LEDs have been used extensively in LCD backlighting in cellphones forseveral years. Today, their usage is expanding into larger LCDapplications including pocket PCs, car navigation GPS, digital pictureframes, portable DVD players and even notebook computers.
LEDs are also beginning to replace traditional incandescent bulbs orhalogen lights in residential, automotive and other general lightingapplications. The driving force behind this trend is the rapidtechnological progress, which has made LEDs brighter, highly efficientand more price competitive.
The justification for using LEDs often is simply their higherreliability and longer lifetime, offering end users a maintenance-freeproduct where the LEDs never need to be replaced.
To provide uniform backlight in color LCD applications, severalwhite LEDs are typically mounted along one side of the LCD panel. Thenumber of LEDs is proportional to the LCD size. For mid-size LCDs(7-inch to 10-inch), a total of 20 to 40 LEDs is commonly used. LEDsare configured in parallel strings of three or more LEDs in series.
To minimize connection points at the interface, many LCDs provideonly a two-terminal interface, where all strings are internally tied inparallel and are biased with a single supply.
Achieving the desired brightness in mid-size LCD panel applicationsrequires the drivers to deliver a regulated current to the LEDs acrossall operating conditions. Typically, two types of LED driver topologiesmay be used: capacitive charge pumps and inductor based switchingregulators. This article focuses on the inductive converter LED drivercircuits delivering between 1W to 6W of power to the LEDs.
Charge pump-type LED drivers are widely used today in cellphones andother small-sized LCD backlight applications because of their highefficiency, low cost and easy implementation. The external componentsrequired with charge pumps consist of only three or four capacitors andno inductor. However, there are limitations in terms of the outputpower.
Although some hig hpower flash LED charge pumps deliver up to 2W,the charge pumps are limited to a maximum output voltage of about 6Vand therefore cannot drive more than two LEDs in series. The number ofLEDs is determined by the number of channels in the charge pump(usually a maximum of six). Therefore, mid-size panel applicationscould be constrained by the charge-pump package, since more channelsmeans more pins and a larger package size.
The combination of LED forward voltage (VF), LED current and thesupply voltage range dictate the type of inductive converter LED drivercircuit required. LED VF varies with current, temperature and LEDmodel.
The maximum VF occurring at the lowest temperature is a criticalparameter when selecting the LED driver architecture: linear, buck orboost, as is the over-voltage protection (OVP) level. In this article,a maximum VF of 3.8V is assumed.
When selecting the LED driver IC, the key parameters are the switchcurrent limit; maximum output voltage; and over-voltage detectionthreshold required to protect against open LED conditions. Externalcomponents such as the inductor and capacitors should also be carefullyselected.
|Figure1: Shown is an example of an 8-inch LCD module backlit with a total ofnine strings of three white LEDs. The total string voltage with LEDforward voltage (VF) of 3.3V typical is about 10V.|
As an example, consider an 8-inch LCD module, which includes backlightwith a total of nine strings of three white LEDs shown in Figure 1 above. The total stringvoltage with LED forward voltage (VF) of 3.3V typical is about 10V (3 x3.3V). For 20mA per LED, the total current is 180mA (9 x 20), the totalLED power dissipated is 1.8W. A 5V power supply is provided from an ACpower adapter. An inductorbased LED driver is well suited for thisapplication.
First, let us find out what device switch current is needed tohandle a 2W load. Assuming an efficiency (?) of 80 percent, the inputcurrent is equal to Vout x Iout / Vin x ? = 10 x 0.18 / 5 x 0.8 =450mA. The CAT4139 inductive boost LED driver has a switch currentlimit of 750mA (minimum), which is well suited for this application.
The current rating of the inductor should be able to handle the peakLED driver switch current without going into saturation. Oncesaturation occurs, some current spikes occur because the inductorbehaves like a resistor and the circuit no longer works as expected.Inductors rated at 800mA or more are fine.
The maximum output voltage of the LED in operation should remainbelow the max output voltage. For three LEDs in series, in coldtemperature, the total forward voltage can be as high as 11.4V (3 x3.8V).
The open LED detection threshold of 24V is well above that limit. Ifthe LEDs become disconnected, the output voltage will increase and stayat about 30V, with the device now in a low-power mode and only drawinga few milliamperes from the supply. 30V rated output capacitors areadequate.
Now, we consider a 6W LED lamp powered from a 12V supply. This canbe achieved using six high-brightness white LEDs connected in seriesand driven at a fixed current of 300mA, with a typical forward voltageof 3.3V.
The LED string voltage is 20V typical increasing to 23V at coldtemperature (6 x 3.8V). This voltage is too high for a device like theCAT4139. A higher voltage boost LED driver such as the CAT4240 isrequired to drive this load. The CAT4240 boost LED driver has a higherover-voltage detection threshold of 40V and is compatible with stringsof up to 10 LEDs in series.
|Figure2: Five 1W LEDs are driven from a 24V power supply using a CAT4201 buckLED driver to provide an accurate average current.|
Choose to step-down
When the power supply voltage is higher than the total LED forwardvoltage, a linear current source or a switching step-down regulator canbe used to provide a constant current through the LEDs.
Linear current sources have one drawback— the power dissipated inthe regulator IC is proportional to the supply-to-load voltagedifferential. Switching solutions have higher efficiency, whichprevents any significant heat from being dissipated in the IC, keepingit close to, or slightly above, the ambient temperature.
The example in Figure 2 above shows how to drive five 1W LEDs from a 24V supply using a CAT4201step-down driver. The LED current is set by the external resistor R1.The CAT4201 buck LED driver provides an accurate average currentachieved in a two-phase switching operation. In the first phase, theinternal CAT4201 FET switch connects the SW pin to ground, so that thecurrent ramps up and charges the inductor.
The voltage applied across the inductor is basically 24V minus theLED voltage drop. As soon as the current reaches a determined peakvalue, the internal switch is turned off and the current continues toflow through the Schottky diode until the inductor is discharged.
When the inductor current drops to zero, the above sequence repeatsso that the inductor current waveform has a triangular shape. In thiscase, the switching frequency is around 260kHz. The capacitor C2 acrossthe LEDs minimizes the LED current ripple.
A larger capacitor size results in a smaller ripple. The overallconverter efficiency (power in LEDs divided by power from VBAT) in thisexample is at a high of about 94 percent.
The LED current remains well in regulation, as long as VBAT ishigher than the total VF plus 3V. Below this level, the LED currentdecreases linearly. Switching regulators and their external componentsmust be properly configured for each specific application.
The high efficiency of switching regulators renders thermaldissipation a non-issue in the power management circuit, with the userbenefiting from a savings in energy.
Linear current regulator ICs offer the benefit of inherent low noiseoperation (no switching), but are mainly suitable for lower currentapplications, due to the package temperature limitation.
When it comes to driving mid-size panels and general lightingapplications, inductive converter LED drivers are the solution ofchoice resulting in well-regulated LEDs and the best overall lightingefficiency. Selecting the right inductive converter— boost orbuck—depends on the application power supply and the LED configuration.
Fabien Francis ApplicationsManager at Catalyst SemiconductorInc.