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Design Articles

Backlight LED Driver System in Small Format Liquid Crystal Displays

It pays to take a system-level approach to powering LED backlights in small-format LCDs.

By Dario Nurzad, Product Marketing Engineer, Mobile Power Devices, National Semiconductor

Light-emitting-diode (LED) technology is widely used to provide illumination for the pixels in small format liquid crystal displays (LCDs) in battery-powered applications. White light, emitted by the LEDs, is transmitted through a polarizer to the LCD where the light can be blocked or attenuated and sent on to RGB color filters to create colored light.

Figure 1 shows a system-level view of a backlight LED driver which consists of a DC/DC converter and one or more regulated current sources. Moreover, RGB-LED-based backlights require a temperature-based feedback control, which translates to a much higher cost than white-LED-based backlights. How much PCB area can be used? What features are needed? How much power will the system consume? Answering these questions can guide the designer in the selection of the appropriate backlight LED driver.

DC/DC Converters for LED Backlight

figure 1
Figure 1: Backlight LED driver system

In portable applications with single-cell Li-Ion source, the sum of the voltage drop across the white, green or blue LED and the current source can be lower or higher than the battery voltage. This means that, whereas a red LED can be powered directly from a single-cell Li-Ion battery, a white, blue or green LED requires the battery voltage to be sometimes boosted.  

The first aspect to consider, when selecting an LED driver for a battery-powered application, is the total area that the IC driver together with the external components will occupy (Figure 2).

Two boost technologies are widely used: step-up DC/DC converter, also called inductive boost, and switched capacitor converter, also charge pump. The charge pump implementation requires only four ceramic capacitors and one low-power resistor which results in a typically much smaller solution size. The capacitor value recommended for these applications is 0.47μF to 1μF with a voltage rating of 10V (to help with DC bias losses).  These capacitors can be found in 0402 or 0603 cases sizes from a number of capacitor manufacturers.  Total solution sizes less than 21mm2 are fairly common with also having the advantage of being very thin, less than 1mm.  Depending on the LED driver package, the capacitors can be the tallest component in the solution.

figure 2L figure 2R
Figure 2: Typical PCB layout examples: charge pump (left), inductive boost (right)

Inductive-boost-based LED drivers tend to have larger solution sizes compared to the switched-capacitor drivers. A typical solution size for an inductive-boost-based LED driver is closer to 30mm2 of board area.  Inductive drivers typically require two capacitors, one on the input and one on the output, with capacitor values of 1μF to 2.2μF and are available in 0603 and 0805 case sizes. Inductive boosts require a rectification element that can handle the peak inductor current and output voltage.  In a synchronous boost, a pass PFET can be integrated into the IC.  However, this integration often causes the size of the IC package to increase over an asynchronous solution.

The power conversion is also less efficient by about 10% in presence of an integrated high voltage PFET or Schottky diode. In an asynchronous topology, the pass element consists of a Schottky diode.  The main area increase of the inductive boost compared to the switch capacitor boost is the inductor itself.  Applications with 6-8 LEDs with currents of 15mA to 20mA typically require one inductor of 10μF to 22μH with a saturation current between 0.4A and 0.5A.  These inductors can be found in footprints smaller than 3.0mm x 3.0mm.  The inductor is also the tallest component in the solution with heights in the range of 0.8mm to 1.2mm.

figure 3
Figure 3: LM3509, Inductive boost LED driver

The easiest way to boost the battery voltage is to use a step-up DC/DC converter (Figure 3). The advantage of this method consists of very high efficiency over all load and input voltage conditions as the input voltage can be boosted to the sum of the LED forward voltage and current source headroom voltage. This significantly optimizes efficiency at the expense of cost and PCB area as previously explained.

Figure 3 shows the principle of operation of a magnetic boost regulator. When the NFET switch is closed (solid arrow), the inductor current iL(t) ramps upward from a minimum value Ia at t = t0 towards a maximum value of Ib at t = t1. During this time the Schottky diode is reverse biased and the load is supported by the energy stored in the output capacitor.

At t = t1 the NFET switch is turned off and the energy stored in the inductor L is now delivered to the output capacitor and to the load (dashed arrow) through the Schottky diode. Consequently the inductor current ramps down during time t2 to the previous value of Ia. The output voltage must be greater than the input: if this voltage relationship were not true, the inductor would not discharge into the output network. In other words, when the NFET is turned off the voltage across the inductor is reversed because the current discharge can not occur instantaneously. The input voltage added by the reversed magnetic voltage leads to the output voltage being higher than the input voltage. When driving 10 LEDs in series the supply voltage required can be as high as 35V. Another advantage of the boost topology is the simplified PCB routing: only two connections between the driver and LED strings are needed.

figure 4
Figure 4: Charge pump circuit with 1x and 1.5x gains

The second method of boosting the battery voltage is to use a charge pump (of which a simple implementation is shown in Figure 4), which utilizes the following property of capacitors: the capacitor charge build-up can not occur instantaneously which means that the initial voltage variation across the capacitor is equal to zero.

The voltage conversion is achieved in two phases. During the first phase switches S1, S2 and S3 are closed, whereas switches S4-S8 are open. Therefore C1 and C2 are stacked and, assuming C1 equal to C2, charged to half the input voltage:

formula1

The output load current is provided by the output capacitor CHOLD. As this capacitor discharges and the output voltage falls below the desired output voltage, the second phase is activated in order to boost the output voltage above this value. During the second phase C1 and C2 are in parallel, tied between VIN and VOUT. Switches S4-S7 are closed, whereas switches S1-S3 and S8 are open. Because the voltage drop across the capacitor can not change instantaneously the output voltage jumps to one and a half times the value of the input voltage:

formula2

In this way voltage boost operation is accomplished. The duty cycle of the switching signal is usually 50% as this value generally yields the optimal charge transfer efficiency.

A voltage conversion with a gain of 1x is achieved by closing switch S8 and leaving switches S1-S7 open. The benefit of the charge pump approach is the absence of the inductor. The inductor is source of EMI noise which can affect performance of the display or of the radio in a mobile phone. 

Input Power and LED Efficiency in Charge Pumps

In a charge pump LED driver, the output power relation that is to be used in the efficiency calculation, assuming all LEDs are identical, is given by:

formula3

The LED drive efficiency is given by:

formula4

where

formula5

and IQ is the supply current of the LED driver.

figure 5
Figure 5: Charge pump LED efficiency

Figure 5 shows a typical efficiency plot with the step indicating a gain transition.

However, for a given a LED current, the forward voltage can vary with process and temperature. This means that the efficiency of the LEDs can vary still maintaining brightness constant, since the latter is dependant only upon the current.

In order to make things clear let us consider an adaptive charge-pump-based LED driver circuit with the following specifications:

formula6

Neglecting the quiescent current IQ, the input current, when the charge pump operates with a gain of 1x, is given by:

formula7

Thus efficiency and input power are equal to:

formula8

Let us now calculate efficiency and input power for the same circuit but with LEDs having 3.3V forward voltage:

formula9

It is clear that efficiency has increased, but input power has remained the same; this means that efficiency of the LEDs, given by formula10, does not affect the power drained from the battery but the amount of power dissipated in the driver circuit. Efficiency is, therefore, not an adequate figure of merit to evaluate power consumption: what must be considered is input power versus LED brightness, i.e. LED current. Input power is the true measure of how many electrons are drained from the battery for a given LED brightness.

With a gain of 1.5x, under the previous conditions, the input power is equal to 333mW regardless of VLED.

Since a charge pump converter has a finite number of voltage gains there will always be, based on the application specifications, a certain amount of wasted power in the driver circuit. It is, therefore, important, in order to minimize input power, to operate the charge pump with the smallest possible gain,

Constant-Current LED Driver

figure 6
Figure 6: Regulated current source

The LED characteristics determine the forward voltage needed to achieve the needed level of current, which determines the amount of emitted light. Due to the variation in LED voltage versus current characteristics, controlling only the voltage across the LED leads to variability in light output. Therefore, most LED drivers use current regulation.

A circuit that accomplishes current regulation is a low-dropout regulator and is illustrated in Figure 6.  The error amplifier takes the voltage across R2, V2, compares it against the reference voltage VREF, and adjusts the LED current, IDX, via the series-pass element, an NFET, to the value required to drive the error signal (VERR=VREF-V2) as close as possible to zero. VREF is equal to:

formula11

where

formula12

Setting VREF =V2, the current through the LED is given by:

formula13

This holds true only if VOUT-VLED is sufficiently high to keep the pass element from saturating. As a matter of fact, the current sources require a minimum voltage, called headroom voltage VHR, across them in order to provide the desired regulated current through the LED. The headroom voltage is typically modeled with a resistor:

formula14

Brightness can be controlled directly by changing the LED current (analog control) or indirectly by shutting on and off the LED fast enough to create the perception of dimming to the human eye (PWM control). In most portable applications analog brightness control is preferred, as the backlight controller is normally distant from the LED driver. Therefore, having to route a PCB trace that carries a PWM signal close to noise-sensitive systems such as a radio transmitter, speaker or display, can cause problems.

figure 7
Figure 7: LP5520, Backlight RGB LED driver

Finally, in applications requiring superior color gamut, red, green and blue LEDs are utilized. The red LED is made of InGaAlP, whereas and both the blue and green are made of InGaN. When the ambient temperature is varied, the dominant wavelength changes dramatically for red compared to blue and green thus requiring some kind of temperature-compensated feedback loop. The LP5520 (Figure 7) adjusts the RGB LED currents for perfect white balance (color accuracy ΔX and ΔY < 0.003) by using an internal calibration memory, which stores the intensity vs temperature data of the LEDs, and an external temperature sensor.

Summary

This paper described the different building-blocks needed to power LED backlights in small format LCDs. A switched capacitor voltage regulator is desirable for applications requiring low cost and small solution size. The constant current regulator approach adds to these features excellent current matching and brightness control. Last but not least the inductive boost solution yields the lowest power consumption over the entire supply voltage range.

We emphasized the criteria to follow when evaluating power consumption of the different topologies: input power consumption, for a given brightness level, is the key factor to consider when evaluating a charge-pump-based LED driver. In switched capacitor circuits input power is directly related to the gain of the device, whereas in magnetic topologies input power is dependant on forward voltage of the LED (diode efficiency).

National Semiconductor Corporation
Santa Clara, CA
(408) 721-5000
www.nsc.com

This article originally appeared in the May, 2008 issue of Portable Design. Reprinted with permission.

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