Chip vendors competing for a share of the red-hot white-LED driver market are pitching advanced charge-pump sources that deliver greater backlighting power for the color displays in larger and more complex portable/wireless devices. They're also promoting chips capable of tighter and tighter brightness-matching (generally to within 1 percent) of the LEDs in the backlighting panel, amid a fair amount of discussion of how good is good enough for the range of display applications. But those gains in most cases remain secondary to the OEM's most fundamental concerns: Higher efficiency in a switched-capacitor (charge-pump) power source usually used for driving LEDs in parallel, and maximizing the best of the tradeoffs with boost (step-up) converters, most often used to drive LEDs in series and series/parallel. Switched-cap devices take up much less space and seem the preferred choice of the major cell phone/small device OEMs. Apart from the different applications in which they're used, some say it's also because of a perceived notion that inductor-based power sources bring potentially insurmountable EMI problems.
In practice, the latest batch of charge pumps take on a topology that appears fairly common; chip differences often owe to subtle but significant tweaks designed to give users an edge in a given application. The typical device operates at a fixed 1 MHz for low noise and EMI and employs a "fractional pump" multiplying voltage architecture that provides 1x (power directly supplied from a lithium-ion battery or the like), 1.5x, and 2x at 15 to 30 mA of constant-current (the preferred driving arrangement for white-LEDs) per LED. As ideally applied, these pumps switch to the fractional/doubling modes only when needed to power a specific LED. Vendors are also finding better ways to match LED brightness levels without wasting power. Interfacing to the outside world has also become much more important for external programming and comprehensive control of charge-pump functions; many of the new devices have an I2C interface or the equivalent.
"Today's devices are being asked to provide backlighting power for main and secondary displays, as well as at least an RGB display," said Tony Armstrong, product marketing manager of the Power Business Unit at Linear Technology (Milpitas, Calif.), noting the expanding nature of the design requirement. Indeed, the company just released their LTC3206, one such charge-pump. The current-accuracy and brightness-matching issue is one that's fairly complicated, he stresses. "Although customers are happy with 10 percent current-accuracy (the difference between the average of LED currents versus a desired set current), they would complain about matching-accuracy (the worst-case difference between any two LED current-sources for a given average current) worse than 5 percent. Having said that, there are a lot of phones out there using our LTC3202 charge pump. If the forward voltage drop (VF) of one LED is only 100 mV different than another, which is very common, current-mismatch is 16 percent! So the light guides must make this not a big enough deal to require 1 percent matching." Beyond that, though, he says, one major issue often overlooked is overall efficiency, and the actual percentage of time a given display can be powered directly from the battery, versus by the charge-pump in its 1.5- or 2x mode when battery voltage begins to drop (beyond the need for a 2x supply, boost-converters usually come into play).
As part of both the efficiency and diode-matching issues, several vendors have several new entries that directly address the variations in a typical white-LEDs VF. Advanced Analogic Technologies (Sunnyvale, Calif.) touts its AAT3140 and AAT3141 as the first to do so. The chip's internal sensing circuitry sets the load switch (i.e., battery input, 1x) or charge-pump modes (1.5x, 2x) to match the operating conditions dictated by the input voltage and the required current-source output voltage, which depends on VF. "When battery voltage drops, drivers typically require that all LEDs in a display run off the less efficient charge pump, even if only one LED requires it, said David Brown, senior manager of applications engineering. "By combining our constant-current source architecture with built-in intelligence, we provide power from the charge-pump only to those LEDs that need it."
The four-output AAT3140 delivers 30 mA constant-current from each of its independent D/A converters that drive its individual LEDs. Users adjust LED brightness via an S2C single-wire interface, which drives the D/A converters through a ROM for 32-bit logarithmic control. The AAT3141 adds addressibility, using two ROMs, one dedicated to three LEDs and the other for a single LED. Fairchild Semiconductor (San Jose, Calif.), with its new FAN5607 adaptive charge pump that drives four LEDs in parallel, eliminates the need for LED pre-selection from another angle: Adjusting the DC output to the lowest minimum value required by the LED having the highest Vf and gradually increasing the output voltage until all devices conduct.
Other charge-pump entries include Arques Technology's (Austin, Tex.) AQ9156, with six-LED capability. It's a 1.5x pump that can deliver up to 120 mA. Texas Instruments' (Dallas) TPS60230 charge pump is suited to driving up to five white LEDs. The device, featuring 1x and 1.5x modes, delivers a constant-current of 25 mA to each LED (total 125 mA). TI says each LED's brightness level, adjusted using PWM control circuitry, is matched to within 0.3 percent. National Semiconductor's (Santa Clara) new LM2794/5 uses fractional (1.5x) conversion to drive up to four white LEDs with 0.5 percent matching. With similar characteristics, National's LM2791 and LM2792, which use a 2x boost converter and an analog brightness control for driving two white LEDs (15 mA to each diode), are matched to 0.3 percent. In addition, Maxim Integrated Products' (Sunnyvale, Calif.) MAX1573, in a 14-ball UCSP measuring 2.1 by 2.1 by 0.6 mm, is billed as the smallest, most efficient charge-pump for driving up to four white LEDs. Yet another device is Sipex' (Milpitas, Calif.) new SP7611/12/14 devices, which are essentially LDO drivers placed between the LED and ground that control two to four LEDs powered directly from the battery. The devices, which can deliver from 40 to 80 mA per output, are specifically suited to the next-generation of low-VF white-LEDs. Other new white-LED drivers include their SP6683, a charge-pump for up to 8 LEDs.
Controlling independent banks of LEDs becomes more important with multiple displays in a cell phone, and most manufactures make provision for this function. The more notable arrivals include Maxim's MAX1575, billed as the first high-efficiency charge pump for dual displays (four LEDs for the main display, two LEDs for the sub-display). Using a fractional charge pump (1x/1.5x) operating at 1 MHz and LDO regulators, it offers 2 percent LED current-matching, individual single-wire, serial-pulse interfaces for dimming LED brightness down to 5 percent, and a peak efficiency of 92 percent. It comes in a 4-by-4 mm, 16-pin TDFN package.
Analog Devices' (Woburn, Mass.) ADM8845, for specifically backlighting micro color TFT-LCDs, has individual control blocks (current-controlled sinks) for driving each of six white LEDs. Users have on/off control of two independent sets of four, and two, LEDs. Brightness matching of the LEDs (in this case, to 1 percent) is achieved by circuitry that senses each LED's current, which sets the individual current sources. Users adjust the actual brightness level via a DC voltage or PWM duty-cyle control.
Charge-pumps having significant current-handling capability are being seen more and more; as a result, they often approach what might be best termed a small general-purpose DC/DC converter suited to both special and general-purpose lighting. Maxim's MAX1574 for instance, in a 3-by-3 mm TDFN package using only four external components, is touted as the smallest solution for cell phones. It drives up to three LEDs, but it can deliver up to 180 mA, and is thus suited to camera flash applications (usually employing a high-intensity type of white LED) and keyboard backlighting. It provides current-matching to 0.6 percent. Its serial control interface facilitates dimming over a 5- to 100-percent range.
Boost converters come into play when driving a large number of LEDs in series. They're sometimes called constant-voltage devices and touted for use in applications where current-matching isn't as important; more often than not, though, these chips call on constant-current sources to drive one or more LED strings. A brief look at some of the more widely acclaimed arrivals for multiple LEDs include Linear Technology's LT3466, a dual step-up DC/DC converter that drives up to 20 white LEDs from a nominal 3.6-volt input. Each converter can drive up to 10 asymmetrically connected LED strings. Switching frequency can be set between 200 kHz and 2 MHz.
Intersil's (Milpitas, Calif) EL7513 is the first in the company's series of Elantec products for white LED applications. Delivering 18 volts from a 2.6 to 13.2 volt input, the chip, in an 8-pin TSOT, can drive up to 12 LEDs in a series/parallel configuration. Capable of powering eight LEDs in series, Micrel's (San Jose, Calif.) MIC2289 boost converter, with an output up to 34 volts from two lithium-ion batteries (2.5- to 10-volt input) is suited to a wide range of applications from cell phones to digital still cameras and camcorder applications.
The company says the chip, in a 2-by-2 mm MLF-8L package that requires only four external components, is the smallest boost solution available on the market. The chip has a 500-mA switch current rating, which reportedly suits it to applications such as camera flash devices. Another just to be released entry is Allegro's (Worcester, Mass.) A8430, a 300 mA boost-converter that can drive up to two strings of eight LEDs. Its QFN package, with the same pin out as an SOT-23, touts a very low profile: only 0.75 mm high. A similar device due soon is their A8431, which includes overvoltage protection, thus eliminating the need to put a zener diode across the load.
Previously, Maxim released its MAX1561, also suited to driving 12 LEDs. They also announced their MAX1582, a 26-volt step-up converter (84 percent efficiency) for driving six white LEDs in series. Others include the MAX1984/5/6, which can drive up to eight, six, and four LEDs, respectively. ON Semiconductor's (Phoenix) NCP5007, with a 22-volt output, can drive five LEDs.
DC/DC converters of the buck-boost variety are also in fair use for display backlighting. In most cases, though, they are generally regarded as general-purpose converters. National Semiconductor's LM3354/LM3355 is deemed a buck-boost constant-voltage driver that delivers up to 90 mA for keyboard lighting and other applications where brightness-matching isn't a priority. Similar devices include Fairchild's FAN5602. Other chips suitable for white-LED backlighting include National's step-up LM2703 (350 mA output) and LM2704 (550 mA output). These devices can power several strings of series-connected LEDs. In an application using a single Li-ion battery, the LM2703 and LM2704 can drive up to four, and eight white LEDs, respectively.
Advanced Analogic Technologies
Maxim Integrated Products