Product category:
Power Supply ICs and Controllers
News Release from: National Semiconductor | Subject: LM2791/2/3/4, LM3354/5
Edited by the Electronicstalk Editorial
Team on 14 February 2002
Frugal drive for backlighting
National Semiconductor has released a range of six white-LED for use in hand-held battery-powered equipment.
National Semiconductor has released a range of six white-LED for use in hand-held battery-powered equipment The LM2791, LM2792, LM2794 and LM2795 are constant-current drivers optimised to light displays with up to four white LEDs
This article was originally published on Electronicstalk on 5 Jan 2001 at 8.00am (UK)
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The LM3354 and LM3355 are constant voltage drivers optimised to drive larger numbers of white LEDs in applications such as keypad backlighting.
The LM2703 and LM2704 are magnetic boost convertors optimised to drive white LEDs in applications where efficiency is paramount.
Together, the eight chips make a complete solution set that addresses all white-LED driver applications and performance requirements found in handheld, portable electronics.
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The LM2794 and LM2795 combine switched capacitor boost technology and matched constant current sources to drive white LEDs.
Four current sources matched to +/-0.5% typical ensure consistent brightness and colour among the LEDs, making the LM2794 and LM2795 ideal for driving display LEDs.
The typical application of four LEDs driven with 15mA each is easily realised with the 80mA available.
The drive current is programmable by a single external resistor and both analogue and PWM methods of varying the drive current for brightness control are accommodated.
Both devices switch at 325kHz minimum to keep the conducted noise spectrum away from sensitive frequencies within portable RF devices.
The LM3354 and LM3355 also apply switched capacitor DC/DC technology but convert a variable battery voltage to a constant voltage, as opposed to the matched constant currents of the LM279x chips.
An innovative regulating buck-boost architecture yields an average efficiency of about 80% over the typical Li-ion discharge range, a much higher efficiency than that delivered by open-loop switched capacitor circuits.
The LM335x devices are ideal for driving large numbers of white LEDs in applications where brightness and colour uniformity among the LEDs is not a primary concern.
A typical application is twelve, 5mA LEDs backlighting a cellphone keypad.
In this application the keypad disperses the light making brightness and colour matching a secondary concern and the added cost of individual matched current sources for so many LEDs unnecessary.
The LM3354 supplies up to 80mA while the LM3355 supplies up to 50mA.
Both devices are also applicable as general-purpose regulated DC/DC buck-boost convertors.
An example application is converting the 2.7 to 4.2V rail supplied by a single-cell Li-ion battery into a regulated 3.3V, 50mA rail.
The LM2703 and LM2704 are magnetic boost DC/DC convertors optimised to drive up to four white LEDs in series and several strings of series LEDs at once.
All the LEDs in a particular string are driven with the same constant current resulting in consistent brightness and colour among those LEDs.
The capability to achieve consistent brightness and colour among several white LEDs coupled with the capability to drive large numbers of white LEDs makes the LM270x devices ideal for driving either display LEDs or keypad backlight LEDs or both at once.
The LM270x devices drive white LEDs more efficiently than any of the switched capacitor-based drivers.
The LM2704 achieves 85% efficiency delivering 20mA at 20V.
The LM2703 has a 300mA peak current limit, and the LM2704 has a 600mA peak current limit.
Both devices are also applicable as general-purpose magnetic boost DC/DC convertors.
An example application is the LM2704 boosting the 2.7 to 4.2V rail supplied by a single-cell Li-ion battery into a regulated 5V, 80mA rail.
National's white-LED drivers are offered in a variety of chip-scale packages including National's revolutionary micro SMD and leadless leadframe (LLP) packages.
In the case of micro SMD, a package National pioneered, the silicon die is the package giving micro SMD several advantages over traditional plastic leaded packages including reductions in footprint areas on the order of three to nine times, higher I/O density, and better electrical performance.
National's LLPs feature industry-leading thermal capability in dramatically smaller, thinner form factors than traditional power packages.
The LM2791 and LM2792 are offered in the LLP-10 package.
The LM2794 and LM2795 are offered in the micro SMD-14 package.
The LM3354 and LM3355 are offered in the MSOP-10 package.
The LM2703 and LM2704 are offered in the SOT23-5 package.
All six new devices are available now.
The LM2794 and LM2795 are priced at $1.30.
The LM3354 is priced at $1.20.
The LM3355 is priced at $0.95.
The LM2703 is priced at $0.85.
The LM2704 is priced at $0.90.
All prices are for a quantity of 1000 units.
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