Resources | LpR Article | Automotive | Driver IC | Jun 01, 2018

Flicker-Free Control of Individual LEDs in Matrix Headlights

LEDs combine design flexibility with practical, robust circuitry, enabling automotive designers to produce striking headlight designs matched by exceptionally long life and performance. Keith Szolusha, Sr. Design Leader for LED driver products at Linear Technology, and now part of Analog Devices explains how automobile designers are increasingly incorporating LEDs in lighting because they can be arranged in distinctive eye-catching designs - helping distinguish new models from old, or high end from economy.

There is no question that automobile LED lighting has arrived, but it has not yet reached its full potential. Future models will feature more LED lights, including new shapes and colors, and more control over the individual LEDs. Simple strings of LEDs will give way to matrices of LEDs that can be individually dimmed via computer control, enabling unlimited real-time pattern control and animation. On the example of the LT3965 matrix LED driver and a combination with the LT3797 boost-then-dual-buck mode drivers it will be shown how advanced driver technology makes it easy to take the next step in automotive lighting design.

I2C Control of Eight Power Switches with a Single IC

A basic LED headlight design operates with uniform LED current, and thus, uniform brightness. But this leaves much of the LEDs’ potential on the table. Matrix headlights take advantage of the innate abilities of LEDs by enabling control of the brightness of individual LEDs within LED strings.

It is not difficult, in theory, to address the individual LEDs in a matrix via computer-controlled power switches, allowing individual LEDs to be turned on or off, or PWM dimmed, to create unique patterns and functions. Each LED (or segment of LEDs) requires either its own converter or its own shunt power switch. It is possible to build a matrix driver with traditional driver/converter ICs that include a serial communications feature, but once more than two or three switches are needed for a matrix of LEDs, designing a discrete component solution becomes challenging, involving a matrix of components that exceeds the size of the LED matrix.

The I.C 8-switch matrix LED dimmer makes it easy to control large or small LED matrices (up to 512 LEDs). Figure 1 shows the driver in action on the demonstration circuit.

Figure 1: LED matrix dimmer demonstration circuit run as a Linduino™ shield. This demonstration circuit runs headlight, turning light, tail light and trim patterns and can be evaluated with a graphical user interface via a USB cable

Its highly integrated design (Figure 2) minimizes component count. The individually addressable channels can be used to control LED matrices in many ways.


Figure 2: 
60 V 8-switch LED matrix dimmer block diagram reveals eight power NMOS shunt switches for brightness control, a fault flag and I2C serial communications interface

Control options for LED matrices:

  • Each driver can control eight dimming channels - eight LEDs or eight clusters - within a string of LEDs
  • The eight channels can control the individual red, green, blue and white light on two RGBW LED modules for adjustable bright- ness or changing color of dashboard or trim lighting
  • Multiple drivers can be individually addressed on a single communications bus to multiply the strings in a large array
  • One driver can control multiple LEDs per channel, or channels can be combined to efficiently control a single LED at higher current

When combined with a suitable constant-current LED driver, the matrix dimmer LED driver allows the individual LEDs to be computer-controlled in headlights, daytime running lights, brake and tail lights, side-bending lights, dashboard display and other trim lighting. A built-in automatic fault detection protects individual LEDs in case of a failure and reports failures to the microcontroller.

The 60 V driver includes eight integrated 330 mΩ power switches, which can be connected to one or more LEDs. The power switches act as shunt devices by turning off or PWM dimming the LEDs on a particular channel. The switches create eight individually controlled brightness channels (up to 256:1 dimming ratio) and eight fault-proof segments of an LED string.

It can handle a string current of 500 mA when all eight power switches are on at the same time (all LEDs off). The switches can be connected in parallel and run at 1 A through four channels of LEDs as shown later in this article. Regardless of the number of LEDs or current, the LED string must be driven by a properly designed converter that has the bandwidth to handle the fast transients of the matrix dimmer. Some reference designs are included in this article.

Boost-then-Dual-Buck Mode Drives Two Strings, 16 LEDs at 500 mA with Two Matrix Drivers

The eight shunt power switches of the matrix LED drivers control the brightness of eight channels of LEDs at 500 mA. The string voltage of the 8-LED matrix dimmer system can be between 0 and 26 V, depending on how many LEDs are on or off at a given time. The recommended converter topology to drive these LEDs is a 30 V step-down converter with high bandwidth and little or no output capacitor. This step-down topology requires that 9–16 V automotive input is “pre-boosted” to a 30 V rail from which the step-down regulators can operate.

The triple output LT3797 LED controller conveniently serves as a single-IC solution for both the “pre-boost” and step‑down functions - it can be configured as a step-up voltage regulator on one channel, followed by step-down LED drivers on the other two channels. Each of two step-down LED drivers can drive a string of matrix-dimmed LEDs. This topology has a number of advantages, most notably, regardless of whether the LED string voltages are above or below the battery voltage, the circuit continues to function optimally.

Figure 3 shows the schematic of the demonstration board shown in figure 1, a boost-then-dual-buck mode matrix dimming headlight system with 16 LEDs at 500 mA. Each LED can be individually controlled to be on, off or PWM dimmed down to 1/256 brightness. The 350 kHz switching frequency is outside the AM band (good for EMI) and the resulting 170 Hz PWM dimming frequency, generated from the same 350 kHz clock, is above the visible range. With the system properly synchronized, the matrix headlight operates flicker-free.

Figure 3: Matrix LED dimmer system with boost-then-dual-buck mode LED drivers and two matrix dimmers that drive 16 LEDs at 500 mA from a car battery. I2C serial communications control the brightness of individual LEDs and check for LED and channel faults

The LT3797 buck mode converters are optimized for extremely fast transients with little or no output capacitor and properly compensated control loops. These >30 kHz bandwidth converters tolerate fast LED transients as the LEDs are turned on and off and PWM dimmed at will. A filter capacitor placed on the LED sense resistor replaces a pole in the control system that is lost when the output capacitor is reduced or removed for the fast transient performance of the matrix dimmer.

A charge pump from the switch node is used to power the matrix LED drivers’ VIN pin more than 7 V above the LED+ voltage to enable the top channel NMOS to be fully enhanced when driven. The low RDs(ON) NMOS switches enable high power operation without the IC getting hot, even when all eight shunt switches are on, turning the entire LED string off. In this case, the LED driver survives the virtual output short created by all eight shunt switches without any issues, and is ready to quickly regulate 500 mA through the next LED that is turned on.

Demonstration circuit from figure 1 features the system shown in figure 3 and operates a matrix headlight with an attached I.C microcontroller via the Linduino™One demo circuit. The used demo Board, operated as a large Linduino shield, has up to 400 kHz serial code that generates different headlight patterns and interfaces with the graphical user interface (Figure 4).

Within the GUI shown in figure 4, LED brightness and fault protection functions can be examined with “All Channel Mode” and “Single Challenge Mode” commands, as well as “Fault Check” read and write commands to check for open and short LEDs. Flicker-free operation, fault protection and transient operation can be examined with this demonstration circuit system. DC2218 can be plugged directly into a 12 V DC source and it can be controlled by a personal computer running the GUI or reprogrammed from a simple USB connection.

Figure 4: The PC-based interface allows designers to access control and monitoring of the LEDs

1 A Matrix LED Driver Using Parallel Channels

The LT3965 can be used to drive matrices of 1A LED channels. It is easy to connect the power switches in parallel so that two power switches split 1 A of LED current and each driver controls four 1 A channels. One way to use parallel power switches for higher current is to run each of the anti-phase parallel switches for only 50% of the PWM period. By alternating and running 1 A through a single NMOS power switch for half the time, the effective heating is about equal to running 500 mA through the same NMOS all of the time.

Figure 5: 1 A matrix LED driver combines anti-phase parallel channels for higher current applications in high power LED headlight systems

Figure 5 shows a 1 A matrix headlight system using eight LEDs driven by two matrix LED drivers and another boost-then-dual-buck mode IC. When PWM dimming, the IC uses a unique 1/8-cycle phasing of the eight switches, as shown in figure 6. In this 1 A matrix system, boost-then-dual-buck mode LED drivers channels are combined in parallel pairs, so that paired channels are anti-phase, 180° from each other; specifically pairing channels 8 and 4, 7 and 3, 6 and 2, and 5 and 1. Parallel channels alternate shunting, effectively doubling the PWM frequency, with the advantage of spreading out the shunted current and heat. For this to work properly, the maximum duty cycle for any single shunt power switch is 50%, because two anti-phase switches that are on 50% of the time (each shunting an LED 50% of the time) turns the LED off 100% of the time.

Figure 6: 1/8 PWM flicker-free phasing of the eight matrix LED driver power switches limits transients during PWM dimming brightness control

Each matrix LED diver controls the brightness of four 1 A LEDs that are driven by two 1 A buck mode LT3797 channels (from the boosted 20 V channel). This high power, robust system can be expanded to power more LEDs with more LT3965s or higher current LEDs with more channels in parallel. It is possible to drive two LEDs per channel at 1 A and drive up the power of this flexible headlight system.

More than One LED per Channel

The matrix driver can support one to four LEDs per channel. Although it can be advantageous to individually control every single LED for fault protection or high resolution patterns, it is not always necessary. Using more than one LED per channel reduces the number of matrix dimmers in a system and is enough to accomplish the patterns or dimming required for some designs. Segments of headlights, signal lights and tail lights can have up to four LEDs with the same brightness. Emergency LED lights can have sets of three and four LEDs that blink and wave with the same pattern.

Figure 7: The flexible matrix LED driver can drive LED channels on independent LED strings and can drive between one and four LEDs per channel. (Complete driver circuit is similar to figure 3, but with only one IC, as shown here)

The circuit in Figure 7 demonstrates a two-LED-per-channel system - it has the same number of LEDs as the circuit in Figure 3, but uses only a single matrix dimmer instead of two.

When an I.C command tells the LT3965 to turn on, off, or dim a channel, it affects the two LEDs that are controlled by that channel’s shunt power switch. To stay within the voltage limitations of the IC, the 16 LEDs at 500 mA still need to be split into two series LED strings as they are in Figure 2. The same boost-then-dual-buck mode circuit in figure 2 can be used, but only a single matrix LED driver controls the brightness of the two strings. This demonstrates how each NMOS shunt power switch inside the IC can be configured independently of the others, allowing an endless variety of matrix designs.

All Channel Mode and Single Channel Mode I.C Commands with Flicker- Free PWM and Fade

The I.C instruction set includes 1-, 2- and 3-word commands. These commands are sent over the serial data line (SDA) alongside the master-generated clock line (SCL) at up to 400 kHz speed. The master microcontroller sends all channel mode (ACM) or single channel mode (SCM) write commands to control the brightness, fade, open-circuit threshold and short-circuit threshold of the LED channels and LT3965 addresses.

Broadcast mode (BCM), ACM and SCM read commands request that the drivers report the content of their registers, including open and short registers for fault diagnostics. The matrix LED driver asserts an ALERT flag when there is a new fault. The micro can respond to the fault by determining which driver reported the fault, as well as the type and channel of fault. In the case that multiple ICs are reporting faults, they can sequence fault reporting to the master to prevent overlap errors. This makes the alert response system reliable and conclusive. A complete list of the registers and command set is given in the data sheet.

ACM write commands instantly turn all of the eight channels of a single LT3965 address on or off with just two I.C words - the channels transition on or off at the same time. Turning a high number of LEDs on or off presents a significant current voltage load step to the DC/DC converter. The converters presented here handle these transients with grace, with little or no output capacitor and high bandwidth.

As shown in Figure 8, an ACM write transitioning a high number of LEDs produces no visible flicker or significant transient on the LED current of other channels. The high bandwidth buck mode converter built around the LT3797 is the reason for such a small and controlled transient.

Figure 8: The LED matrix driver designs shown in this article feature minimal to no cross-channel transient effects. For instance, transitioning half the channels - here, simultaneously turning on two and turning off two - has little to no transient effect on the other four, untouched channels. The non-transitioned channels remain flicker free

Single channel mode writes produce relatively small and fast single-LED transients. SCM writes are used to set the brightness of only one channel at a time to ON, OFF, or PWM dimming with or without fade. PWM dimming values between 1/256 and 255/256 are communicated in 3-word writes while ON and OFF can be communicated in shorter, 2-word commands. A fade bit on a single SCM write command enables the LT3965 to move between two PWM dimming levels with internally determined logarithmic fade and no additional I.C traffic. The open and short thresholds of each channel can be set between one and four LEDs with SCM write commands.

Short and Open LED Fault Protection for Each Channel

Short- and open-circuit protection is an inherent benefit of this matrix dimmer. Each channel’s NMOS power switch can shunt out between one and four series LEDs. Traditional LED strings have protection against the entire string being open or shorted and only some ICs have output diagnostic flags to indicate these fault conditions. In contrast, this driver protects against, and rides through, individual channel shorts and opens, keeping operational channels alive and running while recording and reporting the fault conditions.

When a fault occurs within a string, the driver detects the fault and asserts its ALERT flag, indicating to the microcontroller that there is an issue to be addressed. If the fault is an open-circuit, the driver automatically turns on its corresponding NMOS power switch, bypassing the faulty LED until a full diagnosis occurs or until the fault is removed.

The IC maintains registers of open and short faults for each channel and returns the data to the microcontroller during I.C fault read commands. The command set includes reads that leave the status register unchanged and those that clear the fault registers, allowing user-programmable fault diagnostics.

Registers can be read in the various modes allowed for writes, SCM, ACM, BCM:

  • Single channel mode (SCM) reads return the open and short register bits for a single channel. SCM reads also check the open and short threshold register, the mode control, and the 8-bit PWM dimming value for that channel
  • All channel mode (ACM) reads return the open and short register bits for all channels of a given address without clearing the bits, as well as the ACM ON and OFF bits for all eight channels
  • In more complex systems with many matrix dimmers sharing the same bus, a broadcast mode (BCM) read first requests which, if any, IC address has asserted the fault flag
  • The ACM and SCM reads can be used to check and clear faults and to read all of the registers for a robust I.C communications system

Up to 16 Addressable Drivers on the Same Bus

Every IC features four user-selectable address bits, enabling 16 unique bus addresses. Every ACM and SCM I.C command is sent to the shared communications bus, but action is only taken by the addressed IC. BCM commands are followed by all ICs on the bus. The 4-bit address architecture allows a single microcontroller and a single I.C 2-line communications bus to support up to 8 . 16 = 128 individually controllable channels. With this driver, for all but the most ambitious lighting displays, all individual LEDs in an automobile’s headlight, tail light and trim lights can be controlled by a single I.C communications bus and a single microcontroller. Given that each channel can be connected to up to four LEDs, one relatively easy-to-implement system can support matrix dimming for up to 512 LEDs.

Conclusion

The matrix LED dimmer controls eight LED-brightness channels on a single LED string, giving lighting designers unlimited access to sophisticated and striking automotive lighting designs. The I.C communications interface allows a microprocessor to control the brightness of individual LEDs in the string. Fault protection in the I.C interface ensures LED lighting system robustness. The channels of the matrix dimmer are versatile: each channel can control multiple LEDs; channels can be combined to support higher current LEDs; or high LED-count systems can be produced with up to 16 matrix dimmer ICs on the same communications bus. The time has come to take the next step in designing automotive headlights, tail lights, front, side, dash and trim lights.

(c) Luger Research e.U. - 2018

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