High-Efficiency AC-DC TV Power Solutions: an Intelligent LED

Application Report SLVA474–July 2011

High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent 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 to conventional TV architectures that use cold cathode fluorescent lamp (CCFL) bulbs because of the supreme feature set that these systems offer: greater brightness, higher contrast ratios, stylish thin designs, and significantly lower power consumption. To derive the full potential of the LED-based LCD backlight technology, however, it is important to achieve both good thermal design and precise backlight ON-OFF control for good picture quality by using the greater color gamut and higher contrast ratio of the LED, and suppressing LCD TV motion blur. This application note presents novel power architectures with regard to the ac-dc total power supply for an LCD TV design, achieving higher power efficiencies and lower cost. The revolutionary iHVM™ mechanism enables intelligent LED bias control to minimize power loss. On the other hand, the fast LED drivers provide precise LED ON-OFF control. In this report, two reference designs are discussed. The PMP4298 is the basic topology reference design; this design consists of a primary ac-dc stage, a secondary boost dc-dc converter stage for the LED backlight, and a multi-channel LED driver. The PMP4298A is proposed as an advanced reference design that achieves a much higher efficiency and an overall lower cost by omitting the LED backlight dc-dc secondary stage. The LED driver is instead supplied directly from the ac-dc primary power-based LED backlight solution. Performance results are also presented.

Contents 1 Introduction ...... 2 2 Proposed LED Backlight TV Power Architectures ...... 3 3 iHVM Basics ...... 5 4 Advanced iHVM Feedback Design: Direct Feedback to Isolated AC-DC Power Stage ...... 7 5 Protection Scheme ...... 11 6 Experimental Results ...... 13 7 Conclusion ...... 15 8 References ...... 16 Appendix 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 ...... 14 iHVM is a trademark of Texas Instruments. All other trademarks are the property of their respective owners.

SLVA474–July 2011 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED 1 Submit Documentation Feedback Backlight Driving Scheme Copyright © 2011, Texas Instruments Incorporated Introduction www.ti.com

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 a result of the superior technology adopted by the industry in the manufacturing processes for these systems. Since it was first introduced to the market, periodic improvements have been made in response to user comments and complaints. Many new techniques continue to improve the performance of these units. Some of these techniques are unique to LCD television; light-emitting diode (or LED) backlighting is one of these improvements. This new backlight design has been acknowledged as having enormous potential for television technology and architecture. Some experts even theorize that LED backlight will eventually replace all other backlight techniques in the near future. LEDs are now widely used in many LCD televisions regardless of size. LED implementation facilitates saturated colors and superior color reproduction. Compared to other methods for 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 backlight system in the television industry. 2. Contrast ratio: The fast switching capability is one of the biggest advantages of solid-state backlight compared to conventional backlight such as CCFL bulbs. In an LCD TV, this capability translates into a dynamic contrast ratio, which is an important specification for an LCD TV set to achieve better picture quality. 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 with CCFL 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 twice per decade. LCD TV makes full use of this benefit, not only for picture quality improvement or lower power 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 CCFL backlighting, stylish new TV designs are achievable; for example, an edge-lit, wall-mounted TV set has a 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 to conventional technologies. Therefore, an LED backlight TV set has lower power consumption. EnergyStar v4.0/v5.0 and California Energy Commission (CEC) standards control power consumption performance for LCD TVs with different size panels. For example, for a 42-inch panel, the EnergyStar v4.0 (May 2010) regulation required that the maximum input power should be less than 115 W; the CEC 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, LED backlight 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 to dispose of and more easily recycled than other types of LCD TVs. This is the primary reason that many environmentalists also support LED TVs over other TV technologies.

2 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED SLVA474–July 2011 Backlight Driving Scheme Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated www.ti.com Proposed LED Backlight TV Power Architectures Considering these benefits, as well as others, LED backlight has become the mainstream technology and is quickly replacing conventional CCFL backlight designs for LCD TVs. Because of the low cost and easy design, edge-type LED backlight structures have been widely used in recent LED backlight TVs. Figure 1 shows the typical power conversion architecture for edge-type LED backlight TVs.

LCD Panel

24 V Isolated AC/DC DC DC/DC AC Power Board LED Driver

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 common for a switching boost dc-dc converter used in a multi-string configuration to generate electromagnetic interference (EMI) problems, because multiple switching LED drivers generate additional high-frequency switching, differential-mode (DM) noise at this stage, and therefore affect the measured results of both conducted and radiated EMI in the TV power system. An increased EMI design effort—for example, an RC snubber network at the switching node or an EMI filter at the output side—is then required in order to comply 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 for EMI designs with a reduced BOM cost target. However, the power dissipation of the conventional linear driver approach is normally more excessive than the multiple dc-dc converter approach, because it lacks an LED bias, dc-dc voltage optimization mechanism. This document introduces several innovative topologies for LED backlight TV with two power reference designs (the PMP4298 and PMP4298A) using the TLC5960, an eight-channel, intelligent linear LED driver from Texas Instruments. The TLC5960 intelligent headroom voltage monitor creates a dedicated, closed feedback loop to automatically optimize the LED bias voltage by using the forward voltage information of the LED strings. This application note shows not only a dc-dc LED bias stage but also a direct LED bias control from the primary side ac-dc LLC converter output, without any boost dc-dc required in front of the LED driver. Both topologies can be selected based on the customer’s specific requirements, and contribute 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 is analyzed (see Figure 2); then, a more advanced configuration specific to an ac-dc power supply LED backlight architecture is discussed (refer to Figure 3). The PMP4298A should achieve a higher efficiency and 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 to a certain preferred range. On the other hand, the PMP4298A has no additional stage; the ac-dc output voltage should be set equal to the LED backlight bias voltage variable range, thus achieving higher efficiency and lower cost.

SLVA474–July 2011 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED 3 Submit Documentation Feedback Backlight Driving Scheme Copyright © 2011, Texas Instruments Incorporated 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 stage within the dashed line is the LED light bar driver stage. In this topology, the UCC28051 is a boost

transition 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 feedback to the LLC controller, and 48 V for the LED driver input. The flyback with UCC28610 in parallel generates 5 V/3 A for the system board and 5 V/1 A for standby power. In the LED driver stage, the 48-V input voltage requires another front-end boost stage to boost the output voltage to 80 V, which connects to the eight-channel TLC5960 with 20 LEDs at 120 mA per string. This technique is the most common type of current LED backlight; the LLC design is easily implemented. The intelligent headroom voltage monitor (or iHVM™) automatically minimizes the power loss caused by the LED forward voltage change. Only a single additional resistor regulates the boost dc-dc output voltage.

48 V 80 V DC Boost DC/DC DC TPS40210 LLC Control FB TM PFC UCC25600 UCC28501 R1

Eight-Channel 24 V at R2 Light Bar 2 A for audio and 20 LEDs system in series VREF

iHVM TLC5960 Constant Current LED Driver

System Flyback UCC28610 5 V at 1 A for standby

5 V at 3 A for system Figure 2. PMP4298: Basic Architecture for LED Backlight LCD TV Power Supply

4 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED SLVA474–July 2011 Backlight Driving Scheme Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated www.ti.com iHVM Basics Figure 3 shows the advanced configuration of an LED backlight LCD TV integrated power supply. The PMP4298A does not have a boost dc-dc converter in front of the LED driver; the iHVM directly controls the primary side ac-dc output voltage based on the feedback information of the headroom voltage on the FETs that drive the LEDs. The voltage for the LLC converter is regulated by both the primary LLC feedback loop and the iHVM feedback loop from the TLC5960 LED driver.

~80 VDC

LLC Control TM PFC UCC25600 UCC28501 R1

VREF Eight-Channel R2 Light Bar

20 LEDs 24 V at 2 A for in series audio and system

iHVM

System Flyback UCC28610 5 V at 1 A for standby

5 V at 3 A TLC5960 Constant Current LED Driver for system

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 resistivity of a given package. For instance, the temperature within an LCD TV starts at room temperature, and soon reaches approximately +70ºC. Assuming a typical LED component forward voltage reduction of ~5%, if the backlight total wattage is 100 W, then 5 W is turned directly into additional heat after the system powers on. Of course, this 5 W has an impact on the total power consumption of ~5%; however, a more significant 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 within the TV unit. To suppress the heat, one method is to turn on the cooling fan to lower the overall temperature. Or, a designer may be able to select an additional (larger) heatsink. However, keep in mind that 5 W is only assumed as a typical case; you should consider a higher value for your worst-case scenario in a real design. The innovative iHVM design goal is to free engineers from this tiresome design task. If you want to minimize additional costs such as a larger (or additional) heatsink, you must manage the LED bias voltage according to the LED forward voltage change. The iHVM approach provides the perfect answer to that situation. iHVM detects the LED forward voltage and automatically optimizes the LED bias voltage to diminish the additional heat inside the system.

SLVA474–July 2011 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED 5 Submit Documentation Feedback Backlight Driving Scheme Copyright © 2011, Texas Instruments Incorporated iHVM Basics www.ti.com Equation 1 shows the iHVM-based LED voltage calculation. The dc-dc converter provides the LED bias voltage for all LED strings. The LED driver provides a fast linear switching capability as well as headroom voltage sense feedback. The output voltage of the dc-dc stage is calculated according to Equation 1. R1 + R2 R1 V = V + (V- V ) LED R2 REF R3 REF HVM (1) The left side of Equation 1 describes the typical dc-dc converter output voltage. The right side of the formula is the variable portion based on the iHVM additional feedback loop. The user can adjust the LED

voltage 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 the iHVM-based secondary loop. In general, careful feedback design is required if two feedback loops are present on a single dc-dc converter to ensure stability. With the TLC5960, users do not need to compensate the secondary loop because this adjustment is handled by the fully-integrated digital iHVM control loop.

DC/DC VLED

C Primary Loop OUT Secondary Loop

VFB

VREF Sinking iHVM Process Core TPS40210 R3 D1 Sourcing iHVM G1 R2 S1 TLC5960 R R LED Driver S S

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 has the advantage of automatic LED voltage adjustment; in general, however, the user also must apply careful feedback loop design to ensure stability. iHVM has the capability of intelligently adjusting the secondary loop response, and therefore the user does not need to be concerned about phase compensation design. • Freedom from noise misdetection and retention when sampling the cathode node of LED strings for detecting any LED forward voltage change. iHVM is a fully-developed digital processing technique, and a noise guardband is already integrated for noise immunity in the extremely noisy environment within a TV unit.

• An easily-applied, advanced LED switching sequence. iHVM memorizes the VLED bias voltage once optimized. This feature is not so easy to realize in a conventional analog feedback loop, and it is very beneficial for severe full-on, full-off light sequences that are intended to produce better brightness profiles, which are often required for modern 3-D TV designs. Design note: An iHVM-based system consists of an ideal secondary feedback loop. However, the transient response of the total system depends on the primary loop design of the dc-dc converter or the resonant converter stage.

6 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED SLVA474–July 2011 Backlight Driving Scheme Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated 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 the TPS40210), and is also easily configurable with an isolated ac-dc power stage (using the PMP4298A and the UCC25600 directly). Figure 5 illustrates the PMP4298A application circuitry with an LLC resonant controller with feedback through isolation. Up to four feedback connections can be controlled from TLC5960 iHVM buffers; output levels are automatically adjusted through an internal iHVM mechanism to improve the linear LED driver efficiency. Here, the LLC resonant converter output with variable frequency is regulated based on the shunt current through the TL431 voltage reference device. At the reference node of the TL431 (designated as the FB node), the voltage divider R1 and R2 is connected to define the primary feedback loop voltage. The iHVM

mechanism feeds the LED forward voltage information back to the FB node through RHVM and adjusts the LLC 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 each string first. Next, the TLC5960 processes the obtained information in an intelligent digital power logic block. Finally, the TLC5960 modulates the HVM pin output voltage, which adjusts the UCC25600

operating frequency to achieve an optimized output voltage, VLED.

SLVA474–July 2011 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED 7 Submit Documentation Feedback Backlight Driving Scheme Copyright © 2011, Texas Instruments Incorporated Advanced iHVM Feedback Design: Direct Feedback to Isolated AC-DC Power Stage www.ti.com

in series

20 LEDs

TLC5960

Light Bar

Eight-Channel

D C

iHVM Detection Ch 8

iHVM Detection Ch 1

~80 V

Logic

iHVM

Buffer

HVM

n:1:1

Isolation

SOURCE

SINK

I I

R1 R2

OPTO

Feedback

G1 G2

GS1 GS2

V V

TL431

V V

360 V 400 V

UCC25600

V GND Gate1 Gate2

GS1 GS2

V V

V Figure 5. TLC5960 Application Circuit with LLC Resonant Converter

8 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED SLVA474–July 2011 Backlight Driving Scheme Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated 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 loop and the iHVM secondary feedback loop. The primary feedback loop is the normal voltage feedback loop for the LLC resonant converter with voltage-controlled oscillation (VCO). The LLC resonant converter is based on a serial resonant converter (SRC) architecture. By using the transformer magnetizing inductance, zero-voltage switching can be achieved over a wide range of input voltages and loads. As a result of multiple resonances, zero-voltage switching can also be maintained when the switching frequency of the LLC controller is higher or lower than the resonant frequency. As the switching frequency is lowered, the voltage gain significantly increases. This characteristic allows the converter to maintain regulation when the input voltage falls low or the output current increases.

400 V

VLED

iHVM Voltage COUT Feedback LLC Feedback Loop Controller Loop

R1 13 V

ISINK VFB iHVM Process Core R3 D1 ISOURCE VREF HVM1 G1 R2 S1 TLC5960 R R LED Driver S S

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 in order to optimize the power dissipation caused by the LED string voltage change as a result of the

variance of the LED forward voltage. The VLED output voltage equation is the same as for the iHVM with the non-isolated boost converter feedback loop (see Equation 1).

SLVA474–July 2011 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED 9 Submit Documentation Feedback Backlight Driving Scheme Copyright © 2011, Texas Instruments Incorporated 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 resonant

converter starts to work to output the initial VLED voltage. After the user enables the TLC5960, the iHVM adjustment sequence kicks out the center value (0.7 V) of the HVM output control range (0.14 V to

1.25 V). Thus, the initial VLED voltage is also set to start from the center value (here, 70 V) after the initial startup of the ac-dc power. Then, the TLC5960 begins to sense the LED string total forward voltage, and the HVM output starts to modulate. The UCC25600 then regulates the operating frequency with the

voltage-controlled oscillation (VCO) to set the appropriate voltage of the VLED that corresponds to the LED string total forward voltage.

iHVM Optimization start when TLC5960 is iHVM enabled Optimization EN (V) hold at EN = L 5 TLC5960/EN(I) LED VF Change iHVM Found 0 Detected Settling Point iHVM Output iHVM Found Range VHVM (V) Settling Point 1.25 V 1.2 TLC5960/iHVM(O) 0.7 Default 0.7-V Output 0.14 V 0

iHVM Output

VLED (V) Starts to Adjust AC/DC Output 80 V

AC/DC Default Output Level

70 R1 + R2 V R2 REF

60 V iHVM Adjustable Range

R1 (V - V) R3 REF HVM

0 Initiated AC/DC iHVM Initialization iHVM Optimization iHVM Hold Time (s) Power Supply (EN = L) (EN = H) (EN = L)

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

10 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED SLVA474–July 2011 Backlight Driving Scheme Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated 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 an abnormal situation. LED open detection and LED short detection are basic protection requirements in LED backlight systems. In the PMP4298/A, these LED-related protections are handled intelligently by the TLC5960. First, the TLC5960 monitors the Dn nodes (the cathode nodes of the LED strings) and determines whether an abnormal voltage range is detected or not. Then, the TLC5960 tries to resolve any abnormal situation by using the integrated iHVM feature. Finally, and only if the TLC5960 could not resolve the situation with iHVM, the TLC5960 recognizes an error and automatically shuts off the error string(s), separating them from normal working strings. The detection related diagram is shown in Figure 8.

VIN

VLED

TPS40210 (PMP4298) or R1 UCC25600 (PMP4298A) R3 FB

TLC5960 HVM+

iHVM Feedback iHVM Logic HVM-

On LED LED Short Short

iHVM LED LED Status Open Open Info Dn

LED Short (LSD)

XFLT Error Out

Protection Logic LED Open (LOD)

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

SLVA474–July 2011 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED 11 Submit Documentation Feedback Backlight Driving Scheme Copyright © 2011, Texas Instruments Incorporated Protection Scheme www.ti.com

driving to higher Dn voltages by lowering the HVM output voltage. And when the VLED level reaches its maximum value, the TLC5960 recognizes an actual error status. Figure 9 shows this sequence. The LED short detection (LSD) sequence is also presented in Figure 10. In the case of an LED short, the TLC5960 also recognizes an abnormally higher voltage at Dx node. So, in this case, the iHVM mechanism tries to

set VLED at as low a voltage as possible. If this attempt fails, the TLC5960 recognizes this result as an error and initiates a corrective action: shut off the error string.

iHVM Detection Range Dn Voltage Recovers Error String Dn (V) (Normal Status) Shut Off 2.6 HVM Output 1.6 Range 0.8 LED V LED VF Change F Drives HVM Lower Changes Dn Unable to Again VHVM (V) Recover (Error Status) 1.25 HVM Output 0.7 Reaches Min

0.14

VLED Driven XFLT (V) Higher 5 Error Output

0 iHVM VLED Settles Controls Once VLED Reaches V (V) LED VLED Max Value 80 V Range 70 LED Corresponding to 60 iHVM Range

Normal Status iHVM Normal iHVM Error Optimize Optimize Status

Figure 9. LED Open Detection and VLED Optimization Through iHVM

Error String Shut Off Dn (V) Dn Voltage Recovers (Normal Status) 9.5 iHVM Dn Unable to Detection Recover (Error Status) Range

2.6 LED VF 1.6 Changes

LED VF Change Again HVM Output Drives HVM Higher Reaches Max VHVM (V) 1.25

0.7

0.14 VLED Driven Low XFLT (V) Error Output 5

0 iHVM Controls

VLED VLED (V) VLED Range Corresponding to 80 VLED Settles Once iHVM Range 70 VLED Reaches 60 Min Value

Normal Status iHVM Normal iHVM Error Optimize Optimize Status

Figure 10. LED Short Detection and VLED Optimization Through iHVM

12 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED SLVA474–July 2011 Backlight Driving Scheme Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated www.ti.com Experimental Results These are the basic schemes used by the TLC5960 LED driver, which integrates both iHVM and protection features, to protect the LED backlight system intelligently. Table 2 summarizes the LED open and short conditions. This table also describes the Dn node recognition internal circuitry and how the iHVM information is integrated into the protection recognition routine.

Table 2. LED Open and Short Protection Conditions

iHVM Status to Trigger Error LED Error Status Dn Error Detection Voltage Detection V reaches the maximum value LED Open Dn < 0.8 V LED (HVM = 0.14 V) V reaches the minimum value LED Short Dn > 19.2 V (default) LED (HVM = 1.25 V)

6 Experimental Results In order to verify the proposed LED backlight TV power supply design using intelligent headroom voltage control, 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

SLVA474–July 2011 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED 13 Submit Documentation Feedback Backlight Driving Scheme Copyright © 2011, Texas Instruments Incorporated Experimental Results www.ti.com Figure 11 shows the demonstration fixture PMP4298A configured with an eight-channel LED light bar in the test setup.

5-V System Load

24-V DC Load

5-V Standby Load

PMP4298A Demo Board Eight-String LED Light Bar 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 a full 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, or 2% superior performance than the PMP4298 in efficiency with a lower cost structure. The 3% power-loss savings in a total 150-W design is equivalent to a 4.5-W saving for thermal components. For additional improvements, the user must also consider appropriate transformer and MOSFET selection, because most of the loss comes from the transformer core loss and the FET switching loss. Figure 13 shows the power factor at a full load for the PMP4298A.

EFFICIENCY COMPARISON POWER FACTOR vs INPUT VOLTAGE 0.9 1 0.89 0.99 0.88 0.98

0.87 0.97 0.96 0.86 0.95 0.85 Efficiency (%) 0.94

Power Factor Power 0.84 0.93

0.83 PMP4298 0.92 PMP4298A 0.91 0.82 90 110 130 150 170 190 210 230 250 264 0.9 90 110 130 150 170 190 210 230 250 264 Input Voltage (V) Input Voltage (V) Figure 12. Total Efficiency Comparison at Full Load on Figure 13. Power Factor at Full Load for PMP4298A

VLED, 24 V and 5 V

14 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED SLVA474–July 2011 Backlight Driving Scheme Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated www.ti.com Conclusion Figure 14 shows the waveforms from the LED light bar ON to OFF at 50% and 90% PWM dimming for

VO_LED (blue line), iHVM (red line), and the output LED current (green line). After VLED turns ON, by the heat of the LED itself, the LED forward voltage is lower; iHVM detects that change and controls VLED to be a little lower, cycle by cycle, to achieve maximum total efficiency in power across the board.

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

VO_LED (blue line), iHVM (red line), and the output LED current (green line).

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

applications. 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 originally designed to work well not only for normal dc-dc converter stages (PMP4298), but also with slower response ac-dc resonant converter stages (PMP4298A).

Another key feature is system safety. By combining the VLED control mechanism and the integrated protection mechanisms, the total system is robust even in abnormal situations. Experimental results confirmed 88% peak efficiency including total ac-dc TV power outputs while maintaining a 98% power

factor for a wide input range from 85 VAC to 264 VAC.

SLVA474–July 2011 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED 15 Submit Documentation Feedback Backlight Driving Scheme Copyright © 2011, Texas Instruments Incorporated References www.ti.com 8 References Unless otherwise noted, these documents are available for download through the Texas Instruments website (www.ti.com). • TLC5960 8-channel LED driver controller with integrated intelligent thermal controller. Product data sheet. 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.

16 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED SLVA474–July 2011 Backlight Driving Scheme Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated www.ti.com Appendix A

>12.5V needed

Chassis

85VAC to 265VAC Input Figure 16. PFC Stage Schematic for PMP4298 and PMP4298A

SLVA474–July 2011 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED 17 Submit Documentation Feedback Backlight Driving Scheme Copyright © 2011, Texas Instruments Incorporated Appendix A www.ti.com

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

18 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED SLVA474–July 2011 Backlight Driving Scheme Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated www.ti.com Appendix A

1 2 3 4 5 6 7 8

C73 2.2uF

2.2uF C21

560uF/35V C20

2 [email protected]

[email protected] 560uF/35V C19

C4 330uF

1 2

560uF/35V L3 C18

C3 330uF

1 NA

NA +

C78

NA 1000pF R134

C77 51 10K R128 R94 R136 C57

R127 0 D34

MBR20200CT NA

C65

NA R102

D25 STPS20100CT

NA 4.99k

43K

R91 R87

R101

NA C60

NA R133 10nF C49 R135 C69

C56

220pF

24K

R99

U11

Ref

D24 STPS20100CT D28 MBR20200CT

TL431AIDBZR

R92 12K

1 2

R89

T3 U10

8.2nF/630V 4 3

C54

FOD817A

C50 2.2nF

1.1mH:300uH

C43 8.2nF/630V C59

D33 BAT54

22nF

R149 11K

220 R96

D29

N/A

R100

3.9k

1.8M R112

C76

22k

R97

0.1uF

47K

D23 BAT54

C88 1nF/630V

0.1uF

C117

R111

N/A D13 D22 R109

1N4148W R107

D27 10K 1.8M

BAS16

NA R88 10K

C62 47K 1

C42 U1:A

2.2nF

R110 LM293AD

8 C51 R35 10K

R74 SPP20N60C3 D35

SPP20N60C3 11K

D45

BAT54

1 2 3 4 D26

Q11 Q7 Hiccup OCP BAS16

C63

.47u

DT RT SS

1N4148W

S S

D D

GD1 VCC GND GD2

G G

8 7 6 5

UCC25600D

C53

15K

R76

0.1uF

D12

U1:B

C41 0.1uF

BAS16

LM293AD

10K 10K 5 6

R72 R84

12K

R73

C61

R34

1.1M

OTP (NTC on Pri HS, 95C)

R95

R98

2u2

C33

R131

4.7

C55

R123

4.7

12K

R14 R25

NTC

R42

R75 1M

C14

0.1uF 5.1k R13

12K

R24

C52

1uF

1:1:1

5.1k R71

Ref TL431AIDBZR 0

R47 U6

Figure 18. LLC Converter Stage Schematic for PMP4298

SLVA474–July 2011 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED 19 Submit Documentation Feedback Backlight Driving Scheme Copyright © 2011, Texas Instruments Incorporated Appendix A www.ti.com

Figure 19. LLC Converter Stage Schematic for PMP4298A

Feedback Documentation Submit Scheme Driving Backlight

– 2011 July SLVA474 LED Intelligent an Proposing Solutions: Power TV AC-DC High-Efficiency 21

PMP4298 for Schematic Stage Driver LED TLC5960 20. Figure

FQT4N20L FQT4N20L

Q19

MURA120T3G

D32

30.1k

D D

S S

24V

D3 R8

3 3

4 4

4.99 4.99

R114 R37 G

G R15 1

1 100K UVLO R115 R38

100K 100K

30.1k

MMBT2222 R9 Q6

R116 R31 7.5V

200K 200K D4

FQT4N20L

Q20

MURA120T3G

2 MURA120T3G

Q5 C9

D36

C16

220K

330uF

S D

S C15 220pF

C10

1uF 3

3 470pF

4.99

R41

4.99

R117

0.1uF 1.5K

R118 R43 R20

C13 24K 100K 100K R16 R119 R36

200K 200K

100V

2.2uF

C17

FQT4N20L

0.33uF

FQT4N20L

100K

13K

R17

R18

50V

Q15

Q21

MURA120T3G

D30

MURA120T3G

D37

4.99

R60

4.99

R120

100K

1uF

R44

82K

C12 R21 R121 R65 100K R105

100K 0

100

R39 MMBT2222A 1

R122 R70 47uH L1

200K 200K Q3 FQT4N20L FQT4N20L C11

220pF 2

MMBT2907

3.9

Q18

Q22

MURA120T3G

D38

D31

D S

R10 G

4.99

R86

4.99

R124 G

G 1k

0.33uF

0.03

R12

0.5W

MBR10H100CT C23 50V Q1

R125 R90 D1 R2 IPP80CN10N 100K 100K 10 R129 R93 200K

200K +

100uF

1uF

C24

100K

100uF

100K

R28

2 100uF optional C7

R30

19 18 17 16 15 14 13 12 11 10

4 3 2 1 9 8 7 6 5

D3 G4 S3 D4 G3 S2 G2 D2 S1 G1

HVM1/HVM R29

100V 2.2uF

optional C1

51K

R40

TLC596xDA

HVM4/REFLSD

0 1A @ 80V

XFLT

G8 G7 G6 G5

D7 D8 D6 D5

S7 S6 S5

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 43

10K

R148

8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

390K 100K

R22 R23

J4 J3

100K 100K

R26 R27

A Appendix www.ti.com Appendix A www.ti.com

Figure 21. TLC5960 LED Driver Stage Schematic for PMP4298A

22 High-Efficiency AC-DC TV Power Solutions: Proposing an Intelligent LED SLVA474–July 2011 Backlight Driving Scheme Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where mandated by government requirements, testing of all parameters of each product is not necessarily performed. TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and applications using TI components. To minimize the risks associated with customer products and applications, customers should provide adequate design and operating safeguards. TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right, or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the third party, or a license from TI under the patents or other intellectual property of TI. Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive business practice. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional restrictions. Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not responsible or liable for any such statements. TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would reasonably be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be provided by TI. Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in such safety-critical applications. TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are specifically designated by TI as military-grade or "enhanced plastic." Only products designated by TI as military-grade meet military specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use. TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated products in automotive applications, TI will not be responsible for any failure to meet such requirements. Following are URLs where you can obtain information on other Texas Instruments products and application solutions: Products Applications Audio www.ti.com/audio Communications and Telecom www.ti.com/communications Amplifiers amplifier.ti.com Computers and Peripherals www.ti.com/computers Data Converters dataconverter.ti.com Consumer Electronics www.ti.com/consumer-apps DLP® Products www.dlp.com Energy and Lighting www.ti.com/energy DSP dsp.ti.com Industrial www.ti.com/industrial Clocks and Timers www.ti.com/clocks Medical www.ti.com/medical Interface interface.ti.com Security www.ti.com/security Logic logic.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defense Power Mgmt power.ti.com Transportation and www.ti.com/automotive Automotive Microcontrollers microcontroller.ti.com Video and Imaging www.ti.com/video RFID www.ti-rfid.com Wireless www.ti.com/wireless-apps RF/IF and ZigBee® Solutions www.ti.com/lprf TI E2E Community Home Page e2e.ti.com