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Current used by LED's

Jim Mahaney
Prodigy 110 points
Other Parts Discussed in Thread: DLPC300, TPS63020

I'm trying to figure out how much current is being sent to the LED's.  According to the schematic, the LEDDRV SENSE is the source for each of the MOSFETs which connect to the LEDs.  The last resistor before the MOSFETs is located at position R45, but I cannot find any information about it and the actual resistor on the board has no labeling.  Is there a components list somewhere?

  • 0
    TI__Genius 9935 points

    Jim,

    The BOM for the DLP LightCrafter can be found in the DLP LightCrafter EVM Design Documentation v2.0 which can be downloaded by going to http://www.ti.com/tool/dlplightcrafter and scrolling down the page.


  • 0
    TI__Expert 6995 points

    JIm,

    The LightCrafter LED driver follows the concepts of the LED driver discussed in Section 3 of this app note:

    http://www.ti.com/lit/an/slva419b/slva419b.pdf

    However, the DLPC300 controls a PWM circuit that injects voltage at the feedback node of the regulator.

    As shown in the LightCrafter User's Guide, the current to the LED is roughly:

    LED Current (mA) = 1.8 * (LED PWM Value) + 140

    So for a PWM setting of 274, the LED current is 633mA

    R45 is a zero ohm resistor. The schematics have all the component values. R45 is meant to allow disconnection of the feedback network. R46 is shunt resistor that sinks the LED current whose voltage is being monitored. The Bias voltage of 16V  and resistors dividers is used to feed a constant bias current into the feedback network that feeds the high impedance feedback pin of the TPS63020. This in combination with the PWM voltage and its resistor dividers, controls the LED current. I have created a simple model of the circuit using an excel spreadsheet to show how the PWM and feedback resistors operate. This model is quite simple and does not include all the capacitances and resistances of the FETs, LEDs, and other components. This model is meant as an educational exercise to illustrate the behavior of the LED driver. Note that only one LED is active at any point in time. The equation above was created from actual measurements of different LED PWM values.

    2703.LightCrafter_LED_Driver_Model.xls