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LM36923H: Connect the 3 sinks to drive the 3 LED strings

Elisha Tam
Prodigy 50 points
Part Number: LM36923H

I discussed with a TI application engineer and he said it is possible to connect the 3 current sinks together and then drive 3 strings in parallel. Please see attached image.

The response was: It is OK to combine the sinks together.  The boost control loop will normally regulate the current sink headroom of the highest voltage string, but with them all tied together, it will just regulate all three to the same regulated feedback voltage. 

Can you clarify what the value of the feedback voltage is and how it is set? Also if there is a voltage variation in the 3 LED strings, how does the feedback voltage handle this variance? Is there a variance range I need to design to?

Thank you,

Elisha

  • 0
    TI__Intellectual 2090 points

    Hi Elisha,

    The boost converter will set its output voltage to maintain the proper headroom voltage (Vhr) on the LEDx inputs.  The LEDx headroom voltage (Vhr) is specified in the Electrical Characteristics table on Page 5 of the datasheet.  I see that you have 12 LED in series and it is critical that the maximum forward voltage of each led be less than 3.065V.  Another important issue to realize with this configuration (three strings shorted) the current will be slightly different through each string due to the mismatch of total voltage drop in each LED string.

    Best regards,

    Randy

  • 0 in reply to Randy Dahl
    Prodigy 50 points

    Thank you for the response.

    Page 5 of the datasheet says the typical headroom voltage is 210mV for a current of 25mA. Does this mean the boost converter will set its output voltage to maintain a voltage of 210mV above voltage necessary to drive the LED strings? I'm trying to understand how headroom voltage works in this device.

    Can you describe how you calculated the max forward voltage of 3.065V per LED? The device says it can drive up to 38V, so I thought the max forward voltage of the LEDs would be 3.17V  (38V/12LEDs). My LEDs are < 3.065V but I would like to understand your calculation.

    Thank you

  • 0 in reply to Elisha Tam
    TI__Intellectual 2090 points
    Hi Elisha,

    The minimum Boost voltage can be 37V which is what I used to calculate the forward voltage of 3.065V. Yes when the current sinks are 25 ma the Boost will adjust its output voltage to maintain 210 mV on the LEDx pins. When the current sinks are 10 ma or less the Boost will adjust its output voltage to maintain 100 mV on the LEDx pins.

    Randy