LED driver for automotive exterior lighting application

　　Authors: Romeo Letor, Donato Tagliavia, Luc dang

　　Company: STMicroelectronics

　　Reference Author: Romeo Letor

　　Automotive Electronic Department in Shanghai

　　EMAIL: romeo.letor@st.com

　　Abstract:

　　New automotive front light systems using high brightness white LEDS requires dedicated LED drivers. LED characteristics, driving methodology, low consumption,low emission, fault tolerance, diagnostics and compatibility with existing automotive body controllers are the main topics concerned. This paper illustrates system solutions with a new LED driver designed on purpose.

　1.0 Introduction

　　High brightness white LEDs, that represents the first technology breakthrough after Edison, are making a revolution in today’s lighting systems. Efficiency, reliability, low working temperature, new styling are some of the strong driving factors introducing LEDS in all market segments. Automotive is not free from this innovation process: about 10 years ago high brightness colour LED already started to be introduced in rear signal light systems , furthermore today this innovation process is migrating to front lights with white LEDS for the development of Daytime Running Light (DRL), low and high beam projectors. These last systems involve more sophisticated electronic for LED driving and management. So a new demand of LED driver compliant with automotive specification and compatible with car body module is coming from the car platform that will be in production in the coming years. For this reason, STMicroelectronics is introducing new dedicated LED drivers for the automotive market. This paper shows as an example a new driver dedicated for front light systems putting in evidence the major differences respect to existing driver for the standard market. Main source information of the specification definition process are illustrated and specific features of the device like short circuit protection, burst mode operation and battery line control are highlighted.

　　1.1 REQUIREMENTS FOR

　　AUTOMOTIVE LED DRIVER

　　Driver specification is defined on the base of 3 major necessities:

　　1 - LED driving requirements

　　• Current control

　　• Current trimming based on LED binning

　　• Burst mode operation for light dimming

　　• High conversion efficiency

　　• LED temperature control

　　• Wide output voltage range 5V to 50V 2 - Automotive specification

　　• Battery voltage range 5V to 40V

　　• Ambient temperature range -40C - 125C

　　• Operating junction temperature range -40C –150C

　　• EMI inside automotive standard

　　• Diagnostic and Fail safe

　　• Short circuit protected

　　• Low standby current 3. - Automotive architectures

　　• Remote control from battery line

　　• Compatible for micro interface with SPI/LIN/CAN

　　2.0 Specific features of the LED

　　driver for automotive White LED front light make use of several strings of LEDs in series so that the voltage drop on the LED string is higher than supply battery voltage. A DC-DC converter, which adapts the voltage across the LED string from the battery voltage, ensures steady current and high convertion efficiency. Dimming of LED is performed in burst mode at Frequencies > 200Hz. Hence, all relevant variables of the converter control loop need to be frozen during  burst mode operation in order to speed up the transients and get precise control of the burst mode duty cycle.  Additional functions related to LED characteristics are required: Current adjust for LED binning, short circuit protection and LED temperature control for LED reliability.

　　2.1 Burst mode operation for LED dimming

　　Dimming of LEDs is a wide spread requirement for automotive applications. For example the LED string for daytime running lights can be dimmed down to 10%-20% of the maximum brightness in order to operate as position lights. The LED dimming can be done either by pulse width modulation (PWM) or by the reduction of the forward current (see Figure 2). The PWM method presents the advantage of keeping the LED color unchanged, as the LED’s forward current is kept constant, whereas a reduction of the forward current causes a colour shift, which might be unacceptable.

　　The L99LD01 integrates several features, which optimize the PWM dimming: the memorization of the converter’s duty cycle and the reduction of the current overshoot. As a benefit, it avoids risks of false overcurrent detection and improves the linearity between the dimming’s duty cycle and the brightness of the LEDs, especially at low dimming rates.

　　During the off-time of the PWM period,the output of the operational transconductance amplifier (OTA),which is used as an error amplifier, is disconnected from the compensation network (see Error! Reference source not found.). As a result, the voltage at the compensation network,and the stabilized duty cycle of the converter during the on-time of the PWM period, are memorized. When the LEDs are turned back on, the converter works again with the memorized control voltage and therefore with the previous duty cycle, until the output of the OTA is reconnected to the compensationnetwork. This feature avoids the converter to operate in the first switching cycles with a very high duty cyle, before the LED current settles at its stabilized value.

　　The delay between the beginnings of the PWM dimming’s on-time and the reconnection of the OTA’s output to the compensation network can be set by a SPI command from 0μs to 50μs with a 3.3μs step. This configurability gives the possibility to optimize the delay according to the application conditions such as turn-on time and turn-on delay of the dimming mosfet after the beginning of the PWM dimming period (see Figure 4).

　　2.2 Short circuit protection

　　Automotive applications often require the protection of outputs against a short circuit to the chassis, which is also the system ground. The L99LD01 implements a high side current sensing and the control of a mosfet as high side dimming switch. The high side current sense monitors the LED current. Based on this information, a protection circuit latches off both the converter’s switching mosfet and the dimming mosfet if an overcurrent condition is detected (Figure 5) more than a typical filter time of 16μs. This feature increases the system reliability and fulfills the carmakers’ requirements in term of protection against short circuits to GND for boost-based applications.

　　2.3 LED current trimming

　　The brightness of LEDs depends on the applied current, the type of LEDs, the junction temperature etc.,... just to　　name of few influencing parameters. In several cases, the selection of the LED current by software represents an advantage for automotive applications: platform approach, compensation of the brightness loss caused by the ageing or LED binning classes.

　　Often, a given LED control circuit is developed in the frame of a platform approach. It is used for different car models, and various types of LEDs. Ideally, the hardware should not be modified, which means that the regulated LED must be configured by software, according to the specific application requirements.

　　The brightness of LEDs decreases over operating time. The operating temperature of the LEDs also affects their light intensity.Combining the measurement of the LEDs’ temperature with an ageing law over time, it is possible to keep the brightness of the LEDs constant either by re-ajusting the LED’s forward current, or by increasing the duty cycle of the PWM dimming.

　　Finally, the characteristics of one given type of LEDs variate with the production process. LEDs with similar parameters are collected in binning classes. However, the LED manufacturer may deliver several binning classes (e.g. brightness and  colour classes for given forward currents). Therefore, the LED current must be set according to the binning classes.

　　The L99LD01 is able adjust the LED forward current by a factor +/- 50% with a 3% step, compared to the default value, which is given by the choice of the sense resistor. The adjustment of the LED’s current is done by configuring a specific register, without any changes in the hardware.This feature is an answer to the requirements related to platform approaches, to the ageing compensation and to the adaptation of the LED current to the binning class.

　　2.4 Programmable input current limitation

　　Taking advantage from the peak current mode control, the L99LD01 implements a cycle-by-cycle limitation of the current, which flows through the switching mosfet. The input overcurrent threshold is set by SPI, which offers two application benefits. Firstly, the current limitation can be adapted by software to the inductor’s saturation current. Secondly, this feature results in a limitation of the drop on the supply rail, if the battery line impedance is known.

　　2.5 LED current trimming

　　The LED’s long lifetime is a commonplace. However, it is true only if the LEDs have an appropriate thermal cooling or thermal management. Indeed, unlike incandescent bulbs or xenon lamps, the LEDs‘ lifetime dramatically decreases if the junction temperature exceeds typically 125°C. The control of the LED’s temperature is even more important in front LED applications, because of the high operating temperature, which goes along with the proximity of the LED module to the vehicles motor.

　　Therefore, the L99LD01 features a LED current fold back in order to avoid an overheating. Indeed, the device has an input for a NTC (Negative Thermal Coefficient) resistor, which is placed in the vicinity of the LEDs, as thermal sensor. Below a threshold voltage at this input, the device will decrease the LED current (see characteristics of the NTC current foldback, Figure 6). The LED’s temperature threshold for the activation of this protection and the current-temperature de-rating can be adjusted by the appropriate choice of external resistors.

　　Moreover, the application microcontroller, thanks to an analog multiplexer, can monitor the voltage at the NTC input. This enables the implementation of a strategy to maintain a constant light output,despite the ageing of the LEDs.

　　2.6 Low EMI

　　Switched mode power supplies are sources of electromagnetic emission by nature. Implementing the so-called spread spectrum technique or dithering enables to distribute energy that was originally concentrated over narrow bands of frequencies over wider bands. The total emitted energy does not change. Instead, the noise floor is increased but the peak emission decreases.

　　The L99LD01, despite its architecture of constant frequency peak current mode, can modulate the switching frequency, applying a pseudo-random method. The characteristics of the dithering are selected by SPI: the deviation stretches from 5% to 35% and the modulation frequency can be set from 1.9 kHz to 15.6 kHz.

　　The Figure 7 compares the conducted emission at the input, according to the CISPR25 measurement method, of a DRL application using the L99LD01. The switching frequency was set to 300 kHz. When the dithering is enabled, the deviation was set to 10% and the modulation frequency to 15.6 kHz. Note that no input filters have been used. A reduction up to 10 dBμV in the EMI noise is observed in this specific example.

　　3.0 Car body architecture related features

　　Typical car body architecture has a body control module (BCM) including power actuators driving car body loads(Incandescent bulbs, motors, heaters, etc.) The most convenient way with High brightness LEDS is to manage the LED driver in the same board installed inside the optical system of the car.

　　This will require that the LED module need to be compatible with a standard passive load and needs to be driven by　　the supply voltage using the BCM actuators. Several configurations can be used according with cost and performances required.

　　3.1 Stand alone working mode for

　　low end applications The driver can be settled in stand alone mode, so it can be driven directly by the supply voltage.

　　The Figure 8 shows a typical architecture, where the body modulecontrols the LED optical system via a solid state relay. With such architecture, the body control is compatible both with incandescent lamp and LED optical group including the LED driver. Table 1 shows allowed features in Stand alone mode.

　　3.2 Power Line control with 8 bit micro

　　The LED driver can work in combination with a 8-bit microcontroller maintaining the architecture of Figure 8, where the LED system is driven by the supply voltage. The use of 8 bit micro enhances the application with many additional features. It is possible to send commands from BCM by PWM with very high duty cycle that can be decoded by the micro to set different working mode according to the duration of pulses. Table 1 shows allowed feature in power line mode. Full diagnostic mode can be implemented with specific dummy loads activated by the micro when a short circuit occur. This technique requires a specific programming of the BCM that can decode the diagnostic by current sensing.

　　3.3 LIN/CAN for advanced architectures

　　The Figure 11 shows a typical automotive architecture where the optical group is controller via a can BUS. Very complex functions for dedicated Advanced Front System (AFS) are implemented.

　　LED driver to the micro controller allow correct LED current adjust for light equalization and dedicated diagnostic.See

　　Table 1 shows allowed features with advanced architecture.

　　3.4 LIN/CAN for advanced architectures

　　The Figure 11 shows a typical automotive architecture where the optical group is controller via a can BUS. Very complex functions for dedicated Advanced Front System (AFS) are implemented.LED driver to the micro controller allow correct LED current adjust for light equalization and dedicated diagnostic.See

　　4.0 CONCLUSION

　　The last innovation in automotive lighting has been possible thanks to new technology in high brightness LEDS. Safe and correct drive of LEDS need specific requirement in order to not exceed maximum ratings and guarantee LED life expectation with specified color and brightness. Those requirements and specific needs of car body architecture have driven the development of a new LED driver to target this innovating automotive lighting market.