a matter of light

8/4/2019 A Matter of Light

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A matter of light, Part 1---The ABC's of LEDs

Conserving energy is now a mandate, not a choice, and part of that mandate is the need to go Green. When it comes

to lighting, we can easily imagine the impact of globally improving the efficiency of lighting sources by 10 percent. But

what if it could be improved by 1000 percent? Newly enhanced light emitting diodes (LEDs) have the potential to

achieve these efficiency improvements while maintaining high performance and reliability that supersede many

currently used sources. In the first part of this four-part series, we look at the LED's physical structure, range ofcolors, efficiency, LED drivers, and applications.

Anatomy

Physically, LEDs resemble p-n junction diodes. As with p-n junctions, electrons and holes flow towards the junction

when a positive differential voltage is applied between the anode (p-side), and cathode (n-side). Once an electron is

recombined with a hole, it releases energy. Depending on the physical properties of the p-n junction materials, the

released energy can be non-radiative, as for the typical diode applied in discrete circuits, or in the form of emissions

in the optical range. For an LED, the wavelength of the emitted light (i.e., its color) depends on the band gap

characteristics of its p-n junction material. Performance-wise, LED materials have relatively low reverse breakdown

voltages since they have relatively low band gaps.

Colors

Red LEDs were the first to become commercially available in the late 1960s but their light output was very low.Despite this shortcoming, they were commonly used in seven-segment displays. Thanks to advancements in material

science, nowadays LEDs are commercially available in a variety of colors with some of them having light outputs that

would blind you if you stared directly at them.

Blue LEDs became widely available a few years ago. Mixing blue LEDs with red and green LEDs produces white

light. This technique of generating white light provides a large color gamut, dynamic light tuning, and excellent color

rendering (CRI), which is well suited for high-end backlighting applications. A simpler and more economical way of

producing white light is to use blue LEDs and a phosphor coating that converts some of the blue light to yellow. The

yellow light stimulates the red and green receptors of the eye, and therefore mixing blue and yellow gives the

appearance of white. This scheme can provide good CRI but the LED's light output may suffer from inconsistent color

temperatures due to manufacturing discrepancies and varying thicknesses in the phosphor coating layer.

(Click on Image to Enlarge)

Figure 1: LED color chart for the basic colors
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Efficiency

High efficiency is the buzz word for LED-based light sources. When it comes to lighting, efficiency is defined as the

light output per unit power. Thus, in the metric system, it is measured in lumens (lm) per watt (W). Recently some

LED manufactures introduced LEDs with promised efficiencies hitting the 150 lm/W mark. In comparison,

incandescent comes in at 15 lm/W, and fluorescent provides 70 lm/W. So could LEDs put incandescent and

fluorescent out of business any time soon? Maybe, but, unfortunately some of these LED's efficiency numbers are

subject to specmanship. The problem is that LED inefficiency has to do more with the fact that a considerable portion

of the produced light is reflected at the surface of the packing material back into the LED die. This reflected light is

likely to be absorbed by the semiconductor material and turned into heat.

Anti-reflection coating, and minimizing the reflection angles by using a half-sphere package with the LED placed at

the center, reduces the amount of reflected light and improves efficiency. However, these techniques are subject to

manufacturing variations and may require high premiums to ensure consistent performance. So while LEDs are

rapidly being adopted by industry, they've got a long way to go.

Applications

There are many factors which make LEDs eye-catching for high-performance modern electronics. For example, their

higher light output per watt extends battery life and thus they are well suited for portable applications. In addition, an

LED's fast turn-on/turn-off characteristics fit perfectly with the needs of automotive tail lights, especially the brake

lights, since it improves safety by providing drivers more response time. RGB LEDs in backlighting complies with

ROHS standards, since LEDs do not contain lead or mercury. LED lighting facilitates a full-spectrum light source with

larger color gamut. LEDs have an exceptionally long lifespan, which enables their use in applications where long-term

reliability is highly desirable, such as traffic lights. Machine vision systems require a focused, bright, and

homogeneous light sourceLEDs are a great match. LEDs, with their simple-to-implement dynamic light-tuning,

would also allow you to set the light in your living room to green when you need to relax and to red when it's time for

bullfighting.

Drivers

LEDs are inherently current-driven devices; i.e., their brightness varies with their forward current, IF. Depending on

the color as well as the forward current, the LEDs' forward voltage drop, VF, varies as well. Thus, driving LEDs with a

constant current is essential to achieve the desired color and brightness level. An LED driver scheme can be as

simple as a voltage source and a ballast resistor (Figure 2a). This solution works best for narrow-input range, low-

current applications in which the LED's forward voltage drop is slightly below the input supply voltage. But variationsin the input supply voltage or the LED forward voltage drop will increase the LED current and, therefore, the light

intensity and the color will shift.



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Figure 2: Simplified LED driver schemes

Linear regulators can be used to provide tighter LED current control in small step-down ratio applications (Figure 2b).

In the case of low-current step-up requirements, switching capacitor circuits can be utilized (Figure 3c).

For wide-input range, high-current applications, simple driver schemes such as those mentioned above unfortunately

yield high power dissipation and poor efficiency. Consequently, more efficient and relatively more complex solutions

such as switching regulators are required (Figure 2d). Switching regulators process power by interrupting the power

flow and controlling the conversion duty cycle, which results in pulsating current and voltage. They can be configured

in isolated and non-isolated configurations to realize voltage or current step-down (buck), step-up (boost) or both

(buck-boost) functions.

In general, designers select a switching-regulator topology based on a tradeoff between cost and desired

performance at a given power conversion requirement. On the other hand, in order to properly drive LEDs, the

switching regulators should be configured as constant-current sources. Which switching-regulator topology can be

simply configured as a current source while providing an optimal tradeoff between cost and performance? Stay tuned

for part two of this series.

About the authors

Sameh Sarhan is a staff applications engineer for the Medium Voltage/High Voltage Power Management group in

Santa Clara, CA. He has been involved with power electronics in various forms since 1998, having worked for FRC

Corp. and Vicor Corp. His experience includes the design of hard/soft switching power supplies from a few watts to

600 watts. Sameh received a bachelor's degree in electronics engineering in 1996 from Cairo University (Egypt).

Chris Richardson is an applications engineer in the Power Management Products group, Medium and High Voltage

Division. His responsibilities are divided between lab work, bench evaluation of new ICs, written work such as

datasheets and applications notes, and training for field engineers and seminars. Since joining National

Semiconductor in 2001, Chris has worked mainly on synchronous buck controllers and regulators. In the last three

years he has focused on products for the emerging high brightness LED market in the automotive and industrial

areas. Chris holds a BSEE from the Virginia Polytechnic Institute and State University.

Design ArticleA matter of light, Part 2--- Buck whenever possible

Sameh Sarhan and Chris Richardson, National Semiconductor

5/27/2008 9:22 AM EDT
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Inpart one of this series, we thrashed out the basics of LED lighting sources and their driving requirements. The

performance of simple driving techniques, such as voltage sources/ballast resistors and linear regulators, fall short as

the complexity and input power requirements of LED-based lighting sources increase. Thus a more sophisticated

switch-mode LED driver is required. So what would be the topology of choice? In part 2, we discuss why a constant-

current buck converter should be the first preference when it comes to switch-mode LED drivers or, in other words,

why the buck should be used whenever possible.

The rapid adoption of LEDs in various applications makes simple drive solutions such as linear regulators impractical

in many cases. In general, simple drive schemes continuously deliver power from the input source to the driver's

output while using resistive elements to program the desired LED forward current. For the same LED current, the

losses in these resistive elements increase considerably as the line voltages increase. For example, a linear regulator

based LED driver yields 70 percent efficiency when supplying 1 amp from a 5-volt input source to a typical white

InGaN LED (VF= 3.5V). Under the same operating conditions, the driver's efficiency will drop to approximately 30

percent when the input voltage increases to 12 volts. Such poor efficiencies require impractical thermal management

schemes.

Switching regulatorsSwitching regulators improve the conversion efficiency. They interrupt the power flow while controlling the conversion

duty cycle to program the desired output voltage or output current. Interrupting the power flow results in pulsating

current and voltage and therefore it necessitates the use of energy storage elements (inductors and/or capacitors) to

filter these pulsating waveforms. Contrary to linear regulators, switching regulators can be configured in different

arrangements to realize voltage or current step-down (buck), step-up (boost) or both (buck-boost) functions. They are

also capable of achieving high conversion efficiencies across wide input/output range. Replacing the linear regulator

with a buck-based LED driver in the previous example yields 95 to 98 percent efficiency across the 5-to-12 volt input

range.

The configuration flexibility and the efficiency improvements of switching regulators come at the expense of higher

noise generation caused by the periodic switching events, as well as higher premiums and reduced reliability due to

their perceived complexity. Constant-current LEDs favor regulator topologies that can be simply configured as a

constant-current source. The selected topology should also combine high performance with minimum component-

count to increase the driver's reliability and to reduce cost. It should also facilitate the use of various dimming

techniques to take advantage of the LEDs dynamic light- tuning characteristic. Fortunately, the most basic step-down

(buck) switching topology enjoys all these characteristics, making it the regulator of choice to drive LEDs whenever

possible.

Constant-current power stage

Switching regulators are most commonly known as voltage regulators. Figure 1a illustrates a basic constant-voltage

buck regulator. The buck controller maintains a constant output voltage as the line voltage changes by varying the

operating duty cycle (D) or the switching frequency. The desired output voltage set point is programmed using the

following equation (Eq. 1):
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Fig. 1a: Basic step-down (buck) voltage regulator

The inductor, L, is selected to set the peak-to-peak current ripple, Ipp, while the capacitor, Co, is selected to program

a desired output-voltage ripple and to provide output-voltage hold-up under load transients. The average inductor

current in a buck converter is equal to the load current, and, therefore, we can set the load current by controlling the

peak-to-peak inductor-current ripple. This significantly simplifies the conversion of a constant-voltage source into a

constant-current source. Figure 1b illustrates a basic constant-current buck regulator. Similarly, constant-current buck

regulators provide line regulation by adjusting the conversion duty cycle or the switching frequency, and the LED

current, IF, is programmed using the following equation (Eq. 2):

Fig. 1b: Basic step-down (buck) current regulator

After we set the LED current, IF, we must properly sense the inductor current. Theoretically, multiple current sense

schemes such as MOSFET Rdson sensing and inductor DCR sensing can be used. However, practically, the current

sense precision of some of these would not meet the required LED current set point accuracy (5 to 15 percent for a

high brightness LED (HB-LED). If we directly sense IF through an inline resistor, RFB, we can secure the needed


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precision, but there may be excessive power dissipation in the current-sense resistor. Lowering the feedback

voltage, VFB, allows the use of lower resistance values for the same IF (Eq. 2), which minimizes losses. The newer

dedicated LED drivers generally offer reference voltages (feedback voltages) within the range of 50 to 200 millivolts.

Uniquely, constant-current buck-driven regulators can be configured without output capacitance. The use of the

output capacitor, Co, in these regulators is limited to AC current filtering since they inherently do not experience load

transients and have continuous output currents. When we configure a constant-current buck regulator without output

capacitance, we substantially increase the converter's output impedance and, in turn, boost the converter's ability to

rapidly change its output voltage so that it can maintain a constant current. As a result, the dimming speed and

dimming range of the converter improve significantly. Wide dimming range is valuable feature in applications such as

backlighting and machine vision.

On the other hand, lacking the required output capacitance, AC-current-ripple filtering circuitry necessitates the use of

higher inductance values in order to meet the LED manufacturers recommended ripple current (IF = 5 to 20 percent

of the DC forward current). At the same current rating, higher inductance values increase the size and cost of the

LED driver. Consequently, the use of output capacitors in constant-current buck-based LED drivers is governed by a

tradeoff between cost and size versus dimming speed and dimming range.

For example, in order to drive a single white LED (VF 3.5 volts) at 1 amp with a ripple current, IF, of 5 percent

from an input of 12 volts at 500 kHz requires a 50 microhenry inductor with a current rating of 1.1 amps. However if

the inductor ripple-current is allowed to increase to 30 percent, then the inductance required is less than 10

microhenries. For the same core material and at approximately the same current rating, a 10 microhenry inductor will

be typically offered at roughly half the size and cost of a 50 microhenry inductor. To attain the desired IF (5

percent) using the 10 microhenry inductor, the output capacitance required is calculated based on the dynamic

resistance, rD, of the LED, the sense resistance, RFB, and the impedance of the capacitor at the switching frequency,

using the following expression (Eq. 3):

where (Eq. 4):

Control-loop schemes

Buck-based power stages are well-matched to several control-loop schemes and free of stability limitations such as

right-half-plane zeros. They uniquely facilitate the shunt PWM dimming approach in addition to being compatible with

other dimming methods. This provides the system designer with configuration flexibility when designing an LED driver

for specific requirements. Hysteretic control is well-suited for applications such as light bulbs and traffic lights, in


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which variable switching frequencies are tolerated or where narrow input-voltage range supplies are used. Hysteretic

control doest not experience control-loop bandwidth restrictions, which eliminates the need for loop compensation

because of its inherent stability. Utilizing hysteretic control to drive a buck-based LED driver (Fig. 2a) greatly

simplifies the design as well as reduces the component count, and the cost of the driver. This configuration also

yields superior PWM dimming ranges that outperform other buck-based schemes. Using hysteretic buck-based LED

drivers with the shunt-dimming approach is well-suited for applications that require ultra-wide dimming ranges at high

dimming frequencies and that can tolerate variable switching frequencies.

Fig. 2a: Basic hysteretic buck-based driver

Quasi-hysteretic buck-based LED drivers offer a good compromise between fixed-frequency operation and hysteretic

control for applications in which variable switching frequencies may not be desired. The controlled on-time (quasi-

hysteretic) buck-based LED driver (Fig. 2b) employs a control scheme based on a hysteretic comparator and a one-

shot on-timer which is used to set a controlled on-time. This controlled on-time is programmed so that it is inversely

proportional to the input voltage, and, therefore, it minimizes the switching frequency variations as the line voltage

changes. Using this scheme also eliminates the need for control-loop bandwidth limitations, enabling it to achieve

wide dimming ranges when used with different dimming configurations.

Fig. 2b: Basic controlled on-time buck-based LED driver

In some cases, as in a number of automotive applications, synchronizing the LED driver(s) to an external clock or to

each other may be required to minimize noise interference. Implementing the frequency synchronization feature with


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the non-clock-based hysteretic and quasi-hysteretic scheme can be challenging. In contrast, this feature can be

simply realized in clock-based regulators such as the fixed-frequency buck LED driver shown in Fig. 2c. Fixed

frequency control generally yields a more complex solution, and it limits the dimming range of the driver regardless of

the dimming approach due to its dynamic response limitations.

Fig. 2c: Basic fixed-frequency buck-based LED driver

In summary, there are many characteristics that make buck-based regulators attractive LED drivers. They are simple

to configure as a current source and can be realized with minimum component counts, which simplifies the design

process, improves the drivers' reliability, and reduces cost. Buck-based LED drivers also provide configuration

flexibility since they are compatible with multiple control schemes. They also allow for high-speed dimming as well as

wide dimming ranges since they can be configured without output capacitance and are well-matched to various

dimming approaches including shunt dimming. All these features make buck-based (step-down) LED drivers the

topology of choice whenever the application permits.

What if the application does not permit their use? Applications such as residential and commercial lighting require

thousands of lumens, creating a need to drive LED strings. The total forward voltage drop of an LED string is equal to

the sum of the forward voltage drops of all the LEDs in the string. In some cases, the input voltage range of the

system can be lower than the forward voltage drop of the LED string, or it can vary so that sometimes it's lower and

sometimes it's higher. These scenarios would require either boost, or buck-boost switching regulators. In the next

installment, we discuss the challenges of using boost and buck-boost topologies to drive LEDs.

About the authors












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Design ArticleA matter of light, Part 3---When to boost and buck-boost


5/31/2008 10:52 AM EDT

Inpart one of this series, we looked at the basics of LED lighting sources and their driving requirements. Inpart 2, wediscussed why a constant-current buck converter should be your first preference when it comes to switch-mode LED

drivers. In this third installment, we investigate larger LED displays and the applications space for other converter

topologies.

Manufacturers and designers of LED lighting often refer to applications with clear advantages for solid state

illumination as "low-hanging fruit." Examples such as garden path lighting or MR16 bulb replacement often require

only a few LEDs, or just one (Fig. 1). The most common voltages for low-voltage lighting are 12 VDC, 24 VDC, and

12 VAC. These applications often use a buck regulator. Although the buck is preferred, as previously discussed, the

boost regulator is finding more use as the number of LEDs increase for LED lighting applications. Not content to pick

off flashlights or single bulb replacements anymore, designers are targeting large-scale general illumination, systemsthat require thousands of lumens. Examples include street lighting, residential and commercial lighting, stadium

lighting, and decorative or architectural lighting of spaces both interior and exterior.


Figure 1: Buck and boost LED drivers with Vocalculation; buck: VO = n x VF, VO < VIN; boost: VO = n x VF, Vo >

VIN

Constant-current still required

As with linear and buck-derived LED drivers, the main technical challenge in boost LED driver design is providing a

controlled forward current, IF, to each LED of the array. Ideally every LED would be placed in a single series chain,

ensuring that the same current flows through each device. A boost regulator is the simplest choice when stepping up

a DC input voltage up to a higher DC output voltage, as it allows more LEDs to be placed in series from a given input

voltage. A system designer for general illumination usually draws line power at 110 VAC or 220 VAC. If power factor

correction (PFC), galvanic isolation, and line harmonic filtering aren't required, then single stage, non-isolated

switching converters (buck, boost, or various buck-boost topologies) can use the rectified output of AC line voltage

and directly drive long strings of series connected LEDs.

In many cases, however, an intermediate DC bus voltage is used, derived from an AC/DC regulator that takes a

universal AC input and provides PFC, isolation, and filtering. Besides solving legal requirements, a lower intermediate

voltage bus reduces problems with dielectric breakdown, arcing, and improves the safety of service people working
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with lighting. The European Union leads the world with the strictest legal requirements, including PFC for any lighting

over 25 watts. North America and Asia consistently follow Europe's lead, albeit some years later. Safety standards

and electrical codes such as UL and CE limit the output voltage of the AC/DC power supply that forms the input of the

boost LED driver. Common rails are 12 and 24 volts, and in some cases 48 volts. Rarely are these intermediate bus

rails higher than 60 volts, which is the cutoff for DC voltages under UL Class 2.

The boost challenge

Boost regulators are more difficult to design than buck regulators, regardless of whether we control the output voltage

or the output current. The average inductor current in a continuous conduction mode (CCM) boost converter is equal

to the load current (LED current) multiplied by 1/(1 - D), where D is the duty cycle. Boost voltage regulators require

design review at the limits of input voltage to ensure correct design of the inductor, especially the peak current rating.

A boost LED driver adds a variable output voltage that influences duty cycle and therefore the inductance and current

rating of the main inductor. To prevent inductor saturation, the maximum average and peak currents must be

evaluated at both VIN-MIN and VO-MAX. For example, over the range of process, drive current, and die temperature, a

typical white InGaN LED's VFcan vary from 3 to 4 volts. The more LEDs are placed in series, the greater the gap

betweenVO-MIN and VO-MAX.

Unlike the buck regulator with its output inductor, the boost converter has a discontinuous output current. For this

reason an output capacitor is required to keep the output voltage (and hence the output current) continuous. Where

the output capacitor in a voltage regulator is designed to both filter and hold up the output voltage during load

transients, in a current regulator it functions as an AC current filter only. The capacitance is made as low as possible,

consistent with maintaining the desired LED ripple current. The lower the output capacitance (which keeps cost and

size to a minimum), the faster the converter's response to changes in output current, and consequently the LED's

dimming response is better.

Another serious challenge for boost converters is the control loop. Buck regulators are available with voltage mode

PWM control, peak current mode PWM control, constant/controlled on-time, and hysteretic control among others.

Boost regulators in CCM (with the exception of low-power/portable equipment) are almost universally constrained to

peak current mode PWM control, owing to their right-half plane zeroes and the fact that they deliver power to the

output when the control switch is off. To design a boost LED driver that controls output current, the control loop must

be analyzed using LEDs as the load, a case much different from the typical load of a boost voltage regulator. In peak

current mode control, the impedance of the load has a strong effect on both the DC gain and the low-frequency pole

of the control-to-output transfer function. For voltage regulators the load impedance is determined by dividing output

voltage by output current. LEDs are diodes, with a dynamic resistance. This dynamic resistance can only be

determined by plotting the VFversus IFcurve and then taking the tangent line to find the slope at the desired forward

current. As shown in Fig. 1, the current regulator uses the load itself as a feedback divider to close the control loop.

This reduces the DC gain by a factor of (RSNS/ (RSNS+ rD)). It is tempting to compensate a boost LED driver with a

simple integrator, sacrificing bandwidth for stability. The reality is that many, if not most LED driver applications

require dimming. Whether dimming is done by linear adjustment ofIF (analog dimming) or by turning the output on


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and off at high frequency (digital, or PWM dimming) the system requires high bandwidth and fast transient response

just as a voltage regulator does.

The buck-boost challenge

LEDs for lighting are being adopted much faster than the standards for solid state illumination have developed. A

wide variety of input voltages power a wide variety of LEDs. The number of LEDs in series, the type of LEDs, and the

variation ofVFwith both process and die temperature all contribute to a wide range of output voltage. For example,

high-end automobiles are converting to LEDs for their daytime running lamps. Three 3-watt white LEDs present a

load of about 12 volts at a current of 1 amp. Automotive voltage systems usually require continuous operation over a

range of 9 to 16 volts, with an extended range of 6 to 42 volts where performance is reduced but the system can

operate without suffering damage. In general, the buck regulator makes the best LED driver, followed by the boost,

but neither is appropriate for this case. If a buck-boost regulator must be used, the most difficult decision to make is

often which topology to use.

One fundamental difference between buck-boost regulators of any topology and the buck regulator or the boost

regulator is that the buck-boosts never connect the input power supply directly to the output. Both the buck and theboost regulator connect VIN to VO (across the inductor and switch/diode) during a portion of their switching cycles, and

this direct connection gives them better efficiency. All buck-boost regulators store the entire energy delivered to the

load in either a magnetic field (inductor or transformer) or in an electric field (in a capacitor), which results in higher

peak currents or higher voltage in the power switches. In particular, evaluation of the converter at the corners of both

input voltage and output voltage is necessary because peak switch current occurs atVIN-MIN and VO-MAX, but peak

switch voltage occurs at VIN-MAX and VIN-MAX andVO-MAX. In general this means that a buck-boost regulator of a certain

output power will be larger and less efficient than a buck or boost regulator of equal output power.

The single inductor buck-boost can be built with the same parts count as a buck regulator or boost regulator, making

it attractive from a system cost standpoint. One disadvantage of this topology is that the polarity ofVo is inverted

(Figure 2a) or regulated with respect to VIN (Figure 2b). Level-shifting or polarity inverting circuitry must be employed

in these converters. Like the boost converter, they have a discontinuous output current, and require an output

capacitor to maintain a continuous LED current. The power MOSFET suffers a peak current ofIINplus IFand a peak

voltage ofVINplus VO.


Figure 2: High-side buck-boost (a); low-side buck-boost (b)

Other topologies

The SEPIC converter has the advantages of a continuous input current due to the input inductor and positive output

voltage. Like the boost and single inductor buck-boost it requires an output capacitor to maintain a smooth LED
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current. A further advantage of SEPIC converters is that almost any low-side regulator or controller can be configured

as a SEPIC without the need of polarity inversion or level shift circuitry.

Figure 3: SEPIC LED driver

Rarely used in voltage regulation, the Cuk converter has emerged as an LED driver. Input and output currents are

continuous. The polarity of the output voltage is reversed, as with the high-side buck-boost, but the output capacitor

can be eliminated like the buck converter. The Cuk is the only other practical non-isolated regulator with this ability.

Figure 4: Cuk regulator

Neither the boost nor the buck-boost regulator is preferred for switching LED drivers, owing to their higher complexity

and parts count, lower efficiency (especially for the buck-boosts) and scant choice of control topologies. However,

both are 'necessary evils' as LEDs push into more and more lighting applications. In some cases the system

architecture can be altered to allow buck or even linear regulator-based LED drivers. Examples include very large

light sources, such as street lights, where a hundred or more 1W+ LEDs are required. In general, LEDs for general

illumination are working their way from lower to higher power, and in the intermediate arena, such as automotive

headlights and small lighting fixtures, boost and buck-boost regulators represent the best choice for constant-current

driving.

About the authors



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Design ArticleA matter of light, Part 4 --- PWM dimming


6/5/2008 11:17 PM EDT

Inpart one of this series, we looked at the basics of LED lighting sources and their driving requirements. Inpart two,

we discussed why a constant-current buck converter should be your first preference when it comes to switch-mode

LED drivers. Inpart 3, we investigated larger LED displays and the applications space for other converter topologies.

Here in the concluding part of this series, the authors take a look at how to best implement the dimming function.

Whether you drive LEDs with a buck, boost, buck-boost or linear regulator, the common thread is drive circuitry to

control the light output. A few applications are as simple as ON and OFF, but the greater number of applications call

for dimming the output between zero and 100 percent, often with fine resolution. The designer has two main choices:

adjust the LED current linearly (analog dimming), or use switching circuitry that works at a frequency high enough for

the eye to average the light output (digital dimming). Using pulse-width modulation (PWM) to set the period and duty

cycle (Fig. 1) is perhaps the easiest way to accomplish digital dimming, and a buck regulator topology will often

provide the best performance.


Figure 1: LED driver using PWM dimming, with waveforms

PWM dimming preferred

Analog dimming is often simpler to implement. We vary the output of the LED driver in proportion to a control voltage.Analog dimming introduces no new frequencies as potential sources of EMC/EMI. However, PWM dimming is used in

most designs, owing to a fundamental property of LEDs: the character of the light emitted shifts in proportion to the

average drive current. For monochromatic LEDs, the dominant wavelength changes. For white LEDs, the correlated

color temperature (CCT) changes. It's difficult for the human eye to detect a change of a few nanometers in a red,

green, or blue LED, especially when the light intensity is also changing. A change in color temperature of white light,

however, is easily detected.
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Most white LEDs consist of a die that emits photons in the blue spectrum, which strike a phosphor coating that in turn

emits photons over a broad range of visible light. At low currents the phosphor dominates and the light tends to be

more yellow. At high currents the blue emission of the LED dominates, giving the light a blue cast, leading to a higher

CCT. In applications with more than one white LED, a difference in CCT between two adjacent LEDs can be both

obvious and unpleasant. That concept extends to light sources that blend light from multiple monochromatic LEDs.

When we have more than one light source, any difference between them jars the senses.

LED manufacturers specify a certain drive current in the electrical characteristics tables of their products, and they

guarantee the dominant wavelength or CCT only at those specified currents. Dimming with PWM ensures that the

LEDs emit the color that the lighting designer needs, regardless of the intensity. Such precise control is particularly

important in RGB applications where we blend light of different colors to produce white.

From the driver IC perspective, analog dimming presents a serious challenge to the output current accuracy. Almost

every LED driver uses a resistor of some type in series with the output to sense current. The current-sense

voltage, VSNS, is selected as a compromise to maintain low power dissipation while keeping a high signal-to-noise

ratio (SNR). Tolerances, offsets, and delays in the driver introduce an error that remains relatively fixed. To reduce

output current in a closed-loop system, VSNS, must be reduced. That in turn reduces the output current accuracy and

ultimately the output current cannot be specified, controlled, or guaranteed. In general, dimming with PWM allows

more accurate, linear control over the light output down to much lower levels than analog dimming.

Dimming frequency vs. contrast ratio

The LED driver's finite response time to a PWM dimming signal creates design issues. There are three main types of

delay (Fig. 2). The longer these delays, the lower the achievable contrast ratio (a measure of control over lighting

intensity).


Figure 2: Dimming delays

As shown, tn represents the propagation delay from the time logic signal VDIMgoes high to the time that the LED driver

begins to increase the output current. In addition, tsu is the time needed for the output current to slew from zero to the

target level, and tsn is the time needed for the output current to slew from the target level back down to zero. In

general, the lower the dimming frequency, fDIM, the higher contrast ratio, as these fixed delays consume a smaller

portion of the dimming period, TDIM.The lower limit forfDIM is approximately 120 Hz, below which the eye no longer
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blends the pulses into a perceived continuous light. The upper limit is determined by the minimum contrast ratio that

is required.

Contrast ratio is typically expressed as the inverse of the minimum on-time, i.e.,

CR = 1 / tON-MIN : 1

where tON-MIN = tD + tSU. Applications in machine vision and industrial inspection often require much higher PWMdimming frequencies because the high-speed cameras and sensors used respond much more quickly than the

human eye. In such applications the goal of rapid turn-on and turn-off of the LED light source is not to reduce the

average light output, but to synchronize the light output with the sensor or camera capture times.

Dimming with a switching regulator

Switching regulator-based LED drivers require special consideration in order to be shut off and turned on at hundreds

or thousands of times per second. Regulators designed for standard power supplies often have an enable pin or

shutdown pin to which a logic-level PWM signal can be applied, but the associated delay, tD, is often quite long. This

is because the silicon design emphasizes low shutdown current over response time. Dedicated switching regulations

for driving LEDs will do the opposite, keeping their internal control circuits active while the enable pin is logic low tominimize tD, while suffering a higher operating current while the LEDs are off.

Optimizing light control with PWM requires minimum slew-up and slew-down delays not only for best contrast ratio,

but to minimize the time that the LED spends between zero and the target level (where the dominant wavelength and

CCT are not guaranteed). A standard switching regulator will have a soft-start and often a soft-shutdown, but

dedicated LED drivers do everything within their control to reduce these slew rates. Reducing tSUand tSN involves both

the silicon design and the topology of switching regulator that is used.

Buck regulators are superior to all other switching topologies with respect to fast slew rates for two distinct reasons.

First, the buck regulator is the only switching converter that delivers power to the output while the control switch is on.

This makes the control loops of buck regulators with voltage-mode or current-mode PWM (not to be confused with

the dimming via PWM) faster than the boost regulator or the various buck-boost topologies. Power delivery during the

control switch's on-time also adapts easily to hysteretic control, which is even faster than the best voltage-mode or

current-mode control loops. Second, the buck regulator's inductor is connected to the output during the entire

switching cycle. This ensures a continuous output current and means that the output capacitor can be eliminated.

Without an output capacitor the buck regulator becomes a true, high impedance current source, capable of slewing

the output voltage very quickly. Cuk and zeta converters can claim continuous output inductors, but fall behind when

their slower control loops (and lower efficiency) are factored in.

Faster than the enable pin

Even a pure hysteretic buck regulator without an output capacitor will not be capable of meeting the requirements of

some PWM dimming systems. These applications need high PWM dimming frequency and high contrast ratio, which

in turn requires fast slew rates and short delay times. Along with machine vision and industrial inspection, examples

of systems that need high performance include backlighting of LCD panels and video projection. In some cases the

PWM dimming frequency must be pushed to beyond the audio band, to 25 kHz or more. With the total dimming

period reduced to a matter of microseconds, total rise and fall times for the LED current, including propagation


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delays, must be reduced to the nanosecond range.

Consider a fast buck regulator with no output capacitor. The delays in turning the output current on and off come from

the IC's propagation delay and the physical properties of the output inductor. For truly high speed PWM dimming,

both must be bypassed. The best way to accomplish this is by using a power switch in parallel with the LED chain

(Fig. 3). To turn the LEDs off, the drive current is shunted through the switch, which is typically an n-MOSFET. The IC

continues to operate and the inductor current continues to flow. The main disadvantage of this method is that power

is wasted while the LEDs are off, even through the output voltage drops to equal the current sense voltage during this

time.


Figure 3: Shunt FET circuit, with waveforms

Dimming with a shunt FET causes rapid shifts in the output voltage, to which the IC's control loop must respond in an

attempt to keep the output current constant. As with logic-pin dimming, the faster the control loop, the better the

response, and buck regulators with hysteretic control provide the best response.

Fast PWM with boost and buck-boost

Neither the boost regulator nor any of the buck-boost topologies are well suited to PWM dimming. That's because in

the continuous conduction mode (CCM), each one exhibits a right-half plane zero, which makes it difficult to achieve

the high control loop bandwidth needed in clocked regulators. The time-domain effects of the right-half plane zero

also make it much more difficult to use hysteretic control for boost or buck-boost circuits. In addition, the boost

regulator cannot tolerate an output voltage that falls below the input voltage. Such a condition causes a short circuit

at the input, and makes dimming with a parallel FET impossible. Among the buck-boost topologies, parallel FET

dimming is still impossible or at best impractical due to the requirement for an output capacitor (the SEPIC, buck-

boost and flyback), or the uncontrolled input inductor current during output short circuits (Cuk and zeta). When true

fast PWM dimming is required, the best solution is a two-stage system that uses a buck regulator as the second, LED

driving stage. When space and cost do not permit this approach, the next best choice is a series switch (Fig. 4).



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Figure 4: Boost regulator with series DIM switch

LED current can be shut off immediately. On the other hand, special consideration must be given to the system

response. Such an open circuit is in effect a fast, extreme unloading transient that also disconnects the feedback loop

and will cause the regulator's output voltage to rise without bound. Clamping circuits for the output and/or the error

amplifier are required to prevent failure due to over-voltage. These clamps are difficult to realize with external

circuitry, hence series FET dimming is practical only with dedicated boost/buck-boost LED driver ICs.

In summary, proper control of LED lighting requires careful attention right from the start of the design process. The

more sophisticated the light source, the more likely that PWM dimming will be used. This in turn requires the system

designer to carefully consider the LED driver topology. Buck regulators offer many advantages for PWM dimming. If

the dimming frequency must be high, or the slew rates must be fast, or both, then the buck regulator is the way to go.

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