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LM3463: OutP pin and primary power supply

Part Number: LM3463


with reference to Figure 27 on data sheet, how does OutP pin work?

R1, R2, 2.5 V voltage reference and comparator are inside Primary power supply or are they added externally?

In first case, Figure 27 is referring to a specific model and manufacturer for the power supply?


The most important questions are:

Can we use a power supply without a voltage feedback pin?
If yes, how should we connect OutP pin? Can we leave OutP pin unconnected?

Thank you.

  • Hello,

    A voltage regulated power supply will have a resistor divider that compares to a reference for voltage regulation via the control loop. OutP sums into the divider to be able to pull down on the feedback node to adjust the dynamic headroom. Generally if you purchase a power supply this is internal, R1, R2 and the 2.5V reference.

    The circuit is common for voltage regulating power supplies, having access to this circuit in a purchased power supply may not be easy to find. If you designed the power supply you would know where this divider is. It may also be possible to find this circuit inside of a purchased power supply.

    If the dynamic headroom is not controlled the voltage will have to be set high enough for all operating conditions. This can create excess heat in the LM3463 design lowering the efficiency whichs depends on LED string current and how much headroom has to be added. It is best if you have a power supply with access to the feedback node.


  • Supposing to use a power supply without voltage feedback pin and to manage the heat properly:

    -          Does LM3463 work anyway?

    -          How should be connected OutP pin? Can OutP pin be left unconnected?

    -          What is the best choice for RFB1 and RFB1?

    -          VDHC_READY can be set less than 5 V higher than the VLED-MAX-COLD, eg 0.6 V higher?


  • Hello,

    I have this question to another colleague at TI on not operating this as a DHC control.

    Rfb1 and Rfb2 allow the LM3463 to turn on (turn-on threshold) at 2.5 volts from the divider.  Calculate the divider to allow turn-on when the current source output voltage reaches the level you want the LM3463 to start operating.  It has 1.21 volts of hysteresis.  This is part of Figure 26 in the datasheet which assumes you are using DHC.

    The lowest Vdrain is 0.95 volts so the output range needs to be minimum to maximum string voltage plus 0.95 plus margin.  This includes all variances in LED Vf, LED temperature, operating current (if dimming or adjusting).

    0.6 volts is part of the equation to calculate the DHC resistor since there is a diode drop in series with it.  The 0.15 volts is as low as Outp can pull the pin which is also part of the equation.

  • Hello,

    Response is leave OutP open and it should work fine but you will lose efficiency.

  • Hello,

    the question about VDHC_READY arises from this consideration: using a 9 LED string with VLED-MAX-COLD = 9 x 2.6 V = 23.4 V, is it possible to use a fixed VRAIL equal to 24 V? The 0.6 V margin mentioned above is the difference between 24 and 23.4 V.
    According to the data sheet, the optimal choice for VDHC_READY is 28.4 V and for VRAIL(peak) is 33.4 V, does LM3463 work with VRAIL = 24 V instead? in this case, what is the optimal value for VDHC_READY and then for RFB1 and RFB1?

  • Not really but it could work under most operating conditions or all if that's the extreme end of the string voltage. Is that cold with maximum Vf tolerance at the highest current you will be operating at?

    VDHC_READY cannot be 28.4 volts since it will never be ready, it needs to be set lower than 24 volts. Vrail(peak) means nothing since OutP isn't being used. VDHC_READY is a resistor divider, set it to some level below 24 volts. You may run into issues with how it comes up if there isn't enough headroom. Best to try it.