PowerManagement ICs
Alternatively, choose a charge-pump (this low-EMI device can be step-up or step-down, although often with less current-handling capability) to drive parallel-connected LEDs, which are often how the LEDs are connected in handheld devices. The driver IC should be capable of handling the forward voltage requirements for each LED in the chain or, depending on the LED wiring configuration, include "matched" current sources to maintain near constant brightness levels from each LED in parallel.
Today's drivers often provide dimming control capability to
several thousand gradations, as per customer demand. Multiplexing schemes for driving LEDs are also sought-after features, although some of these functions are related to what the consumer wants in the way of extra bells and whistles (e.g., "fun lights" for portables).
Some notable LED driver
arrivals include the ISL97671/2/3/4 series of 6- channel devices from Intersil, which touts linear dimming capability down to 0.007 percent of full brightness to extend battery life in notebook applications. As a further aid to energy-savings, the devices' proprietary dynamic headroom control circuitry detects the highest voltage LED string and regulates its boost output to the minimum required voltage.
string with the highest forward voltage. TI's TLC59282, for video displays and message boards, is a 16-channel constant-current driver with four-channel grouped delay.
In addition, the H-grade (industrial) version of the LT3496 three-
channel LED driver fromLinear Technology can drive up to eight 500mA LEDs in series (total of 24 LEDs). Each channel can be dimmed independently to ratios as high as 3000:1.
When it comes to higher power residential/commercial
applications, the LM3450 from National Semiconductor (Santa Clara, Calif.) provides what the company says is the industry's best phase-based dimming performance. The driver, a part of the company's PowerWise family, integrates active power factor correction (PFC) and is suited to LED applications rated at from 10 to 100 watts. In addition, Texas Instruments' TPS92020 is a resonant-switching driver controller for high-efficiency street and stadium lighting applications.
Linear Technology's LT3799 is an isolated LED
controller with active power factor correction (PFC) specifically designed for driving 4 to 100 watts worth of LEDs from a 90-265 VAC input. Lack of suitable LED dimming capability is often an issue in these applications, but this one is compatible with standard in-wall triac dimmers.
Fairchild Semiconductor's FAN5622/24/26 can save up to 60 percent board space compared to alternative solutions by eliminating additional passive components used in charge pumps and series boost LED drivers
More systems on chip
Fully integrated systems---a collection of most of the major power-management IC functions--- are also finding more dedicated applications. A power-
Other chips include the ADP8866 from Analog Devices, a
charge-pump driven LED driver whose nine channels can be individually programmed for current, fade time, and blinking rate. Similarly, the CAT3649 from ON Semiconductor (Phoenix, Ariz.) is a six-channel LED driver with fractional charge pump that the company says will increase device efficiency by up to 10 percent over a traditional 1.5x charge pump and with no added external capacitors.
In addition, the FAN5622/24/26 from Fairchild Semiconductor
(San Jose, Calif.) are two-, four-, and six-channel linear current- sink LED drivers that the company says cuts up to 60 percent of the PCB space compared to alternate solutions.
TI's TPS61199, a white LED driver for large-area LCD
backlighting, is suitable for driving up to eight LED strings with multiple LEDs in series. Each string is independently regulated for current. Current matching between strings is three percent. The chip automatically minimizes output voltage to the LED
with-power management IC from austriamicrosystems (Unterpremstaetten, Austria), the AS3607, is suited to handheld, portable navigation devices, and media players. It integrates three DC/DC step-down converters (each 700 mA), a step-up converter, supervisory circuitry that has configurable start-up sequences, five LDOs, 1-amp Li-ion battery charger, and LED backlight driver and dimmer.
The AS3607 device's DC/DC step-up converter can deliver up
to 30 volts to drive strings of eight LEDs or an organic LED (OLED) display. An external PWMsignal can be applied to control dimming. A general-purpose interface is included for system- wide dissemination of the chip's charger status and low-battery and power-good conditions.
Similarly, TI's TPS658623 includes three programmable step-
down converters, 11 programmable general-purpose LDOs, dual RGB LED drivers, a constant current white LED driver, and host interface.
April 2011 | 27
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52