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Managing individual LEDs in matrix headlights

28 Dec 2015  | Keith Szolusha

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Broadcast mode (BCM), ACM and SCM read commands request that the LED dimmers report the content of their registers, including open and short registers for fault diagnostics. The LED dimmer asserts an ALERT flag when there is a new fault. The micro can respond to the fault by determining which LED dimmer reported the fault, as well as the type and channel of fault. In the case that multiple LED dimmer ICs are reporting faults, the LT3965s 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 LT3965 data sheet.

ACM write commands instantly turn all of the eight channels of a single LED dimmer address on or off with just two I2C 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 LED controller 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 I2C 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 the 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, the LT3965 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 LED dimmer 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 LED dimmer automatically turns on its corresponding NMOS power switch, bypassing the faulty LED until a full diagnosis occurs or until the fault is removed.

The LT3965 maintains registers of open and short faults for each channel and returns the data to the microcontroller during I2C 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 8bit 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 LT3965 matrix dimmers sharing the same bus, a broadcast mode (BCM) read first requests which, if any, LT3965 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 I2C communications system.


Up to 16 addressable LED dimmers on the same bus
Every LT3965 features four user-selectable address bits, enabling 16 unique bus addresses. Every ACM and SCM I2C command is sent to the shared communications bus, but action is only taken by the addressed LED dimmer. BCM commands are followed by all ICs on the bus. The 4bit address architecture allows a single microcontroller and a single I2C 2-line communications bus to support up to 8 × 16 = 128 individually controllable channels. With the LED dimmer, 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 I2C 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 LT3965 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 I2C communications interface allows a microprocessor to control the brightness of individual LEDs in the string. Fault protection in the I2C interface is touted to ensure 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.


About the author
Keith Szolusha is Senior Design Lead for LED Driver Products at Linear Technology.


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