Technical Article Driving time-multiplexed LED arrays at high current: a

new approach

Joel Gehlin

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Driving time-multiplexed LED arrays at high current : a new approach Joel Gehlin

System designers have adopted time-multiplexed architectures for large-scale LED matrices in re-cent years in order to achieve a large reduction in the number of current sinks/sources required. This architecture reduces the size and cost of the electronics circuitry in end products with large LED arrays, such as smart commercial lighting and RGB signage.

This kind of design, however, is more difficult to implement when a high current is required at the LEDs. This is because of the refresh rate applied by the LED driver in order to spread the current evenly throughout the LED matrix if more than one LED is on at the same time. As a result, design-ers have found it hard to combine high current output (producing high brightness) with high efficien-cy, low cost and small size using conventional LED driver ICs. (The meaning of ‘refresh rate’ is ex-plained below.)

Interestingly, conventional LED driver ICs tend to be able to drive a high number of LEDs in a matrix configuration. Close examination of the datasheets, however, reveals the problem: the constant current at each sink/source in matrix configurations is typically in the range 10-40mA; only a few can deliver even as much as 150mA.

In fact, for many large display applications, a real 150mA supply would be adequate – but the re-fresh rate applied in time-multiplexed architectures means that the effective peak current at the LED is often one-half or one-third of the chip’s nominal peak.

This article describes a solution to the problem using a device type that is far from the most obvious choice: LED driver ICs for TV backlighting. The rapid growth in the market for LED TVs has spawned a new generation of sophisticated and highly efficient driver ICs that provide a high current capability. The article will explore the operation of time-multiplexing schemes and show the extent to which the capabilities of backlighting driver ICs match the requirements of large lighting and signage systems.

Basic operation of time-multiplexed matrices

Time-multiplexing is a technique for driving LEDs in a matrix without requiring a source for every LED. Figure 1 shows the operation of a time-multiplexing scheme. To control the LED D1, Source.1 needs to be supplied with a voltage higher than the maximum forward voltage (VF) of the LED; Sink.1 needs to be connected to a resistor or other type of current sink to draw the current through the LED. LED D5 is controlled in the same way via Source.2 and Sink.2.

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Fig.1: the operation of an LED time-multiplexing scheme

So far, so easy. But what if D1 and D5 need to be ON at the same time? If Sink.1/Sink.2 and Source.1/Source.2 are all ON, D2 and D4 will be turned ON as well. To overcome the problem, the concept of time-multiplexing must be used. Instead of turning Source.1, Source.2, Sink.1 and Sink.2 ON continuously, the driver multiplexes between Source.1/Sink.1 and Source.2/Sink.2.

Provided the flickering of LEDs D1 and D5 is at a frequency of 50Hz or higher, the light will appear to the human eye to be continuously ON. This time-multiplexing technique using an effective refresh rate faster than 50Hz thus permits D1 and D5 to be lit without lighting D2 or D4.

There is, of course, a drawback: the time-multiplexing with the associated refresh rate reduces the total LED current passing through the LEDs. Let’s say that a given matrix refresh rate for a given set of lit LEDs produces an effective 50% duty cycle applied to the LEDs: at a current set to 100mA via the current sink, the effective constant current through each LED is 50mA.

There might appear to be an obvious way to combat this effect: double the current at Sink.1 and Sink.2 to 200mA to provide a constant current of 100mA though the LEDs. Unfortunately, a current output of 200mA is beyond the capability of the conventional LED driver ICs on the market today.

Time-multiplexing control scheme

The refresh rate describes the number of times per second that the current through each lit LED in the matrix is reset. An example of a matrix control scheme is shown in Figure 2. Here, D1, D5 and D9 are being lit with a current of 100mA through each LED.

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Fig.2: time-multiplexing control scheme

Provided a multiplexing scheme is looped fast enough – between 200 and 1,000 times per second, depending on the number of LEDs to be lit simultaneously – the LEDs will appear to the human eye to be continuously ON. In Figure 2, a refresh rate of 200Hz for the entire matrix means that each LED will be switched at around 67Hz, which corresponds to a duty cycle at each LED of 33%. This means that each sink needs to handle at least 300mA in order to produce the constant current equivalent of 100mA at each LED.

Time-multiplexing also enables the creation of animations. The animation may be created in soft-ware code with a pre-defined series of bitmap images: these are usually arrays of n-bytes, in which each bit represents one LED in the LED matrix. To realize the picture, the controller must scan through each array one byte at a time, displaying one column after another.

A new approach to driving time-multiplexed LED arch itectures at high current

TV backlighting designs have almost universally replaced incandescent light sources (CCFL tubes) with LEDs. A huge market segment, TV backlighting has induced a surge in the number of special-ized backlighting LED driver ICs. Because of the requirement for high brightness in TVs, these ICs must be able to control LEDs at high currents, either via external MOSFETs or via FETs embedded in the driver chip.

An example of such a device is the AS3693B from ams, a 16-channel, high-precision LED controller with built-in PWM generators for driving external FETs. (A sister part, the AS3693A, features inte-grated MOSFETs.) While the AS3693 family was specially designed to meet the precise current-

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control requirements of TV manufacturers, the devices may also be used to source/sink and control LEDs in other applications.

Particularly useful is the ability to program output currents, which is also available in the AS382x family. The options include:

• Independent digital current control for each channel with a PWM generator

• Linear current control with an 8-bit DAC

• Linear current control with an external analogue voltage

Together with the ability to control external MOSFETs, the AS3693B gives the designer the freedom to set an appropriate maximum current and adapt the output to the needs of a variety of applica-tions, such as:

• Multi-pixel advertising boards

• Traffic signals

• Backlit signage

• General illumination

• Accent lighting with RGB color-changing capability

Demonstration: LED ‘EXIT’ sign

Safety EXIT signs powered by LEDs are up to 90% more efficient than traditional incandescent signs. Operating 24 hours a day, the cost and energy savings to be made by switching to LEDs are thus very considerable.

LED EXIT signs also offer savings in maintenance and repair, since their re-lamping cycle is typical-ly a very long period of around 10 years. In addition, LED signs can offer better optical performance.

Figure 3 shows how a single AS3693B – a 16-channel driver IC with an integrated PWM generator – can be configured to control a time-multiplexed matrix of 60 white LEDs. (An almost identical cir-cuit could also control 3 x 20 RGB LEDs.) The use of such a device in a time-multiplexed architec-ture offers the system designer considerable bill-of-materials and space savings compared to a conventional design, which would require four 16-channel driver ICs of the conventional type, which has no PWM generator. This second, conventional design will occupy a much larger PCB area and incur a much higher bill-of-materials cost.

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The constant current through each LED is provided by NMOS transistors. Selection of an appropri-ate NMOS device will provide for high efficiency and high brightness. The maximum current can be limited by the RSET resistor connected to the source of the external MOSFET in each current sink.

Fig. 3: 60 time-multiplexed white LEDs controlled in a time-multiplexing architecture by a single AS3693B from ams

To reduce power consumption even more the AS3693’s current settings can also adapt to the am-bient light. During daytime, the application can be dimmed; once the light becomes too dark, the system can boost the brightness to produce higher visibility and contrast.

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Improving efficiency through intelligent DC-DC conv ersion

The LEDs may be connected directly to, for example, a 12V power supply, but this will reduce the design’s efficiency as power is dissipated as waste heat in the NMOS transistors. To optimize effi-ciency for battery-powered equipment, the LEDs can equally be powered from an external DC-DC converter that can dynamically change its output voltage to match the needs of the LEDs’ VF.

The AS3693B offers three different paths for feedback, and each LED can be assigned to a specific LED supply. The operation of such a circuit is shown in Figure 4. The AS1341 is an efficient step-down converter with adjustable output voltages ranging from 1.25V up to the input voltage (20V maximum). It senses its output voltage with a resistive voltage divider. This voltage divider can be modified to set the output voltage between a minimum output voltage (VMIN) and a maximum out-put voltage (VMAX) which is the basis of the device’s dynamic feedback control.

Fig. 4: AS3693B feedback block for adjustable voltage control

The output of pins FBR, FBG and FBB of the AS3693B can be used to control any external power supply. Each PWM generator in the AS3693B can be independently selected to use any of the three feedback pins.

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Alternatively, by tying all the feedback pins together as a single feedback from AS3693B, the default register setting will function as a general feedback to the power supply.

Conclusion

By implementing a time-multiplexing scheme with an LED backlight driver IC and combining this architecture with an adaptive power supply, the designer of a large LED array can gain impressive savings in board area, bill-of-materials cost and power consumption. Moreover, this design involves the use of readily available standard parts that are well supported by relevant documentation and specifications.

For more information about the ams portfolio of LED drivers, including the AS369x- and AS382x-families, visit www.ams.com/eng/Products/Lighting-Management/Large-LCD-Panel-Backlighting-LED-Drivers.

For further information

ams AG

Tel: +43 (0) 3136 500

info@ams.com

www.ams.com