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Thursday 28 August 2008
Components
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How to select the right white LED driver
Thursday 22 May 2008
The growing adoption of LEDs as a lighting source in different applications has simultaneously driven the demand for LED driver ICs to power them.
To understand the obstacles for the design and manufacture of these LED driver ICs, it is necessary to understand what a white LED requires in order to produce light. A white LED must be driven by a constant current source so that the white point of the light does not shift (that is, it must be uniform).
Furthermore, since the white LED is a diode, its internal forward voltage (Vf) drop has to be overcome. This Vf varies with the current rating of the white LED and will also change with temperature. A typical 20mA white LED has a Vf that varies between 2.5V and 3.9V over the entire operating temperature range.
Most applications use more than one white LED and can also have these LEDs configured in parallel, in series, or a combination of both – for example parallel strings of LEDs in series.
This means that white LED driver ICs must be capable of delivering sufficient current and voltage for the specific configuration of LEDs, and in a conversion topology which satisfies both the input voltage range and required output voltage and current requirements.
White LEDs versus RBGs
A RGB LED actually expands the range of visible light when compared to a cold cathode fluorescent light (CCFL), which have a limited color spectrum and lack color vividness. CCFLs exhibit about 80 per cent of the National Television System Committee (NTSC) defined colours while RGB can reveal up to 110 per cent of the NTSC colour spectrum – enabling a more accurate representation of images on the screen. Furthermore, the largest possible colour spectrum is achieved by using three monochromatic light sources such as red, blue and green lasers.
Meanwhile, white LED backlighting is well suited for handheld and mobile display panels since they have small form factors, are simpler to drive, are less sensitive to mechanical stress and have twice the life expectancy when compared to CCFL. However, white LEDs share the same disadvantage in colour spectrum as CCFL because a white LED is equivalent to a broadband light source.
A white LED is a blue diode covered with phosphor to convert a portion of the blue light to yellow light. This combined spectrum is perceived as white light.
Still, RGB LEDs come closer to delivering a narrow-band spectrum at a fraction of the cost of monochromatic light sources. Not only do RGB LEDs improve the color spectrum, but they also improve efficiency as well because RGB LEDs only emit optical energy as needed – red, green and blue.
Broadband light sources such as white LEDs and CCFL have a relatively high presence of unwanted colors that deteriorate the color spectrum and therefore cause a loss in efficiency. Since individual colors can be driven independently, the white point, or the color temperature of an RGB LED, can be corrected – whereas both CCFLs and white LEDs have a fixed white point.
In some instances, even though more RGB LEDs and therefore RGB LED drivers might be needed to backlight a display when compared to white LEDs, the color spectrum and more accurate color image can be worth the additional design complexity to a system designer.
It is justifiable to use RGB LEDs instead of white LEDs due to the expanded color spectrum that significantly improves picture quality because consumers will pay a premium for color vividness when deciding between LCD TV models.
Nevertheless, using RGB LEDs involves a larger, more complex and more costly solution. Thus, in applications where the expanded color spectrum is not going to allow the end product to have a price premium, the white LED solution as the backlighting source is an acceptable one.
For example, Linear Technology’s LTC3219 is an inductorless LED driver which provides nine individually configurable “universal” current sources for Main, Sub, AUX and RGB displays in mobile phones.
Display currents are set via a precision internal current reference. The universal current sources can be digitally controlled with independent dimming, brightness, blinking, and gradation control, programmable via a simple two-wire I2C serial interface.
The device’s multimode charge pump features low-noise constant-frequency operation, automatically optimising efficiency based on the voltages across the LED current sources. The device powers up in 1x mode and automatically switches to boost mode (1.5x) when any enabled LED current source approaches dropout; a subsequent dropout switches the device into doubler (2x) mode. Internal circuitry prevents inrush current and excessive input noise during start-up and mode switching.
Another inductorless charge pumps is the LTC3220/-1. This IC provides 18 individually configurable “universal” current sources for Main, Sub, AUX and RGB displays in mobile phones and for other general-purpose lighting.
Tony Armstrong is product marketing manager for power products at Linear Technology
To understand the obstacles for the design and manufacture of these LED driver ICs, it is necessary to understand what a white LED requires in order to produce light. A white LED must be driven by a constant current source so that the white point of the light does not shift (that is, it must be uniform).
Furthermore, since the white LED is a diode, its internal forward voltage (Vf) drop has to be overcome. This Vf varies with the current rating of the white LED and will also change with temperature. A typical 20mA white LED has a Vf that varies between 2.5V and 3.9V over the entire operating temperature range.
Most applications use more than one white LED and can also have these LEDs configured in parallel, in series, or a combination of both – for example parallel strings of LEDs in series.
This means that white LED driver ICs must be capable of delivering sufficient current and voltage for the specific configuration of LEDs, and in a conversion topology which satisfies both the input voltage range and required output voltage and current requirements.
White LEDs versus RBGs
A RGB LED actually expands the range of visible light when compared to a cold cathode fluorescent light (CCFL), which have a limited color spectrum and lack color vividness. CCFLs exhibit about 80 per cent of the National Television System Committee (NTSC) defined colours while RGB can reveal up to 110 per cent of the NTSC colour spectrum – enabling a more accurate representation of images on the screen. Furthermore, the largest possible colour spectrum is achieved by using three monochromatic light sources such as red, blue and green lasers.
Meanwhile, white LED backlighting is well suited for handheld and mobile display panels since they have small form factors, are simpler to drive, are less sensitive to mechanical stress and have twice the life expectancy when compared to CCFL. However, white LEDs share the same disadvantage in colour spectrum as CCFL because a white LED is equivalent to a broadband light source.
A white LED is a blue diode covered with phosphor to convert a portion of the blue light to yellow light. This combined spectrum is perceived as white light.
Still, RGB LEDs come closer to delivering a narrow-band spectrum at a fraction of the cost of monochromatic light sources. Not only do RGB LEDs improve the color spectrum, but they also improve efficiency as well because RGB LEDs only emit optical energy as needed – red, green and blue.
Broadband light sources such as white LEDs and CCFL have a relatively high presence of unwanted colors that deteriorate the color spectrum and therefore cause a loss in efficiency. Since individual colors can be driven independently, the white point, or the color temperature of an RGB LED, can be corrected – whereas both CCFLs and white LEDs have a fixed white point.
In some instances, even though more RGB LEDs and therefore RGB LED drivers might be needed to backlight a display when compared to white LEDs, the color spectrum and more accurate color image can be worth the additional design complexity to a system designer.
It is justifiable to use RGB LEDs instead of white LEDs due to the expanded color spectrum that significantly improves picture quality because consumers will pay a premium for color vividness when deciding between LCD TV models.
Nevertheless, using RGB LEDs involves a larger, more complex and more costly solution. Thus, in applications where the expanded color spectrum is not going to allow the end product to have a price premium, the white LED solution as the backlighting source is an acceptable one.
For example, Linear Technology’s LTC3219 is an inductorless LED driver which provides nine individually configurable “universal” current sources for Main, Sub, AUX and RGB displays in mobile phones.
Display currents are set via a precision internal current reference. The universal current sources can be digitally controlled with independent dimming, brightness, blinking, and gradation control, programmable via a simple two-wire I2C serial interface.
The device’s multimode charge pump features low-noise constant-frequency operation, automatically optimising efficiency based on the voltages across the LED current sources. The device powers up in 1x mode and automatically switches to boost mode (1.5x) when any enabled LED current source approaches dropout; a subsequent dropout switches the device into doubler (2x) mode. Internal circuitry prevents inrush current and excessive input noise during start-up and mode switching.
Another inductorless charge pumps is the LTC3220/-1. This IC provides 18 individually configurable “universal” current sources for Main, Sub, AUX and RGB displays in mobile phones and for other general-purpose lighting.
Tony Armstrong is product marketing manager for power products at Linear Technology
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