FEATURE POWER ELECTRONICS
POWERING LEDS FOR AUTOMOTIVE DAYTIME RUNNING LIGHTS
Simon Ramsdale, European analogue marketing manager at Diodes Incorporated explores how the latest LED controller is providing a versatile yet easy to use solution for driving LED-based daytime running lights
A
utomotive daytime running lights are now a familiar sight on our roads.
New cars have been required to have them since February 2011 and they have been mandatory for commercial vehicles since August 2012. But while the use of LEDs has not been a prerequisite, advances in this technology have provided numerous design benefits, not least the ability for manufacturers to distinguish their brand through styling of LED lamps. Key to the use of LEDs for automotive
lighting, other than for cabin or instrument displays, has been the dramatic improvement in the quantum efficiency of the core process technologies that has realised high- brightness LEDs that can achieve the reliability criteria demanded by automotive applications. This reliability also translates to a long service life of more than 10 years meaning that, unlike incandescent bulbs, accessibility for replacement is no longer an important consideration. Additionally, LEDs consume
significantly less energy than incandescent bulbs - this can lower the fuel consumption of conventional vehicles, saving as much as two litres of petrol per 1000km, or extend the range of hybrid or electric vehicles. When used for daytime running lights,
LEDs offer further benefits as their brightness can be increased in response to poor visibility driving conditions or dimmed when they are required to act as parking lights. Such versatility requires the use of suitable power controller and driver circuits, which in turn must function correctly within the electrical and environmental operating constraints that apply to automotive systems. For example, under normal conditions a
vehicle’s supply voltage will range from 9V to 16V but can easily extend beyond these limits under transient conditions such as jump starting, which can double the normal 12V battery voltage, and load dumping, which may occur if a loose cable temporarily disconnects the battery from the alternator and can result in a spike of up to 60V.
Engine stop/start technology can also
result in the supply voltage dropping below 9V during restart. LED driver
22 DECEMBER/JANUARY 2017 | ELECTRONICS
circuits need to both cope with these challenging conditions and include protection against transient and other fault conditions such as over-voltage, under-voltage, over-current, short- circuit and over-temperature. Ultimately the goal for an LED
controller/driver is to deliver a well- regulated output voltage and LED current so that even when suffering voltage spikes of 30V or more, the light produced by the LEDs does not change. This cannot be achieved with a single regulator topology: The long LED strings typically used for daytime running lights require a boost regulator to provide an output voltage that is higher than the vehicle’s supply voltage, especially under start- stop conditions. However buck regulation may be required, either to handle a temporary supply voltage increase or to power lower voltage LED chains.
Figure 1:
Distinctive styling has become a feature of LED-based daytime running lights
Figure 2:
Functional block diagram of ZXLD1371Q LED Driver-Controller
To avoid the need for the automotive
designer to work with different LED controllers for different layout configurations, a device such as the ZXLD1371Q from Diodes offers the ability to work in buck, boost or buck-boost modes. The buck-boost mode makes it possible to respond effectively to changes in supply that oscillate around the voltage needed to drive the LED string and a switching frequency of 400kHz provides high-efficiency voltage regulation. Current accuracy is vital for ensuring
consistent colour and output level matching across changes in supply voltage and between LED strings. The Diodes’ device employs a modified hysteretic control algorithm with a single current-sense resistor to provide high accuracy, with a deviation of no more than 5% in any of its three operating modes. On-board support for pulse- width-modulation (PWM) dimming makes it possible to control the brightness of the LED string over a 1000:1 ratio. External sense resistors are used to
switch the controller between its different power-control modes and to determine other operating characteristics. For effective integration into the rest of the vehicle electronics’, the device features two pins for fault diagnosis; one to flag a fault, with the other multilevel status pin providing further information on the nature of the fault, such as over-temperature. An LED controller such as the
ZXLD1371Q, which offers the capability of operating over a 5V to 60V supply range at temperatures up to 125°C, provides automotive designers with a versatile yet easy to use solution for driving LED- based daytime running lights. By supporting multiple regulation
modes and needing only a minimal number of external components, this highly integrated controller can reduce manufacturers’ inventory requirements across a range of vehicle designs while its approval to AEC-Q100 and TS16949 standards will ensure ready acceptance in new automotive designs.
Diodes Incorporated
www.diodes.com 01784 275000
/ ELECTRONICS
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