Advances in WLED/RGB LED Drivers
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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 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

through an integrated output disconnect switch. An optional external thermistor can be used to limit LED current based on panel temperature. Introduction The Allegro A8501 is a multioutput WLED/RGB driver for backlighting medium-size displays. It integrates a boost converter and four 100 mA current sinks. LED channels can be tied together for up to 400 mA sink capability. It can work from a single power supply of 8 to 21 V and withstand transients as low as 6.8 V and up to 40 V. The boost converter is a constant frequency, current-mode converter. The device is supplied in a surface mount,...

Power Limiting at Start-up Many of the new applications, such as notebook computers, are often operated in a battery-powered mode. Even in the case of devices, such as televisions that are intended to be operated from the utility mains, environment-friendly standards are driving designers to reduce power consumption wherever possible. In the A8501, these requirements are met by a combination of soft start and compensation techniques. At startup, the output capacitor, COUT in figure 3, is discharged and the device enters soft start. The boost current is limited to 0.6 A and all active LEDx...

Power Management in Normal Operation While power regulation at start-up is important, continuing the control seamlessly into normal operating modes is critical. The A8501 uses an innovative power management scheme that regulates based on current. The maximum LED current through the LEDs, referred as the 100% current, can be set externally. This is done by connecting a resistor, RISET, between the ISET pin and analog (power) ground. While the device can provide up to 100 mA per channel, the 100% current level is set to a reference current level, IISET , according to the following formula:...

Dimming LED Output in Normal Operation From a customer acceptance standpoint, the ability to control light output levels has an immediate impact on satisfaction. Backlighting levels must be adjustable to ambient light conditions and to the displayed contents, in order to adapt to user viewing capabilities and preferences. In addition, power-conservation mode when under battery power is essential for extended portable use. Such diverse requirements can be optimized by providing different approaches to the dimming function. With the A8501, the LED current can be reduced from the 100% current...

Protection Functions In previous sections of this application note, the essential function of power level management was described, both for powering-up and for normal operation. The industry-leading driver ICs, however, also offer circuit management functions. In this category, the most important are the system protection functions. The A8501 offers several useful protection features, described in this section. LED Open Detect When any LED string opens, the boost circuit increases the output voltage until it reaches the overvolt- age protection level. The OVP event causes any LED string...