Advances in WLED/RGB LED Drivers0 pages

Application Information
Advances in WLED/RGB LED Drivers

Abstract
The increasing use of LEDs for backlighting TFT LCD panels
in automotive navigation/infotainment, computer monitors,
LCD TVs, and notebook computer screens, has generated
a demand for optimization of power regulation techniques
for those LEDs. The potential savings in system design and
increase in overall performance can have a growing impact
on customer acceptance as these applications spread across
a wide variety of market segments such as: Automotive,
Consumer, Industrial, Medical, and others. This application
note discusses an important trend in the successful implementation of these LED arrays—the increasing integration
of control functionality in sophisticated optimized control
devices. The Allegro™ A8501 LED driver is examined as
an example. It offers 2 MHz, 4 channel × 100 mA WLED/
RGB LED driving, with add-on features such as Output
Disconnect, and integrating a boost converter, 40 V DMOS
switch with load dump (for automotive) and overvoltage
protection (OVP).

Integration Level
An important consideration for the rapid development of
complex power management designs is the integration of
multiple features in the control electronics. Not only does this
save in PCB footprint area and peripheral parts matching and
qualification, but it also allows the extensive reuse of these
critical components in modular design elements, improving
the designs of derivative applications.
The Allegro A8501 demonstrates superior integration of
an extensive set of complementary features. The primary
features of interest include the following:
▪ 600 kHz to 2.2 MHz switching frequency—ability to operate above the AM band in Automotive applications
▪ Internal bias supply for single-supply operation (VIN = 8 to 21 V)
▪ Ability to ride through transient input voltages as low as 6.8 V
and as high as 40 V
▪ Boost converter with integrated 40 V DMOS switch and
OVP–load-dump protection

296074-AN

Figure 1. The A8501 is provided in a thin industrystandard 28-pin TSSOP package, with an exposed
pad for enhanced thermal dissipation.

▪ 3.5 μA shutdown current—limits battery drain
▪ Active current sharing between LED strings for
0.8% current matching and 0.7% accuracy
▪ Drive up to 9 series LEDs in 4 parallel strings, 36 LEDs
maximum (Vf = 3.5 V, If = 100 mA)
▪ LED sinks rated for 100 mA each (400 mA total)
▪ PWM dimming with LED PWM duty cycle control
▪ 4000:1 dimming range
▪ Extensive fault mode protection schemes:
▫ Shorted LED protection against misconnected loads—
with true output disconnect
▫ Open LED disconnect protects against LED failures
▫ External thermistor sensing to limit LED temperature
▫ Output overvoltage protection (OVP): 19.5 V default
can be adjusted as high as 38 V
▫ Open Schottky and open OVP resistor protection
against external component failure
▫ Input under- and overvoltage protection (UVLO and
OVLO) against VIN variation
▫ Boost current limit, output short circuit limit, overtemperature protection (OTP), and soft start