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LM3644: pulse driver,

Yang zhang
Prodigy 110 points
Part Number: LM3644

I want to pulse a LD, whose vf is about 2-2.4V, at 1.5A~2A.

The pulse is 30Hz-200Hz, 10% duty cycle.

Input voltage is 3.3V or 5V.

First, I wonder whether LM3644 acts as LD driver here if series connect a diode with the LD to fullfil the head room？

Then, is there any better advice ？

  • 0
    TI__Intellectual 2140 points
    Hi,

    The LM3644 will operate in pass mode when VIN > (VLED + VHR + ILOAD*0.15 Ohm), where VHR is approximately 300mV and 0.15 is the approximate internal PMOS resistance of the boost plus the resistance of the inductor. The only issue when operating in pass mode is the thermal dissipation due to the excess power dissipated across the current source. At VIN = 5V, IOUT = 2A and Vf = 2.2V, the power dissipation is 2A*4.7V = 9.4W. (4.7V is VIN - LOAD*0.15 Ohm). This is indeed very high power consumption so your proposed solution of connecting diode(s) in series with the LD is a good one. The LM3644 drives a current source, so it won't have any issues driving this load. Also, keep in mind that since these will be a pulsed events, heating due to power dissipation will be much lower than long flash duration dissipation. You can try increasing the duty cycle until thermal shutdown is triggered to get an idea how close you might be to triggering thermal shutdown in your application.

    Regards,
    Angelos Tsiros
  • 0 in reply to Angelos Tsiros
    TI__Intellectual 2140 points
    Hi,

    I realized that in my calculations above I made a mistake. The power consumed on the current source is not 2A*4.7V but 2A*(4.7V - 2.2V) = 5W. Still, this is very high power consumption. Everything else in my precious response applies.

    Regards,
    Angelos Tsiros
  • 0 in reply to Angelos Tsiros
    Prodigy 110 points
    Thanks for your reply. If the power supply only can offer 500mA, can I realize a pulse driver circuit for a laser-diode? Should the pulse width have to be set to ns level ?
  • 0 in reply to Yang zhang
    TI__Intellectual 2140 points
    This will depend on the input capacitor you are using. I would suspect that you need very high input capacitance to provide the required energy to drive the output with (1 - 2)A. You can shorten the pulse duration but you cannot go down to a few ns because it takes a little time for the current to ramp up and down. I suggest that you try it. You cannot damage the device under these conditions, it is well protected.