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PWMing of White LED LCD back light

Discussion in 'Electronic Design' started by Sylvain Munaut, Jul 23, 2005.

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  1. Hello,

    I have a CPU with programmable PWM output and a STN LCD with witheled
    back lighting. All the whiteleds are in // so it's powered by classic
    5V and draw about 150mA (the current limiting resistors are included in
    the panel).

    Currently, I turn on and off the panel with something like this :

    5V ----------------.____,--------- LCD BackLight +
    | ----
    \ |
    R / |
    1k \ |
    / |
    PWM Inp _/\/\_|/ NPN
    |\ BC???

    What I'd like to do is to control the intensity of the backlighting
    by modulating the input. It doesn't need to be linear, nor precise,
    just no artefact visible to the naked eyes. The PWM frequency output
    can be from about 1Mhz to about 5kHz and from 0 to 100% duty in at
    least 64 steps.

    I know that for a simple led, I can just directly use that to PWM and it
    works fine. But for the LCD backlight I'd like to know if inducing such
    a discontinuous current might have on the life span on the back light
    (for a simple led I don't care but for a LCD back light, that matters ;)

    I was thinking of adding a simple LC filter like :

    _|_ |
    /_\ |
    | ###
    / ### C
    \ R |
    / |
    | |
    _|_ _|_

    My question ares :
    - Any one has some experience to share in this domain ?
    - Is the LC filter necessary ?
    - Will the proposed filter be OK ? Is the Diode really necessary ?
    - Finally, what advice could you give in choosing the parameters :
    pwm frequency, L C and R values ...

    Thanks for any insight you might have,

  2. Guest

    The LC filter probably isn't necessary. In fact, it could create a
    problem. Depending on the PWM frequency, the LC component values along with
    their parasitic components, and at some on/off ratios, you could get a higher
    output voltage across the LED than the 5 volts you are switching.

    The safest thing to do is to simply PWM the supply voltage without any

  3. It is very normal procedure to dim LED backlights with a PWM. Don't filter.
    Just switch the FET at a few kHz.

  4. Mochuelo

    Mochuelo Guest

    First, you don't need the NPN. If your MCU is fed also with 5 V, the
    PWM output can directly command the PMOS. You don't even need a
    resistor. You might want to include it, in series with the gate of the
    PMOS, if you were concerned about radiation (EMI). The PMOS must have
    a gate threshold voltage lower than 5 V (as it was also in your
    circuit). Just take into account that the LEDs will be on while your
    MCU output is 0. No problem at all. However, if you still want to
    complement it, most MCUs with PWM allow you to invert the output
    inside the MCU itself.
    Right. Ignoring interaction with the scan rate of the LCD, any pulse
    repetition frequency (PRF) of the PWM signal higher than about 1 kHz
    is ok. I have been designing circuits for lighting products based on
    high-power LEDs (1.2 W/LED), and using 3.9 kHz as the PRF, and no
    problem. The frequency is high enough not to be noticed by our eyes
    (even if you move your sight with respect to the source of light), and
    low enough to be able to use very small MOSFETs (with very low total
    gate charge). However, I would encourage you to take advantage of more
    duty ratio steps. I was using 256. At the low-light side of the range,
    the (linear) steps appear to be larger to our sight.
    No, no, don't worry. If you guarantee that there is no 't' for which
    v(t) or i(t) is going to exceed the ratings, the LED does not care
    about the shape of the excitation. Most LEDs in this world are being
    excited with rectangular waves.

    No, forget about it. In fact, delivering the average voltage to the
    LED would be worse in terms of chromaticity. It is better to send him
    the full rectangular wave. I can explain this better, but just as a
    summary: the instantaneous current flowing through an LED determines
    not only the luminous flux, but also the color of the light.
    No. It is worse if you use it.
    PRF between 1 kHz and 5 kHz is probably best. You may need to choose
    some especific frequency related to the LED scan rate, not to have a
    bad interaction.
  5. Hello,

    Ok, first thank you all for you answers.

    See some comment below about Mochuelo answers.

    No, the MCU is a Au1100 (32bit MIPS) and has 3.3V IO and when in sleep
    or early boot phase it's output might be Tristated or low and the
    backlight must be off.

    In fact I said 64 as a worst-case value. It depends on the divider.
    Basically I have a 6Mhz clock that I can divide up to 4096 by setting a
    divider. Then I can choose until which step it's off or on. So by
    setting the frequency at 1.5 kHz, I would have up to 4096 steps to
    choose from.

    Great, good to know, even easier then ;p

  6. Mochuelo

    Mochuelo Guest

    Ok, if the MCU IO is 3.3 V and the backlight input is 0 to 5 V, it
    makes sense using an extra transistor. However, if you have access to
    the positive and the negative node of the backlight subcircuit (i.e.,
    if it is not internally grounded), I would use a single NMOS (with a
    gate threshold voltage lower than 3.3 V) and a high-ohmic resistor
    from gate to GND. Source to GND; drain to backlight negative;
    backlight positive to 5 V. The resistor would guarantee that the gate
    is low when your MCU is in sleep mode (regardless on whether the PWM
    output is high-z or low).
  7. In this case, even 100% PWM would be OK because of the limiting resistors.
    If you are driving LEDs directly, you do need to be careful about the peak
    current. The specs I found on the Nichia website for a white LED on show a maximum
    pulse current of 100 mA, and the I/V curve shows this limit to be exceeded
    above about 4.7 VDC. Other color LEDs have a lower forward drop and probably
    would need current limiting if driven from a 5 VDC supply with PWM. The
    inductor for current limiting would be a good idea if power consumption were
    a major issue.

    Paul E. Schoen, President
    P S Technology, Inc.
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