High-Efficiency AC-DC TV Power Solutions: An Intelligent ... · Application Report SLVA474–July 2011 High-EfficiencyAC-DCTV Power Solutions: Proposing an Intelligent LED Backlight

Application ReportSLVA474–July 2011

High-Efficiency AC-DC TV Power Solutions: Proposing anIntelligent LED Backlight Driving Scheme

Jimmy Liu, Pony Ma, Yoshio Katsura ........................................................... Power Management Products

ABSTRACT

LCD televisions designed with LED backlight are continuing to grow in popularity compared toconventional TV architectures that use cold cathode fluorescent lamp (CCFL) bulbs because of thesupreme feature set that these systems offer: greater brightness, higher contrast ratios, stylish thindesigns, and significantly lower power consumption. To derive the full potential of the LED-based LCDbacklight technology, however, it is important to achieve both good thermal design and precise backlightON-OFF control for good picture quality by using the greater color gamut and higher contrast ratio of theLED, and suppressing LCD TV motion blur.

This application note presents novel power architectures with regard to the ac-dc total power supply for anLCD TV design, achieving higher power efficiencies and lower cost. The revolutionary iHVM™ mechanismenables intelligent LED bias control to minimize power loss. On the other hand, the fast LED driversprovide precise LED ON-OFF control. In this report, two reference designs are discussed. The PMP4298is the basic topology reference design; this design consists of a primary ac-dc stage, a secondary boostdc-dc converter stage for the LED backlight, and a multi-channel LED driver. The PMP4298A is proposedas an advanced reference design that achieves a much higher efficiency and an overall lower cost byomitting the LED backlight dc-dc secondary stage. The LED driver is instead supplied directly from theac-dc primary power-based LED backlight solution. Performance results are also presented.

Contents1 Introduction .................................................................................................................. 22 Proposed LED Backlight TV Power Architectures ...................................................................... 33 iHVM Basics ................................................................................................................. 54 Advanced iHVM Feedback Design: Direct Feedback to Isolated AC-DC Power Stage ........................... 75 Protection Scheme ........................................................................................................ 116 Experimental Results ..................................................................................................... 137 Conclusion .................................................................................................................. 158 References ................................................................................................................. 16Appendix A Schematics ........................................................................................................ 16

List of Figures

1 Edge-Type LED Backlight LCD TV Power Conversion Scheme ..................................................... 3

2 PMP4298: Basic Architecture for LED Backlight LCD TV Power Supply............................................ 4

3 PMP4298A: Advanced Power Architecture for LED Backlight LCD TV ............................................. 5

4 PMP4298: iHVM Feedback Loop Integration into DC-DC Primary Feedback Loop ............................... 6

5 TLC5960 Application Circuit with LLC Resonant Converter........................................................... 8

6 AC-DC Power-Stage Direct Feedback Loop Integrated with iHVM Secondary Feedback ........................ 9

7 PMP4298A: AC-DC Power Stage Startup and iHVM Optimization Sequence .................................... 10

8 TLC5960 Total iHVM and Protection Scheme......................................................................... 11

9 LED Open Detection and VLED Optimization Through iHVM ......................................................... 12

10 LED Short Detection and VLED Optimization Through iHVM ......................................................... 12

11 PMP4298A Demonstration Fixture Test Setup ........................................................................ 14iHVM is a trademark of Texas Instruments.All other trademarks are the property of their respective owners.

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12 Total Efficiency Comparison at Full Load on VLED, 24 V and 5 V.................................................... 14

13 Power Factor at Full Load for PMP4298A ............................................................................. 14

14 Output Headroom Voltage Waveforms for PMP4298A: LED ON to OFF.......................................... 15

15 Output Headroom Voltage Waveforms for PMP4298A: LED OFF to ON.......................................... 15

16 PFC Stage Schematic for PMP4298 and PMP4298A ................................................................ 17

17 Auxiliary Flyback Converter Stage Schematic for PMP4298 and PMP4298A .................................... 18

18 LLC Converter Stage Schematic for PMP4298 ....................................................................... 19

19 LLC Converter Stage Schematic for PMP4298A...................................................................... 20

20 TLC5960 LED Driver Stage Schematic for PMP4298 ................................................................ 21

21 TLC5960 LED Driver Stage Schematic for PMP4298A .............................................................. 22

1 Introduction

Liquid crystal display (LCD) television has become a hot favorite for consumers worldwide, mostly as aresult of the superior technology adopted by the industry in the manufacturing processes for thesesystems. Since it was first introduced to the market, periodic improvements have been made in responseto user comments and complaints. Many new techniques continue to improve the performance of theseunits. Some of these techniques are unique to LCD television; light-emitting diode (or LED) backlighting isone of these improvements.

This new backlight design has been acknowledged as having enormous potential for television technologyand architecture. Some experts even theorize that LED backlight will eventually replace all other backlighttechniques in the near future. LEDs are now widely used in many LCD televisions regardless of size. LEDimplementation facilitates saturated colors and superior color reproduction. Compared to other methodsfor backlights, LEDs are the most popular system because of these advantages, among others:

1. Long lifetime: With a life span of 50,000 hours or more, LED technology is the most durable backlightsystem in the television industry.

2. Contrast ratio: The fast switching capability is one of the biggest advantages of solid-state backlightcompared to conventional backlight such as CCFL bulbs. In an LCD TV, this capability translates into adynamic contrast ratio, which is an important specification for an LCD TV set to achieve better picturequality.

3. Wide color gamut: LED backlight has the potential to provide a superior, wide range color gamut,especially with RGB LED backlight. Even with WLED backlight, color purity is much better than withCCFL backlighting; this benefit also increases the overall system picture quality.

4. Brightness: The light efficacy of LED technology is continuously improving, almost at the rate of twiceper decade. LCD TV makes full use of this benefit, not only for picture quality improvement or lowerpower consumption, but also in areas such as 3D-TV (theoretically, the brightness is halved to 2D-TV)and using LCD TVs out-of-doors.

5. TV design flexibility: Because the LED itself has a smaller dimension compared to traditional CCFLbacklighting, stylish new TV designs are achievable; for example, an edge-lit, wall-mounted TV set hasa thickness of less than 10 mm. These types of innovations are promoting new styles in many homes.

6. Low power consumption: As noted earlier, the LED has much higher light efficacy compared toconventional technologies. Therefore, an LED backlight TV set has lower power consumption.EnergyStar v4.0/v5.0 and California Energy Commission (CEC) standards control power consumptionperformance for LCD TVs with different size panels. For example, for a 42-inch panel, the EnergyStarv4.0 (May 2010) regulation required that the maximum input power should be less than 115 W; theCEC Tier1 (January 2011) standard requires a maximum input power of less than 183 W.Conventional CCFL backlight technology is unable to meet these regulations; as a result, LEDbacklight technology is the first choice for system designers.

7. Environmentally friendly TV: An LED backlight LCD TV set contains no mercury, so it is easier todispose of and more easily recycled than other types of LCD TVs. This is the primary reason thatmany environmentalists also support LED TVs over other TV technologies.

2 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED SLVA474–July 2011Backlight Driving Scheme Submit Documentation Feedback


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Isolated AC/DC

Power Board

DC/DC

LED Driver

24 VDC

LCD Panel

AC

www.ti.com Proposed LED Backlight TV Power Architectures

Considering these benefits, as well as others, LED backlight has become the mainstream technology andis quickly replacing conventional CCFL backlight designs for LCD TVs. Because of the low cost and easydesign, edge-type LED backlight structures have been widely used in recent LED backlight TVs. Figure 1shows the typical power conversion architecture for edge-type LED backlight TVs.

Figure 1. Edge-Type LED Backlight LCD TV Power Conversion Scheme

This topology uses a switching, high-voltage boost dc-dc converter for each LCD string. It is very commonfor a switching boost dc-dc converter used in a multi-string configuration to generate electromagneticinterference (EMI) problems, because multiple switching LED drivers generate additional high-frequencyswitching, differential-mode (DM) noise at this stage, and therefore affect the measured results of bothconducted and radiated EMI in the TV power system. An increased EMI design effort—for example, anRC snubber network at the switching node or an EMI filter at the output side—is then required in order tocomply with the EMI standards.

Instead of this additional constraint, a linear constant-current regulator topology is sometimes selected.This topology does not generate any high-frequency switching noise; therefore, it is the perfect solution forEMI designs with a reduced BOM cost target. However, the power dissipation of the conventional lineardriver approach is normally more excessive than the multiple dc-dc converter approach, because it lacksan LED bias, dc-dc voltage optimization mechanism.

This document introduces several innovative topologies for LED backlight TV with two power referencedesigns (the PMP4298 and PMP4298A) using the TLC5960, an eight-channel, intelligent linear LED driverfrom Texas Instruments. The TLC5960 intelligent headroom voltage monitor creates a dedicated, closedfeedback loop to automatically optimize the LED bias voltage by using the forward voltage information ofthe LED strings. This application note shows not only a dc-dc LED bias stage but also a direct LED biascontrol from the primary side ac-dc LLC converter output, without any boost dc-dc required in front of theLED driver. Both topologies can be selected based on the customer’s specific requirements, andcontribute to improved system efficiency while also reducing the total BOM cost.

2 Proposed LED Backlight TV Power Architectures

Two power architectures are proposed. First, the basic architecture of a LED backlight TV power supply isanalyzed (see Figure 2); then, a more advanced configuration specific to an ac-dc power supply LEDbacklight architecture is discussed (refer to Figure 3). The PMP4298A should achieve a higher efficiencyand lower cost because it omits the unnecessary boost dc-dc stage for the LED backlight bias generation.The PMP4298 also has an additional dc-dc stage, enabling users to easily set the ac-dc output voltage toa certain preferred range. On the other hand, the PMP4298A has no additional stage; the ac-dc outputvoltage should be set equal to the LED backlight bias voltage variable range, thus achieving higherefficiency and lower cost.



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TM PFC

UCC28501

LLC Control

UCC25600

System

Flyback

UCC28610

Boost DC/DC

TPS40210

VREF

TLC5960 Constant Current LED Driver

Eight-Channel

Light Bar

20 LEDs

in series

R2

R1

80 VDC48 VDC

24 V at

2 A for

audio and

system

FB

iHVM

5 V at 3 A

for system

5 V at 1 A

for standby

Proposed LED Backlight TV Power Architectures www.ti.com

Table 1 presents a brief comparison of the two architectures.

Table 1. Comparison of Two Reference Designs

Reference Design

Characteristic/Feature PMP4298 PMP4298A

Primary stage ac-dc design TM PFC + LLC, Flyback TM PFC + LLC, Flyback

Secondary stage design DC-DC + LED Driver with iHVM LED Driver with iHVM

Front-end boost dc-dc converter Yes No

Efficiency 85% 87%

Cost Medium Low

Primary stage ac-dc output voltage 48 V 80 V ±10%

Figure 2 shows the basic architecture of a generic, edge-type LED backlight TV power supply. The stagewithin the dashed line is the LED light bar driver stage. In this topology, the UCC28051 is a boosttransition mode (TM) power factor corrector (PFC), which can rectify an ac input line voltage to 380 VDC;the resonant LLC stage UCC25600 regulates the output to 24 V/2 A for an audio amplifier with feedbackto the LLC controller, and 48 V for the LED driver input. The flyback with UCC28610 in parallel generates5 V/3 A for the system board and 5 V/1 A for standby power. In the LED driver stage, the 48-V inputvoltage requires another front-end boost stage to boost the output voltage to 80 V, which connects to theeight-channel TLC5960 with 20 LEDs at 120 mA per string. This technique is the most common type ofcurrent LED backlight; the LLC design is easily implemented. The intelligent headroom voltage monitor (oriHVM™) automatically minimizes the power loss caused by the LED forward voltage change. Only asingle additional resistor regulates the boost dc-dc output voltage.

Figure 2. PMP4298: Basic Architecture for LED Backlight LCD TV Power Supply



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TM PFC

UCC28501

LLC Control

UCC25600

System

Flyback

UCC28610

VREF

TLC5960 Constant Current LED Driver

Eight-Channel

Light Bar

20 LEDs

in series

R2

R1

~80 VDC

24 V at 2 A for

audio and

system

5 V at 3 A

for system

5 V at 1 A

for standby

iHVM

www.ti.com iHVM Basics

Figure 3 shows the advanced configuration of an LED backlight LCD TV integrated power supply. ThePMP4298A does not have a boost dc-dc converter in front of the LED driver; the iHVM directly controlsthe primary side ac-dc output voltage based on the feedback information of the headroom voltage on theFETs that drive the LEDs. The voltage for the LLC converter is regulated by both the primary LLCfeedback loop and the iHVM feedback loop from the TLC5960 LED driver.

Figure 3. PMP4298A: Advanced Power Architecture for LED Backlight LCD TV

3 iHVM Basics

The LED forward voltage is subject to change depending on the forward current and the thermal resistivityof a given package. For instance, the temperature within an LCD TV starts at room temperature, and soonreaches approximately +70ºC. Assuming a typical LED component forward voltage reduction of ~5%, ifthe backlight total wattage is 100 W, then 5 W is turned directly into additional heat after the systempowers on. Of course, this 5 W has an impact on the total power consumption of ~5%; however, a moresignificant impact is made on the thermal design. Imagine the additional 5 W heat inside a state-of-the-art,edge-lit TV with only 10-mm thickness. Even worse, this 5 W concentrates in some specific areas withinthe TV unit. To suppress the heat, one method is to turn on the cooling fan to lower the overalltemperature. Or, a designer may be able to select an additional (larger) heatsink. However, keep in mindthat 5 W is only assumed as a typical case; you should consider a higher value for your worst-casescenario in a real design.

The innovative iHVM design goal is to free engineers from this tiresome design task. If you want tominimize additional costs such as a larger (or additional) heatsink, you must manage the LED bias voltageaccording to the LED forward voltage change. The iHVM approach provides the perfect answer to thatsituation. iHVM detects the LED forward voltage and automatically optimizes the LED bias voltage todiminish the additional heat inside the system.



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V =LED V +REF (V V )REF - HVM

R1 + R2

R2

R1

R3

RS RS

D1

G1

S1

iHVM

iHVM

Process

Core

TLC5960

LED Driver

COUT

R1

R2

R3

VLED

VFB

VREF

TPS40210

Primary Loop Secondary Loop

Sinking

Sourcing

DC/DC

iHVM Basics www.ti.com

Equation 1 shows the iHVM-based LED voltage calculation. The dc-dc converter provides the LED biasvoltage for all LED strings. The LED driver provides a fast linear switching capability as well as headroomvoltage sense feedback. The output voltage of the dc-dc stage is calculated according to Equation 1.

(1)

The left side of Equation 1 describes the typical dc-dc converter output voltage. The right side of theformula is the variable portion based on the iHVM additional feedback loop. The user can adjust the LEDvoltage with an additional resistor, R3, to fit it to the system-required VLED bias voltage range within ±20%.

As Figure 4 shows, the total power system consists of two feedback loops: the primary loop and theiHVM-based secondary loop. In general, careful feedback design is required if two feedback loops arepresent on a single dc-dc converter to ensure stability. With the TLC5960, users do not need tocompensate the secondary loop because this adjustment is handled by the fully-integrated digital iHVMcontrol loop.

Figure 4. PMP4298: iHVM Feedback Loop Integration into DC-DC Primary Feedback Loop

Some of the benefits of the additional closed-loop feedback with iHVM include:

• Freedom from concern about any LED forward voltage change with a temperature or current change;the closed feedback loop always regulates the LED bias voltage to the appropriate level.

• Freedom from any additional feedback compensation design. The secondary closed-loop system hasthe advantage of automatic LED voltage adjustment; in general, however, the user also must applycareful feedback loop design to ensure stability. iHVM has the capability of intelligently adjusting thesecondary loop response, and therefore the user does not need to be concerned about phasecompensation design.

• Freedom from noise misdetection and retention when sampling the cathode node of LED strings fordetecting any LED forward voltage change. iHVM is a fully-developed digital processing technique, anda noise guardband is already integrated for noise immunity in the extremely noisy environment within aTV unit.

• An easily-applied, advanced LED switching sequence. iHVM memorizes the VLED bias voltage onceoptimized. This feature is not so easy to realize in a conventional analog feedback loop, and it is verybeneficial for severe full-on, full-off light sequences that are intended to produce better brightnessprofiles, which are often required for modern 3-D TV designs.

Design note: An iHVM-based system consists of an ideal secondary feedback loop. However, thetransient response of the total system depends on the primary loop design of the dc-dc converter orthe resonant converter stage.



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www.ti.com Advanced iHVM Feedback Design: Direct Feedback to Isolated AC-DC Power Stage

4 Advanced iHVM Feedback Design: Direct Feedback to Isolated AC-DC Power Stage

iHVM is designed to be flexible and to work well with various configurations of primary feedback loops.iHVM can work with a non-isolated dc-dc power stage design (based on the PMP4298 and theTPS40210), and is also easily configurable with an isolated ac-dc power stage (using the PMP4298A andthe UCC25600 directly). Figure 5 illustrates the PMP4298A application circuitry with an LLC resonantcontroller with feedback through isolation. Up to four feedback connections can be controlled fromTLC5960 iHVM buffers; output levels are automatically adjusted through an internal iHVM mechanism toimprove the linear LED driver efficiency.

Here, the LLC resonant converter output with variable frequency is regulated based on the shunt currentthrough the TL431 voltage reference device. At the reference node of the TL431 (designated as the FBnode), the voltage divider R1 and R2 is connected to define the primary feedback loop voltage. The iHVMmechanism feeds the LED forward voltage information back to the FB node through RHVM and adjusts theLLC resonant converter output voltage according to the required LED forward voltage.

To sense the LED forward voltage in strings, the TLC5960 monitors the FET drain node voltage of eachstring first. Next, the TLC5960 processes the obtained information in an intelligent digital power logicblock. Finally, the TLC5960 modulates the HVM pin output voltage, which adjusts the UCC25600operating frequency to achieve an optimized output voltage, VLED.



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Advanced iHVM Feedback Design: Direct Feedback to Isolated AC-DC Power Stage www.ti.com

Figure 5. TLC5960 Application Circuit with LLC Resonant Converter



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RS RS

D1

G1

S1

HVM1

iHVM

Process

Core

TLC5960

LED Driver

COUT

R1

R2

R3

VLED

VFB

VREF

Voltage

Feedback

Loop

iHVM

Feedback

Loop

ISINK

ISOURCE

400 V

LLC

Controller

13 V

www.ti.com Advanced iHVM Feedback Design: Direct Feedback to Isolated AC-DC Power Stage

Figure 6 shows a block diagram of a total system that consists of an isolated ac-dc primary feedback loopand the iHVM secondary feedback loop. The primary feedback loop is the normal voltage feedback loopfor the LLC resonant converter with voltage-controlled oscillation (VCO). The LLC resonant converter isbased on a serial resonant converter (SRC) architecture. By using the transformer magnetizinginductance, zero-voltage switching can be achieved over a wide range of input voltages and loads. As aresult of multiple resonances, zero-voltage switching can also be maintained when the switchingfrequency of the LLC controller is higher or lower than the resonant frequency. As the switching frequencyis lowered, the voltage gain significantly increases. This characteristic allows the converter to maintainregulation when the input voltage falls low or the output current increases.

Figure 6. AC-DC Power-Stage Direct Feedback Loop Integrated with iHVM Secondary Feedback

The secondary feedback loop is intended to integrate the cathode node information of the LED string inorder to optimize the power dissipation caused by the LED string voltage change as a result of thevariance of the LED forward voltage. The VLED output voltage equation is the same as for the iHVM withthe non-isolated boost converter feedback loop (see Equation 1).



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0

70

0

5

1.2

0.7

0

V (V)HVM

V (V)LED

EN (V)

TLC5960/iHVM(O)

TLC5960/EN(I)

iHVM

Optimization

start when

TLC5960 is

enabled

iHVM

Optimization

hold at

EN = L

iHVM Found

Settling Point

LED V Change

DetectedF

iHVM Found

Settling Point

Default 0.7-V Output

1.25 V

0.14 V

AC/DC Default

Output Level

R1 + R2

R2VREF

R1

R3(VREF HVMV )-

80 V

60 V iHVM Adjustable

Range

iHVM Output

Starts to Adjust

AC/DC Output

Initiated AC/DC

Power Supply

iHVM Optimization

(EN = H)

iHVM Hold

(EN = L)

iHVM Initialization

(EN = L)

Time (s)

iHVM Output

Range

Advanced iHVM Feedback Design: Direct Feedback to Isolated AC-DC Power Stage www.ti.com

4.1 PMP4298A Start-Up Sequence of LLC Direct Feedback Through iHVM

Figure 7 illustrates the start-up sequence of this system. When the ac line powers on, the LLC resonantconverter starts to work to output the initial VLED voltage. After the user enables the TLC5960, the iHVMadjustment sequence kicks out the center value (0.7 V) of the HVM output control range (0.14 V to1.25 V). Thus, the initial VLED voltage is also set to start from the center value (here, 70 V) after the initialstartup of the ac-dc power. Then, the TLC5960 begins to sense the LED string total forward voltage, andthe HVM output starts to modulate. The UCC25600 then regulates the operating frequency with thevoltage-controlled oscillation (VCO) to set the appropriate voltage of the VLED that corresponds to the LEDstring total forward voltage.

Figure 7. PMP4298A: AC-DC Power Stage Startup and iHVM Optimization Sequence



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TLC5960

LED

Short

LED

Short

LED

Open

LED

Open

On

On

On

LED Open (LOD)

LED Short (LSD)

HVM-

HVM+

iHVM Logic

Protection

Logic

Dn

iHVM

Feedback

XFLT

Error Out

iHVM

Status

Info

R1R3

R2

FB

TPS40210

UCC25600

(PMP4298)

or

(PMP4298A)

VLED

VIN

www.ti.com Protection Scheme

5 Protection Scheme

Protection is one of the key design parameters required to keep the total system safe in case of anabnormal situation. LED open detection and LED short detection are basic protection requirements in LEDbacklight systems. In the PMP4298/A, these LED-related protections are handled intelligently by theTLC5960. First, the TLC5960 monitors the Dn nodes (the cathode nodes of the LED strings) anddetermines whether an abnormal voltage range is detected or not. Then, the TLC5960 tries to resolve anyabnormal situation by using the integrated iHVM feature. Finally, and only if the TLC5960 could notresolve the situation with iHVM, the TLC5960 recognizes an error and automatically shuts off the errorstring(s), separating them from normal working strings. The detection related diagram is shown inFigure 8.

Figure 8. TLC5960 Total iHVM and Protection Scheme

For instance, an LED Open Detection (LOD) flag is triggered when the voltage Dn is lower than 0.8 V.When the TLC5960 recognizes this low voltage, the device tries to resolve the abnormal situation by



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0.81.6

2.6

Dn (V)

0.14

0.7

1.25

V (V)HVM

5

0

XFLT (V)

80

70

60

V (V)LED

iHVM

Detection

Range

V Range

Corresponding to

iHVM Range

LED

V Reaches

Max ValueLED

Error Output

HVM Output

Reaches Min

Dn Unable to

Recover (Error Status)

Error String

Shut OffDn Voltage Recovers

(Normal Status)

LED V

Changes

Again

FLED V Change

Drives HVM LowerF

V Driven

HigherLED

V Settles

OnceLED

iHVM

Controls

VLED

Normal Status iHVM

Optimize

iHVM

Optimize

Normal Error

Status

HVM Output

Range

iHVM

Detection

Range

V Range

Corresponding to

iHVM Range

LED

V Reaches

Min ValueLED

Error Output

HVM Output

Reaches Max

Dn Unable to

Recover (Error Status)

Error String

Shut Off

Dn Voltage Recovers

(Normal Status)

LED V

Changes

Again

F

1.6

2.6

9.5

Dn (V)

1.25

0.7

0.14

XFLT (V)

5

0

V (V)LED

80

70

60

LED V Change

Drives HVM HigherF

V (V)HVM

V Driven

LowLED

V Settles

OnceLED

iHVM

Controls

VLED

Normal Status iHVM

Optimize

iHVM

Optimize

Normal Error

Status

Protection Scheme www.ti.com

driving to higher Dn voltages by lowering the HVM output voltage. And when the VLED level reaches itsmaximum value, the TLC5960 recognizes an actual error status. Figure 9 shows this sequence. The LEDshort detection (LSD) sequence is also presented in Figure 10. In the case of an LED short, the TLC5960also recognizes an abnormally higher voltage at Dx node. So, in this case, the iHVM mechanism tries toset VLED at as low a voltage as possible. If this attempt fails, the TLC5960 recognizes this result as anerror and initiates a corrective action: shut off the error string.

Figure 9. LED Open Detection and VLED Optimization Through iHVM

Figure 10. LED Short Detection and VLED Optimization Through iHVM



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www.ti.com Experimental Results

These are the basic schemes used by the TLC5960 LED driver, which integrates both iHVM andprotection features, to protect the LED backlight system intelligently. Table 2 summarizes the LED openand short conditions. This table also describes the Dn node recognition internal circuitry and how theiHVM information is integrated into the protection recognition routine.

Table 2. LED Open and Short Protection Conditions

iHVM Status to Trigger ErrorLED Error Status Dn Error Detection Voltage Detection

VLED reaches the maximum valueLED Open Dn < 0.8 V (HVM = 0.14 V)

VLED reaches the minimum valueLED Short Dn > 19.2 V (default) (HVM = 1.25 V)

6 Experimental Results

In order to verify the proposed LED backlight TV power supply design using intelligent headroom voltagecontrol, two reference design demonstration fixtures are constructed using the PMP4298 and PMP4298A.Typical specifications of the reference design include:

• Input: 150-W universal ac input power supply (85 VAC to 264 VAC, at 47 Hz to 63 Hz)• Greater than 85% efficiency from ac to LED backlight at 220 VAC input• Power output:

– LED strings: Eight channels x 80 V with 20 120-mA LEDs per string– Audio and system: 24 V at 2 A– System: 5 V at 3 A– Standby power: 5 V at 1 A

• Minimum 20-ms hold-up time when the input line is shut off• Input standby power Less than 500 mW with 5-V/30-mA output (onboard switch can trigger standby

mode)• Printed circuit board (PCB) specifications:

– Single layer board– X-Y dimensions (max):10 inches x 10 inches (25.40 cm x 25.40 cm)– Height (max): .394 inches (10 mm)—not including PCB thickness

• Dimming range: 1% to 100% (250-Hz dimming frequency)• LED current matching specification: Less than 3% for full dimming range• Flyback stage output regulation tolerance: Less than ±5% over the load range• LED protection (short, open, overcurrent, undercurrent, undervoltage, etc.)• OCP and OVP protection for the Flyback stage• In Standby mode, system disables PFC and LLC stages to reduce standby power• TLC5960 headroom voltage mode (iHVM™) to control the UCC25600 LLC output and optimize

efficiency• Meets IEC 61000-3-2 for total harmonic distortion• Meets EN55022 Class B EMI requirements• Operating temperature range: 0ºC to +50ºC; storage temperature range: –20ºC to +80ºC



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5-V System Load

5-V Standby Load

Eight-String

LED Light Bar

PMP4298A Demo Board

24-V DC Load

90 110 130 150 170 190 210 230 250

Input Voltage (V)

0.9

0.89

0.88

0.87

0.86

0.85

0.84

0.83

0.82

Effic

iency (

%)

EFFICIENCY COMPARISON

PMP4298

PMP4298A

264

Input Voltage (V)

1

0.99

0.98

0.97

0.96

0.95

0.94

0.93

0.92

0.91

0.9

Pow

er

Facto

r

POWER FACTOR vs INPUT VOLTAGE

90 110 130 150 170 190 210 230 250 264

Experimental Results www.ti.com

Figure 11 shows the demonstration fixture PMP4298A configured with an eight-channel LED light bar inthe test setup.

Figure 11. PMP4298A Demonstration Fixture Test Setup

Figure 12 presents a total efficiency comparison with an input voltage range from 90 VAC to 264 VAC at afull load for both the PMP4298 and PMP4298A designs. Overall, the PMP4298 shows good efficiency(85%). Furthermore, by omitting the front-end boost dc-dc stage, the PMP4298A shows an 88% peak, or2% superior performance than the PMP4298 in efficiency with a lower cost structure. The 3% power-losssavings in a total 150-W design is equivalent to a 4.5-W saving for thermal components. For additionalimprovements, the user must also consider appropriate transformer and MOSFET selection, becausemost of the loss comes from the transformer core loss and the FET switching loss. Figure 13 shows thepower factor at a full load for the PMP4298A.

Figure 12. Total Efficiency Comparison at Full Load on Figure 13. Power Factor at Full Load for PMP4298AVLED, 24 V and 5 V



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(a) 50% PWM Dimming (b) 90% PWM Dimming

(a) 50% PWM Dimming (b) 90% PWM Dimming

www.ti.com Conclusion

Figure 14 shows the waveforms from the LED light bar ON to OFF at 50% and 90% PWM dimming forVO_LED (blue line), iHVM (red line), and the output LED current (green line). After VLED turns ON, by theheat of the LED itself, the LED forward voltage is lower; iHVM detects that change and controls VLED to bea little lower, cycle by cycle, to achieve maximum total efficiency in power across the board.

Figure 14. Output Headroom Voltage Waveforms for PMP4298A: LED ON to OFF

Figure 15 shows the waveforms from the LED light bar OFF to ON at 50% and 90% PWM dimming forVO_LED (blue line), iHVM (red line), and the output LED current (green line).

Figure 15. Output Headroom Voltage Waveforms for PMP4298A: LED OFF to ON

See Appendix A for schematics for the PMP4298 and PMP4298A.

7 Conclusion

This application note shows two reference designs, which are both applicable to various LED backlightapplications. One of the key features of these recommended designs is the VLED control methodology(iHVM) that reduces power dissipation, additional costs, and overall design effort. The iHVM was originallydesigned to work well not only for normal dc-dc converter stages (PMP4298), but also with slowerresponse ac-dc resonant converter stages (PMP4298A).

Another key feature is system safety. By combining the VLED control mechanism and the integratedprotection mechanisms, the total system is robust even in abnormal situations. Experimental resultsconfirmed 88% peak efficiency including total ac-dc TV power outputs while maintaining a 98% powerfactor for a wide input range from 85 VAC to 264 VAC.



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References www.ti.com

8 References

Unless otherwise noted, these documents are available for download through the Texas Instrumentswebsite (www.ti.com).

• TLC5960 8-channel LED driver controller with integrated intelligent thermal controller. Product datasheet. Literature number SBVS147.

• UCC28610 Green-mode flyback controller. Product data sheet. Literature number SLUS888D.• UCC25600 8-pin high-performance resonant mode controller. Product data sheet. Literature number

SLUS846A.• UCC28051 PFC controller for low to medium power applications requiring compliance with IEC

1000-3-2. Product data sheet. Literature number SLUS515F.

Appendix A Schematics

Schematics for the PMP4298 and PMP4298A are given in Figure 16 to Figure 21.



http://www.ti.com

http://www.ti.com

http://www.ti.com/lit/pdf/SBVS147

http://www.ti.com/lit/pdf/SLUS888




NL

++

>12.5

Vneeded

Chassis

85V

AC

to265V

AC

Input

+

www.ti.com Appendix A

Figure 16. PFC Stage Schematic for PMP4298 and PMP4298A



http://www.ti.com


+

+

+

+

Appendix A www.ti.com

Figure 17. Auxiliary Flyback Converter Stage Schematic for PMP4298 and PMP4298A



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R98

12

765

LM

293A

D

U1:B

R76

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C41

0.1

uF

D26BAS16

R74

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C63

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BA

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R149

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kR

100

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14FO

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NA

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48V

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A

D33

BA

T54


Figure 18. LLC Converter Stage Schematic for PMP4298



http://www.ti.com


+

+

+

+

+


Figure 19. LLC Converter Stage Schematic for PMP4298A



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R15100K

100KR38

R41

4.9

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R119200K

R65100K

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R121100K

R44

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R129200K

R93200K

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R29optional

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R116200K

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4.9

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Q3

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10K

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51K

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R26

100K

R27

100K

R22

100K

R114

4.9

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Q19

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RA

120T

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Q21

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20L

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4.9

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C15470pF

R18

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KR

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C13

0.1

uF

MU

RA

120T

3G

D6

0.3

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C8

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R6

220K

2.2

uF

C17

100V

21+

C16

330uF

220pF

C9

R122200K

R125100K

1 G3

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4D

Q22

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T4N

20L

R70200K

MU

RA

120T

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C5

MB

R10H

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D1

SDG

IPP80CN10NQ1

MM

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2907

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100

R30optional

R28

100K

2.2

uF

C1

100V

21+

C7

100uF

R23

390K

18

G4

15

G3

12

G2

9G

1

63

21

G8

24

G7

27

G6

30

G5

36

19

S4

16

S3

13

S2

17

D4

14

D3

7 8

10

S1

11

D2

1 52

20

S8

23

S7

26

S6

22

D8

25

D7

29

S5

32

XF

LT

28

D6

31

D5

38

34

37

4H

VM

1/H

VM

35

HV

M4/R

EF

LS

D

33

TLC

596xD

AU

4

MU

RA

120T

3G

D37

1 G3

S

4D

Q5

FQ

T4N

20L

100K

R3

1 G3

S

4D

Q20

FQ

T4N

20L

MU

RA

120T

3G

D30

741 6 8532

J3

741 6 8532

J4


Figure 20. TLC5960 LED Driver Stage Schematic for PMP4298



http://www.ti.com


++

+


Figure 21. TLC5960 LED Driver Stage Schematic for PMP4298A



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