The WLD3343 needs a voltage drop of at least 1.8V or so. With a long connection
cable to the laser head and the substantial voltage drop of the laser diode,
this means that an input voltage of 5V is not enough, so one needs an adjustable
power supply and not just a fixed 5V supply to run the circuit. On the other
hand, the voltage drop should be as small as possible, for minimal power dissipation.
I adjust the input voltage (ca 7.5V) such that the voltage drop is 2.3V at
2A LD current, which means 4.6W dissipation for the WLD3343; this is roughly
one-half of the allowed maximum, when using a large heat sink.
When the voltage drop over the WLD3343
is close to 1.8V, oscillation can occur with an amplitude beyond what the
laser diode can survive. I found that a 1uF capacitor across the output cures
the problem. At any rate, it is strongly advised to test any driver circuit
with a dummy load (consisting of 2 or 3 ordinary high power diodes) with an
oscilloscope for all possible operating conditions (undervoltage, turn-on
and shutdown etc), before attaching the laser diode !
By doing so I didn't see any of the advertized soft start behavior the
WLD3343
is supposed to have - after power on, the current goes up instantaneously.
A 220uF cap across the current control input (along with a 5.6K series resistor)
does produce a smooth turn-on. There is also a shutdown pin which is connected
to the LTC1923 driver board - when it senses a too high LD thermistor temperature
(or a wire interruption), the LD driver is shut down and stays down until
power is siwtched off and on again. Note that undoing the shutdown manually
is not recommended as the circuit does not soft-start this way.
The internal current monitor is not very precise, so perhaps an extra 0,1Ohm
1% shunt resistor on the negative end of the LD diode would be better. It
would also give a direct reading of the LD current in terms of mV (convenient
for using a panel meter), rather than having a screwy conversion factor.
Otherwise the circuit I used is as given in the data sheet, plus an extra
DAC/ADC pair plus a voltage reference, plus a relay for switching between
manual (pot) and digital control mode. Shunt resistor is 5W 0.47 Ohms with
extra 0.5W 3Ohms in parallel.
The digital circuitry can be activated by triggering the relay K1.It consists
of a MAX1236 ADC plus a MAX5812 DAC, both 12 bit. The MAX1236 has one channel
free which can be accessed via J-5, eg for an external temperature sensor
etc.
The connector J-3 is used for testing purposes, and also offers a shutdown
pin eg for connecting with a temperature protection circuit.
JP-3 is used to connect to an external pot for manual LD current control.
