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LM3421 LED driver IC...questions

Discussion in 'LEDs and Optoelectronics' started by eem2am, Jun 2, 2013.

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  1. eem2am

    eem2am

    422
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    Aug 3, 2009
    Hello,

    Please could you advise on the operation of the LM3421 LED driver IC?

    LM3421 datasheet:
    http://www.ti.com/lit/ds/snvs574e/snvs574e.pdf
    (block diagram is on page 10)

    1….Off time duration:
    Is the off time duration simply the length of time that it takes the “CT” capacitor, connected to pin 5, “RCT”, to charge up from 0V to Vin/25 Volts?
    (page 30 shows the “RCT” pin and buckboost connection.)
    I presume that during the main FET ON time, the RCT pin is held at 0V by the internal FET inside the LM3421?


    2….’Vin proportionality’ of off time:
    On page 12 it states that when the ‘RT’ resistor is connected to the switching node, the “Vin proportionality” of the off time setting means that boosts and buckboosts switch at constant frequency. However, surely there is no “Vin proportionality” to the off time setting?……..indeed, “Vin” has nothing to do with off time setting when the ‘RT’ resistor is connected to the switching node. So why does it say “vin proportionality”?
    If the ‘RT’ resistor is connected to the switching node, then when say a “buckboost” converter is on its off time, then the ‘RT’ resistor is effectively connected to the output voltage (ignoring the diode drop)….so surely it should say “vout proportionality”?

    3….RT resistor connection:
    Can you confirm that (in a buckboost converter) connecting the “RT” resistor to Vout would have the same effect as connecting the RT resistor to the switching node? (except for the fact that there would be slightly more dissipation in the RT resistor if the RT resistor is connected to Vout.)

    4….Discontinuous inductor current buckboost operation:
    Supposing I have Vin = 5V to 8V, and Vout could be any LED lamp from 5V to 100V (5W max) and I am using LM3421 in a buckboost…..at the high vout, I am going to end up with discontinuous inductor current…..i cannot simply increase the inductor size since I will get not enough dv on the source current sense resistor when the output is driving just a 5V lamp.
    Can the LM3421 handle discontinuous inductor currents?
    For the case of discontinuous inductor current, does it mean that connecting the RT resistor to the switching node is no longer possible?
    For discontinuous inductor current, does the RT resistor have to be connected to either the input voltage , or to the output voltage? (eg connecting the RT resistor to the output voltage would give a shorter off time than connecting the RT resistor to the (lower) input voltage(?)).

    5. LED flashing beacon operation:
    The nDIM pin can be used to turn off the LED driver periodically as in for flashing LED beacon operation.
    In a flashing LED beacon, when the LED is flashed OFF, its nice if the compensation capacitor “Ccmp” can be “disconnected” during the flash-OFF, since disconnection stops this compensation capacitor from discharging, and thus instigating an unwanted current surge when the beacon is flashed ON again.
    Does the compensation capacitor get “disconnected” in this way whenever the nDIM pin is taken high? (nDIM being taken high turns off the main switching FET)

    6….Current accuracy at low Vin:
    Figures 6 & 7 on page 8 of the datasheet appear to show the LED current regulation quickly worsening, and one extrapolates that at input voltages as low as 5V or 6V , the LED current regulation will be very poor indeed. Is this the case? What would the LED current accuracy have been like at Vin = 5V?

    7….Soft Start:
    I’m thinking of the lowest component count method of implementing a soft start. I am thinking that some variation of the analong dimming circuit on page 14 would be the best way?…eg as follows….

    Soft start for LM3421…
    http://i44.tinypic.com/57g5x.jpg


    Notes on LM3421
    I have noticed that the LM3421 LED driver IC is one of the only LED driver IC’s in the world which comprises both “constant off time” operation and comprises an internal error amplifier…….(it might be the world’s only one like this?)
    The internal error amplifier aids accuracy, and the constant off time is superb for driving long series LED strings as slope compensation is not needed.
     
    Last edited: Jun 2, 2013
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