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Op-amp/feedback selection: Constant current LED driver using switching regulator IC

Discussion in 'Electronic Design' started by Joel Kolstad, Oct 27, 2003.

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  1. Joel Kolstad

    Joel Kolstad Guest

    I'm attempting to design a small, constant current LED driver using a Linear
    Technologies LTC1174HV switching power supplied controller IC. I'm using a
    Luxeon LED that can take up to 375mA of drive current and attempting to use
    this as a reading light at a cabin that has a 12V solar power system --
    hence the desire for high efficiency with the switching regulator. I have a
    question about which op-amp I should be able to use and if I might need
    additional feedback compensation.

    The schematic is here:

    The op-amp is used to allow the variable current selection as well as to
    reduce the power dissapated by the shunt resistors that measure the current
    through the LED (the LTC1174 uses a reference voltage of 1.25V). I first
    attempted to use an LT1490 op-amp because it boasts rail to rail output and
    very low power consumption. Unfortunately, it didn't work so well, with the
    switching output of the regulator pretty wildly jumping around -- the
    current through the LED tended to jump around wildly (I initially used a 10
    ohm power resistor in place of the LED for testing -- the LED is rather
    spendy!). I therefore decided to just build a constant voltage power supply
    using the LT1490 as a unity gain follower coming from a resistive devider --
    this didn't work either! I changed the LT1490 for an LM358 (that I had
    lying around) and finally did achieve a nice, constant square wave output at
    the LTC1174's 'switch' pin. At this point I began to wonder what the
    problem with the LT1490 might be... the only thing I can think of is that it
    has a UGBW of 200kHz. I didn't think this would be a problem, since I'm not
    interested in the transient response of this circuit as the load will always
    be the same... but I'm now thinking that an op-amp that slow perhaps allows
    the LTC1174 to initially overshoot its target... followed by
    undershooting... etc., and therefore the entire system is unstable. (The
    LTC1174 is a 'constant off-time' switcher with a 4us off-time. It
    automatically switches to a voltage-driven hysteretic mode -- 'burst mode'
    well under light loads.)

    Anyway, thinking I had the problem identified, I went back to the constant
    current configuration as shown in the schematic. With the LM358, it "pretty
    much" works in that the switch pin runs for awhile... then stops... and then
    runs again. The current through the LED is relatively constant, so I think
    what's happening is that the system still overshoots... the LTC1174 waits
    for the voltage to drop to something 'reasonable' again... and then the
    cycle repeats. Under heavier loads, there's a combination of the 'several
    switching cycles-brief pause' as well as longer pauses inbetween these.

    My feeling is that the LTC1174 is getting confused between whether it should
    be in 'burst mode' or its regular mode and this makes the entire circuit
    somewhat less than optimally. However -- with the LM356 -- it does seem to
    be workable.

    I've stuck some waveforms up as well: -- This is under a constant
    current of ~75mA. Notice the switch pin is on twce in rapid succession (no
    4us gap here!), turns off for awhile, the current through the catch diode
    stops about 7us later, and then everything 'sleeps' for about 35us before
    the process repeats. This is a constant current
    of ~335mA. Similar to the above, but there are some really short spiky
    turn-ons of the switch as well. Strange...

    Anyway, I'm wondering... what would a good op-amp be to use? Is it the lack
    of speed on the LT1490's part that caused the initial troubles? I'm very
    much wanting a single supply op-amp. Its output doesn't need to go all the
    way down to 0V, as it'll normally be sitting right around 1.25V (the
    LTC1174's reference voltage).

    Is there any easy way to introduce some filtering somewhere to get the
    switch output to be a 'nice' square wave? Should I even bother trying?
    Putting a capacitor across the op-amp's feedback resistor to drop the high
    frequency response doesn't really improve things much, nor does placing one
    from the op-amp's output to ground.

    Thanks in advance for any assistance!

    ---Joel Kolstad
  2. GPG

    GPG Guest

    Try shifting filter to output of amp

    -|-\ ___
    | >--|___|| Pin1
    -|+/ ---
    |/| ---
    created by Andy´s ASCII-Circuit v1.24.140803 Beta
  3. GPG

    GPG Guest

    The led does not need cap filter, from look at sheet pin1 does (close to chip)
    Let me know
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