LEDs have emerged as the driver of innovation for exterior lighting for cars
They offer attractive design possibilities as well as promote the image and brand of the automotive manufacturer. They are energy efficient and contribute to reduction of CO2 emissions. In this article, Infineon Technologies discusses how fast integration by automotive producers is possible.
Integrated LED drivers triumphed over discrete solutions in the control of LED taillights, starting in the European market about five years ago. Apart from luxury class vehicles, they are increasingly being used also in smaller vehicle platforms. Advantages include increased reliability and cost saving, in addition to offering better diagnosis functions, some of which are already required by law. Further features, such as power shift and pulse width modulation (PWM) give the system developer more playroom in lighting design (1).
MEETING INCREASING DEMANDS
Infineon’s new LED driver family LITIX™ Basic+ (1) provides an answer to the increasing design demands. The company complemented the existing LITIX™ Basic family, as was introduced in 2013, with further seven new devices. Both driver families use high-side linear current sources for low and medium power exterior lighting and are also suitable for applications in car interior and in industrial LED lighting.
The seven devices of the LITIX™ Basic+ LED driver family (one-channel, two-channel and three-channel versions) provide the well-known scalability of LITIX™ Basic: the almost identical pin-out of the LITIX™ Basic+ devices allows easy design adaptations. Once one is familiar with one device, one is familiar with the whole family. All devices are qualified for automotive and are delivered in a 14-pin package (TSDSO-14 EP), (2).
All drivers are equipped with integrated protective functions at overload, short circuit and over temperature and in addition are protected against reverse-polarity (reverse battery protection). The input voltage of the family ranges from 5.5 V up to 40 V, whereby the modules support an LED current range of 5 mA up to 360 mA. The high current accuracy of LITIX™ Basic+ fulfils the global demands of automotive suppliers, (2). The current accuracy is defined by the k-factor, which is the ratio of output current to reference current. In some regions, an accuracy of 5 % is required for LED current. Favourable EMC behaviour of the device reduces the necessity for additional filter components for a stable system. Thus, further system expenses are saved.
DIAGNOSTIC FUNCTIONS ARE SAFETY RELEVANT
Diagnosis of light functions in cars is essential. If the LEDs fail without notice, then road traffic safety is severely compromised. Reliable load diagnosis of the LEDs helps to permanently fulfil the legal requirements of lighting, (3).
Standard diagnostic functions of LITIX™ Basic+ LED drivers detect short circuits to ground (SC, short circuit) and load interruptions of the LED chain (OL, open load). When several LED drivers are used for a lighting function, they can be automatically deactivated by the integrated N-1 function in case of failure. This means that with at least one defective LED chain, the complete lighting function will be deactivated.
The synchronisation takes place via a failure-state-network ERRN, (3). In such a network, up to 16 LITIX™ Basic+ devices can be connected directly, whereby only one external pull-up resistor is required. If there is a failure on the circuit board, the power consumption is significantly reduced to 850 µA per device. The device that detects the failure shows a power consumption up to 1.25 mA. In effect, the devices produce a small total failure current as is required for the diagnosis in the body control module (BCM).
SHORT CIRCUIT DETECTION
In addition to the diagnostic functions described above, LITIX™ Basic+ devices can also diagnose single LED short circuits (SLS, single LED short) very flexibly. This diagnostic function, which is required by the American market and many German automotive producers, is fulfilled by three devices of the driver family: TLD2132-1EP with one input and one output channel (1in/1out), TLD2131-3EP (1in/3out) and TLD2331-3EP (3in/3out).
The failure diagnosis circuit compares the voltages of the LED chains with a configurable reference voltage. Should one LED within an LED chain show a short circuit, the output voltage of the channel that drives the defective LED chain, is reduced. This reduction corresponds approximately to the forward bias of one LED, (4) (a). The output current keeps on flowing through the LEDs because a closed electric circuit still exists. Since the output voltage sinks below the reference voltage value VOUT (SLS_REF), the device detects the failure.
Depending on the LEDs that are being used and their operating points (LED current, temperature), the reference voltage needs to be adapted. This occurs via a resistance on the SLS_REF pin, (4) (b). According to the system requirements, the behaviour in case of failure can be configured via two pins (D and DS) in the following way: either the LED driver reports the failure case at the ERRN pin or it reports the failure and additionally switches-off. The switch-off can be delayed by a small capacity of about 10 nF.
LITIX™ Basic+ has an automatic restart function for short circuits after ground and load interruption. An active retry mode is added as a new functionality, taking into account that individual LED short circuits can only be detected, when the device is switched on.
In active retry mode, the LED driver briefly reactivates the switched-off LEDs in the event of a single LED short circuit to perform a new diagnosis. If the error then no longer exists, all LED drivers are reactivated. The sampling frequency can be flexibly adjusted with a small capacitance (approx. 220 nF). With these configuration options, LITIX™ Basic+ offers the most flexible single LED short-circuit diagnosis currently available on the market.
LITIX™ Basic+ has a combined Enable/Diagnosis Enable Pin (EN/DEN) that can be configured with a simple voltage divider to switch the device and the diagnostic functionality on or off. The voltage divider, as well as other configuration components, can be used simultaneously by several LITIX™ Basic+ modules, (5). System costs will be reduced significantly, (4).
POWER SHIFT FUNCTION
The TLD1114-1EP (1in/1out) with its integrated power-shift function supports a widely usable output current range of up to 360 mA. TLD1114-1EP is ideal for use in stop, reverse and rear-fog lights. These light functions cause an increased power loss in the LED driver. The integrated power-shift function allows for a large share of power loss to be dissipated by external resistors. This means that the device can also be operated at maximum current. The resistors can be distributed evenly over the circuit board to make optimum use of its available cooling area.
The power shift function in the TLD1114-1EP device is based on the fact that part of the current is routed externally past the power section of the device. The external current flows through a resistor/MOSFET circuit, (5). The resistors R1 and R2 carry the largest part of the power loss, while the external MOSFET serves only to switch the power circuit.
The diagram shows the split of the LED current into the internal current IOUTS and the external power-shift current IPS. As soon as the supply voltage VS has been exceeded by the output voltage VfLED and the threshold voltage VGS(th), the power shift circuit becomes activated. Exact monitoring of the LED current takes place by directing this current back into the control loop of the LED driver and integrating it into the control circuit. The control loop measures the external provided current and adds additional current to reach the defined LED current.
The degree of current distribution can be set by the size of the external resistors. The external MOSFET can be eliminated in many light functions – at average currents of approximately 250 mA and in the case where the light functions are only switched via the supply voltage, (5).
APPLICATION EXAMPLE: COMBINED TAIL & STOP LIGHT
In many cars, various taillight functions are implemented by means of the same LEDs. This can be found, for example, in combined stop and taillights. An adaptation of the LED current results in the required different light functions. Common practice to achieve this is the use of pulse width modulation (PWM). The LITIX™ Basic+ family offers three components with integrated PWM timer module, TLD2141-3EP (1in/3out), TLD2252-2EP (2in/2out) and TLD2142-1EP (1in/1out). Combined taillight function is achieved cost efficiently and by changing the pulse width: for example, 100 % for stop light and 20 % for taillight.
(6) depicts an application example using a combination of three LITIX™ Basic+ building blocks (one TLD2141-3EP and two TLD2131-3EP). They configure a combined tail and stop light, which consists of nine LED chains each with three LEDs.
The TLD2141-3EP operates as a timer building block producing the PWM signal for all three drivers. The LED drivers are automatically activated as soon as voltage is applied on one of the supply lines, VTAIL or VSTOP, depicted in (6). Simultaneously, the PWM driver is activated. The PWM frequency and pulse width is set by two resistors and a capacitor. A PWM output pin conveys the PWM signal from TLD2141-3EP to the two other building blocks. Consequently, fewer external components are necessary and all LED drivers function synchronously to produce an even change in brightness.
The PWM driver stage is set to 100 % by means of a transistor switch so that the brightness of the LED for the stop light increases, when stop supply line VSTOP is switched. The right-hand side of (6) shows the LED current, which is dependent on the input signal for the taillight VTAIL as well as for the stop light VSTOP.
Several of the LITIX™ Basic+ modules offer an output pin that copies the LED reference current and provides it as a reference current to the following LITIX™ Basic+ modules. Therefore, the OUT_SET pin of the first driver and the IN_SET pin of the second driver are connected. This connection possibility allows system engineers to reduce the number of further resistors, (7).
The LITIX™ Basic+ family belongs to the family of LITIX™ LED drivers for automotive LED applications. In addition to LITIX™ Basic+ for low and medium power exterior lighting, LITIX™ Basic and LITIX™ Linear address low to high power applications in rear lighting. LITIX™ Basic is also used in automotive interior LED applications, such as LED reading lights or cost-sensitive front light applications such as daylight running light (DRL). The LITIX™ Power family consists of DC/DC converters for LED use in DRL, low beam and high beam lighting. Since 2016, DC/DC converters of the LITIX™ Power Flex family support high power and efficient front light applications.
GERNOT UNTERWEGER is an Application Engineer responsible for Automotive LED Drivers at Infineon Technologies
VOLKER TAGGRUBER is Product Marketing Manager for Automotive LED Drivers responsible for Automotive LED Drivers for rear light applications at Infineon Technologies.