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LP5861T: Higher voltage for green laser diode using LP5861T

Paul Barker
Prodigy 265 points
Part Number: LP5861T


Tool/software:

Hello, I have an application where I will need to connect either LEDs or low power RGB laser diodes to the LP5861T.  For the laser diode case there is no problem with red or blue since they both need well below +5V to operate.  However most green laser diodes require more like 6 to 7 volts to operate.  Would I be able to power the green laser diode channel with a +7V rail voltage and then put a resistor to GND on the drive channel to ensure that even when off there will be a small amount of current that forward biases the Green laser diode so that the voltage on the output drive channel never goes above +5.5V.  I could also use a Zenor diode for extra protection. 

All it takes is a few milliamps on the green laser diode to create a 3 volt drop across the diode so the voltage at the output drive pin would never be higher than 7 - 3 = 4Volts.  I realize this will cause the built in LED open/short function to maybe not work properly but that is not a problem for my application.  I know there are other ICs I could use but the bit depth on the Global / Color Bank / and Individual Current control are features I need to accurately control the output current.  Will this be OK to do?

  • 0
    TI__Mastermind 20515 points

    Hi Paul,

    I'm probably clear about what you think, but I would like to say that if the LED is short-circuited, a voltage of 7V is applied directly to the output pin, which should cause the output pin to break. An I right?

    BR, Jared

  • 0 in reply to Jared Zhou
    TI__Prodigy 265 points

    Yes, you are correct if the laser diode completely shorts then the 7V would be directly applied to the output pin.  I'm not expecting to the green laser diode to become a complete short but I should research if this is possible or not.

    Most of the green laser diodes i'm looking at using should not need more than 5.5Volts to operate at the low power levels we will be using them.  However, i want a backup plan just in case we have to use one that does need a little more voltage.  If I put a low value current limiting resistor in series with the laser diode (ex. 10 ohm) and a 5.5 v Zener diode from the output pin to GND this would protect the output in the rare case the green laser diode completely shorts out to 0 ohms.  I don't think i will need this circuit but will design it in the board as a backup plan.

  • 0 in reply to Paul Barker
    TI__Genius 15986 points

    Hi Paul,

    Our expert is out of office today and will reply to you later.

    BR, Gracie

  • +1 in reply to Paul Barker
    TI__Mastermind 20515 points

    Hi Paul,

    Ok, I got you mean. You want to add some resistor to parallel just like the circuit you attached to guarantee the output will not suffer over than 6V voltage. Am I right, if in this case, I think it can work.

    BR, Jared