In many high-brightness LED (HB-LED) applications, one of the
main issues to be addressed is the powering of the devices from a
wide input voltage source. This is especially true in automotive
applications where the +12V battery power source may fluctuate
between 8V and 18V.
HB-LEDs need to be driven by a current source rather than a voltage
source since the forward voltage rating varies from part to part
and over temperature. To obtain predictable and matched luminosity
and chromacity it is also desirable to drive the LEDs with a
constant current.
To decide the best cost/performance trade-off for driving HB-LEDs,
it is useful to segment applications and their corresponding
solutions according to their approximate LED drive current, number
of
LEDs per string and system
complexity.
I identify three groups:
● Low-power and low-complexity applications such as most of the
rear lighting functions, which are typically addressed with linear
regulators.
● Medium-power applications with low system complexity, such as
vehicle interior lighting. In this category, the most popular
device is a standard switching regulator.
● Medium to high-power front lighting applications with high-end
system requirements to deal with the safety regulations and
standards.
It is possible to use standard buck or boost switching regulators
plus the supporting circuitry for the control of higher power front
lighting applications.
However, application-specific devices can provide a more elegant
approach that incorporates a wide range of important additional
functionality to improve reliability and reduce overall cost.
Able to drive and control more than one HB-LED string and
incorporating a host of diagnostics monitoring and protection,
application specific devices allow the implementation of a
single-chip self-contained lighting module that also negates the
need for a microcontroller.
They are able to generate pulse width modulation (PWM) dimming
above 500Hz, whereby the duty-cycle of the LED current is varied to
achieve a dimming effect. This approach overcomes the problems
associated with analogue dimming, where LED light colour (colour
temperature) varies visibly with reduced applied current.
It is anticipated that future HB-LED-based headlamps will emit more
lumens than their predecessors. It is then likely that replacement
LED lighting modules fitted to a vehicle damaged in an accident may
have better performance and light output than those fitted when the
vehicle was manufactured.
This could cause a visible difference in brightness between the old
and new lighting modules. The PWM dimming feature embedded in an
integrated device is able to compensate for this, as well as for
the natural degradation in light over the lifetime of the
vehicle.
Temperature monitoring and diagnostics are two important functions
that integrated drivers can include.
Temperature monitoring is achieved via an interface to the
appropriate sensors. With the life and reliability of HB-LEDs
closely tied to their junction temperature, this can prove vital in
helping achieve acceptable longevity for the LEDs.
It can also protect the LED module in case of a system failure
caused by, for example, an inoperative cooling fan or a short
circuit in the wiring to the LEDs.
Diagnostics support compliance with safety and regulatory
requirements. Typical problems that integrated diagnostics are able
to detect include shorted and open circuit LED strings and the
detection of a single shorted LED that may cause the light beam to
be non-compliant with regulations.
Application-specific devices can also incorporate EMC filtering to
ensure stringent automotive requirements and standards are
satisfied.
Input current filtering is particularly important for
HB-LEDs during PWM dimming when they
are continuously switched on and off. External filtering for
standard switching regulators can be expensive and difficult to
implement.
A typical boost application will generate high ripple currents from
the battery. During dimming, PWM switching generates fast current
changes in the LED strings which causes them to act like an
antenna. A standard switching regulator approach requires expensive
filtering to overcome these effects.
A design approach that uses an integrated device generates much
lower input ripple, and in most cases can filter out PWM dimming
effects without the need for external components.
Paul Decloedt is business development manager at ON
Semiconductor