Diode
Shut Down
Loop Good Indication
Current limit set
LIO
6
VPS
12
PGND
10
LPGD
7
LDA
9
GND
2
LIS
5
PGND
11
TMPO
8
2P5V
3
LILM
4
SDN
1
ATLS1A103
2
W2
20K
1
2
R1
1M
1
2
C1
1uF to 10uF
Current set
To ADC
To ADC
(Clock-wise)
Figure 3 A Typical Stand-alone Application Schematic
Turning the Controller On and Off
The controller can be turned on and off by setting the SDN
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 SDN pin.
To turn off: turn off the controller by lowering the voltage of
SDN pin, turn off the power by stopping the voltage supply
on the VPS pin.
When not controlling by the SDN pin: leave it unconnected
and turn on and off the controller by the power supply.
In Figure 3, S1 is the shut down switch. The internal
equivalent input circuit of SDN 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 SDN 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 LILM, pin 4. The output current will be:
I_output = 1.1
×LILM(V)/2.5V (A).
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:
I_output = LIS(V)/2.5V (A).
When no modulation is needed, it is suggested to use an RC
low-pass-filter, the R1 and C1 in Figure 3, 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 3. 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 3.
The LIS allows modulating the output current by a signal of up
to 350KHz in bandwidth. That is, when using a sinewave
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 1
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 4 shows such a circuit. The
resistor R1 can be between 10K to 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 3 or connect to 2P5V.
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.
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