Matrix LED dimmer helps LED lights to achieve bright colors

LED semiconductor lighting RGB LED strings are used in projectors, buildings, displays, stage and automotive lighting systems because such systems require high efficiency and bright light output. An RGB LED string produces the desired color, with each LED (red, green, and blue) requiring independent and accurate dimming control. High-end systems can use an optical feedback loop to allow the microcontroller to adjust the RGB LED strings to provide accurate color. Adding a white LED to the RGB LED string to form an RGBW LED string increases the color, saturation, and brightness values ​​available in the color illumination system. Four of the LEDs in each RGBW LED string require accurate dimming. Two RGBW LED strings require eight "channels".

There are many ways to drive RGBW LED strings for color and brightness control. One way to drive the RGBWLED string and adjust its brightness is to use four separate LED drivers for four colors (R, G, B, and W), as shown in Figure 1a. In systems employing this approach, the current (or PWM dimming) of each individual LED or LED string is driven by separate drivers and control signals. However, in this type of solution, the number of LED drivers increases rapidly as the number of RGBW LED strings increases. Any lighting system that uses a large number of RGBWLED strings requires a large number of drivers, and the amount of control for synchronizing the control signals of these drivers is also large.

A much simpler (and more convenient) approach is to drive all LEDs at a fixed current with a single driver/converter while PWM dimming each LED with a parallel power MOSFET matrix for brightness control. The matrix dimmer and single LED driver as shown in Figure 1b reduces the circuit size of the solution of Figure 1a. In addition, using a single communication bus to control the matrix LED dimmer makes the RGBWLED color mixing system relatively simple and compact, while driving large current RGBWLED strings, color and brightness control is also very accurate.

Figure 1a

Figure 1b

Figures 1a and 1b: (1a) In a high-power color mixing application, 8 separate LED drivers and PWM signals can be used to drive two RGBW LED strings, or (1b) a single boost with serial communication capability - Step-down LED drivers and matrix LED dimmers enable a much smaller and more compact solution.

The LT3965 matrix LED dimmer can be used to achieve this design, as shown in Figure 5. Each of the LT3965's eight switch matrix dimmers can be paired with a full two RGBW LED strings, allowing each LED to be individually controlled between zero and 100% brightness in 1/256 PWM steps (red, The brightness of green, blue and white light). The two-wire I2C serial interface command provides color and brightness control for all eight channels. The I2C serial interface code provided to the matrix LED dimmer IC determines the brightness status of all eight LEDs and can be used to check for open or shorted LEDs in the event of a fault.

Since each LED in the RGBW LED string is designed as a single point source, red, green, blue, and white light combine to produce a wide variety of colors, and saturation, color, and brightness are controllable. With the high-speed LT3965 matrix dimmer, the brightness of each LED can be set between zero (0/265) and 100% (256/256) brightness in 1/256 dimming steps.

Accurate 0~256 level RGBW color and brightness control

RGBWLED produces accurate color and brightness by individually PWM dimming the red, green, blue, and white LEDs in the RGBW LED string. Separate PWM brightness control supports dimming ratios of 256:1 or higher. Another way to replace PWM dimming is to simply reduce the drive current of each LED, but this method affects accuracy, so only a 10:1 dimming ratio is allowed, and this method causes the LED itself to produce color. Offset. The matrix dimming method using PWM dimming has higher color and brightness accuracy than the method of reducing the driving current.

The bandwidth and transient response of the LED driver (providing 500mA LED current) can affect color accuracy. The compact step-up converter of Figure 5 has a crossover frequency higher than 10 kHz, with little or no output capacitors, and the switch can be turned on or off as the matrix dimmer turns on or off. The change in the number of LEDs responds quickly.

To illustrate how fast fast response is important to accuracy, we run red, green, and blue LEDs separately at different PWM duty cycles and measure the light output of these LEDs with an RGB light sensor. The results in Figure 3 show that the slope of each color is consistent from 4/256 to 256/256, with the slope slightly changing below this range. Of course, the color performance of red, green and blue LEDs is not perfect, so even when driving only one color LED, some colors will leak out of other bands. However, in general, this is a highly accurate system.

Figure 2: PWM dimming and phase-modulating the current of a 500mA RGBW LED string with the LT3965 matrix dimmer to produce a variety of colors and illumination patterns. The LT3952 step-up converter/ LED driver is very easy to keep up with the rapid changes in LED voltage when performing separate PWM dimming of individual LEDs.

Figure 3: Control of the brightness of red, green, blue, and white light as the PWM dimming duty cycle varies from 0/256–256/256. The PWM dimming duty cycle is controlled by a matrix LED dimmer that is paired with the LT3952 boost-buck LED driver, as shown in Figure 5.

When using a very large bandwidth (>40kHz) buck converter LED driver, the accuracy of the 1/256PWM dimming range can be improved, but to do so, or to add another boost converter to provide A stable and higher than 30V output voltage, thus increasing the cost, or an input voltage source higher than 30V. Unless very high accuracy is required at very low light output, there is no reason to add an additional converter and abandon the general-purpose, simple and compact step-up converter in Figure 5.

The matrix dimming RGBW LED color mixer system described here implements a very wide color gamut, as shown in Figure 4. Adding extra colors, such as amber, can further extend the color gamut. The RGBWA LED string (including an amber LED) can produce deep yellow and deep orange that RGBW LED strings cannot produce. These LEDs can also be driven with a matrix dimmer, but two RGBW LEDs are well matched to an 8-channel matrix dimmer.

Figure 4: The RGB LED string provides a wide color gamut. One way to simplify the color mixing algorithm is to add white LEDs. In some hybrid methods, white LEDs are used to change saturation while setting colors with red, green, and blue LEDs.

The 256-level dimming method of the LT3965 is very easy to correspond to typical RGB shading programs and common color mixing algorithms. For example, if you open a standard PC shader, you'll see that color blending is done with a 256-value RGB system, as shown in Figure 6. As another example, the LED current waveform in Figure 2 produces purple light using an RGBW matrix LED system that is controlled by a basic PC shading program. Since the design described herein produces accurate current drive and PWM control, the RGBW LED string can be color calibrated as desired by adjusting the duty cycle of each LED to easily counteract the inherent LED brightness variations.

Figure 5: The LT3965 matrix LED dimmer is used with the LT3952 boost-buck LED driver to control the color of each of the two 500mARGBWLED strings to control color and illumination patterns in serial communication.

(6a)

(6b)

(6c)

Figure 6: Color can be selected using the standard PC color picker. The 256 values ​​(0-256) used by the matrix dimmer can correspond to 0–255 used in typical RGB systems. For example, RGB (128, 10, 128) produces purple light. As seen in the photos above, matrix dimmers can make a real RGBWLED string produce the desired color, simplifying the work of the lighting designer. (6a) Select a color. (6b) Corresponding to the dimming ratio of the LT3965 LED matrix dimmer. (6c) Set the value of the dimming ratio with a PC, and then you can see the result.

Matrix LED color mixer with boost-buck driver

Matrix dimmers require a suitable LED driver to be able to power a string of 8 LEDs with multiple inputs, such as a standard 12V ± 10% power supply, 9V to 16V (automotive battery) or 6V to 8.4V (lithium ion) battery). One such driver solution is the LT3952 step-up LED driver , from input to LED, which increases or decreases voltage while providing low ripple input and output current. In the device's floating output topology, the output capacitor has little or no output capacitor, so the device can respond quickly to changes in LED voltage when PWM dimming individual LEDs in an on-off mode to control color and brightness. (figure 2).

The LT3952500mA boost-buck LED driver shown in Figure 5 is used with the 8-switch matrix LED dimmer LT3965 and two RGBW 500mA LED strings. This new step-up topology can operate smoothly from 0V to 25V output voltage range when the number of series LEDs varies from 0 to 8. The instantaneous voltage of the series LED changes over time, and how it changes depends on which and how many LEDs the matrix dimmer is activated and disabled at any given instant. The converter/topology's 60V OUT voltage (the sum of VIN and VLED) and converter duty cycle are specified for the entire input range of 6V to 20V and the output range of 0V to 25V/500mA (series LED voltage).

The matrix dimmer uses a parallel power MOSFET to shunt the LEDs to control the brightness of the LEDs. Whether it is a floating output boost-buck LED driver or a matrix LED dimmer, LED grounding is not required. As long as the VIN pin of the LT3965 is connected to SKYHOOK, all parallel MOSFETs can operate normally and the voltage is at least 7.1V higher than LED+. The SKYHOOK voltage can be generated by a charge pump consisting of a switching converter or by a stable power supply. Of course, the supply voltage should be at least 7.1V higher than the expected LED+max voltage (in this case, the maximum value of 20V VIN plus Upper 25V LED maximum). The SKYHOOK voltage is a good choice for the tiny LT8330 boost converter in a 3mm x 2mm DFN package.

An optional external clock device is used to synchronize the system at 350kHz, which is suitable for automotive environments because of its relatively high efficiency and allows for the use of compact components. Although this system can also operate at 2MHz (higher than the AM band), when the matrix dimmer shorts all LEDs and the LED string voltage drops to 330mΩ 500mA 8 = 1.3V , 350kHz (below the AM band) This boost-buck converter can perform the regulation function without using the pulse skip mode. This frequency also supports high dimming ratios without visible LED flicker.

Start sequence when the LED is turned on or off

The matrix LED dimmer system can be set to start when all LEDs are on or off. If enabled when all LEDs are off, the brightness of these LEDs can be gradually ramped up, or with a set color and brightness, such as 10% brightness green-blue. If all of the LEDs are enabled at 500mA full scale current before the serial communication system issues a command to instruct the dimmer what to do, then bright full "white" light may be seen before serial communication is initiated.

In either case, the LT3965 should be powered up before receiving an I2C serial communication command, otherwise the initial communication command may be lost when the device is powered on (POR). POR occurs when the EN/UVLO pin goes up through the 1.2V threshold. Since this voltage is based on the fact that SKYHOOK is at least 7.1V higher than LED+, it can happen any time with a high SKYHOOK voltage, such as 55V with a small boost regulator, or from the LT3952 switch node. The charge pump voltage is high enough to provide the SKYHOOK voltage. In the case where the SKYHOOK voltage is supplied by the charge pump, there may be an LED current before the charge pump supplies the SKYHOOK voltage, so the LED will illuminate before the LT3965's switch turns off the LED. This is a simple solution that designers can use when they want the LED to turn on at maximum brightness.

For the LED to start working, there must be a high SKYHOOK voltage before the LT3952 turns on. As shown in Figure 6, if the PWM pin is held low at startup, the LT3952 will not start until an external signal commands the device to start, such as by the primary microcontroller. Once the SKYHOOK voltage is present, the microcontroller can send an I2C setup command to the LT3965, setting the LT3965's switch to the LEDOFF position, after which current will flow to these switches. Once setup is complete, the LT3952PWM can be asserted and the current begins to flow through the already shorted LT3965 switch, which is turned off. After that, the start of the brightness ramp appears, or the LT3965 dimmer may jump to a specific color or brightness.

Upon reset, the LT3952's PWM must be pulled low again to turn it off and restart at the LED off position. In the case shown in Figure 5, a simple micropower boost converter such as the LT8330 provides a 55V output over a 6V to 20V input voltage range. By asserting the ALERT flag, the microcontroller receives a signal indicating that the LT3965 is powered up and ready to receive serial communication commands. Before any switch is shorted, the current through the LED is zero because the voltage across the switch is zero. This state is interpreted and reported as a short circuit fault. This flag will only be confirmed after powering up the LT3965 via SKYHOOK.

in conclusion

The LT3965 matrix LED dimmer can be paired with a boost-buck LED driver to form a color-accurate RGBWLED color mixer system. The LT3952 can be used to drive two RGBWLED strings at a 350kHz switching frequency and 500mA current from a 6V to 20V input range. This general purpose system can be powered by a car battery, a 12V power supply or a lithium ion battery. High color accuracy is achieved because the patent-pending step-up LED driver topology enables fast transient response and enables the desired dimming control with a 256:1 I2C control matrix system . The LT3965 can be set to start with all LEDs off and start with a gradual brightness or jump directly to a specific color. Optical feedback (via a microcontroller) can be added to improve color accuracy, although it is not necessary.