LINEAR TECHNOLOGY DECEMBER 2006 IN THIS ISSUE… Cover Article Reliable, Efficient LED Backlighting for Large LCD Displays .......................1 Hua (Walker) Bai Linear Technology in the News…..........2 Design Features Precise Current Sense Amplifiers Operate from 4V to 60V........................6 by Jun He Tiny, High Efficiency Monolithic Buck Converters are Perfect for Powering Portable Devices....................9 Phil Juang Supply Supervisor Family Accurately Monitors Multiple Voltages with Independent Undervoltage and Overvoltage Detection........................11 Scott A. Jackson Improve that Mobile Phone Camera: Replace the Anemic LED Flash with a Xenon Flashlamp and a Tiny Photoflash Capacitor Charger............14 Wei Gu High Performance, Feature-Rich Solutions for High Voltage DC/DC Converters...............................16 Kevin Huang OLED Driver Has Low Ripple, Small Footprint and Output Disconnect.......20 Jesus Rosales Hot Swap Controller Controls Power to Two PCI Express Slots....................21 CY Lai DESIGN IDEAS .....................................................25–36 (complete list on page 25) New Device Cameos............................37 Design Tools.......................................39 Sales Offices......................................40 VOLUME XVI NUMBER 4 Reliable, Efficient LED Backlighting for Large LCD Displays Introduction LEDs are rapidly becoming the preferred light source for large LCD displays in computers, TVs, navigation systems, and various automotive and consumer products. LEDs offer several benefits over fluorescent tubes: high luminous efficacy (lm/W), more vivid colors, tunable white point, reduced motion artifacts, low voltage operation and low EMI. However, system engineers face certain problems associated with driving LEDs for LCD backlight applications, including effectively providing sufficient power, regulating the LED current, matching current in multiple LED strings, achieving high LED dimming ratios, and fast LED current turn on/off. All of these issues can be easily addressed in compact and reliable circuits that use the LT3476 high current LED driver and LT3003 3channel ballaster. The LT3476 is a quad output, current mode DC/DC converter operating as a constant current source with up to 96% efficiency. It is ideal for driving up to 1A of current for up to eight-per-channel RGB or white LEDs (such as Luxeon III) in series. That results in a total output power of about 100W. The LT3003 is a 3-channel LED current ballaster, which can be used to triple the number of LEDs driven by a single LT3476 channel. When LED by Hua (Walker) Bai strings are in parallel, special care is required to ensure safe operation and accurate current matching. Otherwise, one string will almost always draw much more current and eventually be damaged. The LT3003 can be used System engineers face a number of problems when designing LED backlights for LCD backlight applications— such as effectively providing sufficient power, accurately regulating the LED current, matching current in multiple LED strings, achieving high LED dimming ratios, and fast LED current turn on/off. All of these issues can be easily addressed in compact and reliable circuits that use the LT3476 high current LED driver and LT3003 3-channel ballaster. with the LT3476 or other LED drivers to regulate current in the LED strings. This is one way to reduce the perLED current and increase brightness uniformity across a large display. For continued on page L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Power, Bat-Track, BodeCAD, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView, µModule, Micropower SwitcherCAD, Multimode Dimming, No Latency ΔΣ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, True Color PWM, UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. DESIGN FEATURES L required for instantaneous setting of the backlight brightness according to the image information and environment in which the device is used. A large dimming ratio also helps reduce motion artifacts. Without adding components and cost, both the LT3476 and the LT3003 can achieve at least 1000:1 PWM dimming ratio with less than 5µs rise/fall time. Additional analog dimming is also possible. LT3476, continued from page example, the 1A output LED current of the LT3476 can be safely shared by three parallel strings of LEDs when the LT3003 is added. Each string carries up to 350mA. The LT3003 guarantees 3% LED current matching. Dimming ratio is defined as the ratio between the highest and the lowest achievable brightness of a system. A large dimming ratio is often R3 100k R2 4.99k R24 100k R25 4.99k R26 100k R27 4.99k R28 100k R29 4.99k C4 1µF E1 18 7 REF R6 0Ω LT3476EUHF 1 VC1 R7 0Ω 38 VC2 R9 0Ω 13 VC3 R10 0Ω 12 C10 1nF C11 1nF 4 PVN CAP2 5 LED2 SW2 27 SW2 26 35 PWM1 34 PWM2 17 PWM3 16 PWM4 C12 1nF VC4 C13 1nF 6 RT R11 21k GND 39 C1 4.7µF 50V (OPT) C2 2.2µF 35V C3 2.2µF 35V D1 DFLS140 3 PVN CAP1 2 LED1 29 SW1 28 SW1 37 VADJ1 36 VADJ2 15 VADJ3 14 VADJ4 PWM1 PWM2 PWM3 PWM4 + 33 VIN SHDN High Side Current Sensing for Versatility and Reliability High side LED current sensing is generally more flexible than low side, in that it supports buck, boost or buck-boost configurations. High side sensing also allows for “one-wire” operation. For example, in a boost circuit with a high side sense resistor, if the LEDs are PVIN 33V MAX VIN 2.8V TO 16V C6 22nF Features 9 PVN CAP3 8 LED3 25 SW3 24 SW3 10 PVN CAP4 11 LED4 23 SW4 22 SW4 N/C N/C 19-21 30-32 C5 0.33µF R1 0.1Ω LED L1 10µH LED D2 DFLS140 C7 0.33µF R4 0.1Ω LED L2 10µH LED D3 DFLS140 C8 0.33µF R5 0.1Ω LED L3 10µH LED D4 DFLS140 C9 0.33µF R8 0.1Ω LED L4 10µH LED Figure 1. The LT3476 delivers 100W in buck mode 0.98 1000 900 800 LED CURRENT (mA) EFFICIENCY (%) 0.96 0.94 PWM 5V/DIV 0.92 0.90 0.88 0.86 200 600 400 800 LED CURRENT (mA) 1000 1200 Figure 2. Efficiency of the buck mode circuit in Figure 1 Linear Technology Magazine • December 2006 600 500 400 300 200 ILED 500mA/ DIV 1 700 100 5µs/DIV PWM FREQ = 100Hz PWM PULSE WIDTH = 10µs Figure 3. 1000:1 PWM dimming 0 0 20 60 40 DUTY CYCLE 80 100 Figure 4. Average LED current vs PWM duty cycle L DESIGN FEATURES Buck, Boost or Buck-Boost Operation Because of the high side current sense scheme, the LT3476 and the LT3003 support buck, boost or buck-boost operation. In buck mode, an LT3476 circuit can achieve 96% efficiency, generating less heat and providing more reliability. For automotive applications where the LEDs must be remote from the driver in some way, such as in a hinged laptop display, the LED current can return to the local display ground, saving a wire in the return path. Low side sensing requires an extra wire, because the LED current must return to the driver side for low noise operation. The one wire setup lowers cost and improves reliability, especially as the channels multiply in high performance displays. PVIN 8V TO 16V 2.2µF L1 10µH L2 10µH L3 10µH D2 D3 D1 CAP1 2.2µF 6–8 LEDS VIN 3.3V CAP2 0.1Ω 2.2µF LED2 350mA 2.2µF D4 CAP4 0.1Ω 0.1Ω LED3 350mA SW1 SW2 CAP1-4 LED1-4 VIN PWM1-4 SHDN PWM1-4 SHDN 2.2µF L4 10µH CAP3 0.1Ω LED1 powered from a lead-acid battery, the LT3476 can be configured for boost mode to drive up to eight LEDs per channel. Furthermore, returning the LED current in a boost configuration to the battery enables buck-boost operation, where the input voltage can be higher or lower than the output voltage. As a result, the LT3476 and LT3003 can accept a variety of power sources. 2.2µF LED4 350mA SW3 LT3476 350mA 1.05V SW4 REF VADJ1-4 66.5k 33.2k VC1-4 RT GND 1k 21k 1nF Figure 5. The LT3476 configured into a boost circuit for automotive applications VIN 3V TO 16V PVIN 33V MAX C2 2.2µF 35V C1 1µF 18 37 3 CAP1 2 LED1 REF R1 0.1Ω PWM1 PWM1 6 R3 21k LED LED LED SW1 RT SW1 GND 39 35 LED LED 2 3 D2 20V VIN LED1 D1 DFLS140 5 VMAX SHDN LT3003EMSE LED2 VEE LED3 9 C5 0.33µF 10 L1 10µH PWM OT1 OT2 GND 6 7 8 11 R4 10k R2 0Ω C6 1nF LED VADJ1 VC1 4 1 VIN LT3476EUHF 1 C4 1µF 35V 33 SHDN 7 C3 2.2µF 35V 29 28 N/C N/C 19-21 30-32 Figure 6. The LT3476 and LT3003 in buck mode Linear Technology Magazine • December 2006 DESIGN FEATURES L PWM and Analog Dimming Dedicated PWM dimming circuitry inside the LT3476 and LT3003 allows a 1000:1 dimming ratio. Additional analog dimming is possible through the VADJ pins. This allows for a significant number of hues and tones, resulting in finer and more exact color definition. Small Packages The LT3476 is available in a 5mm × 7mm QFN package. The LT3003 comes in a small MS10 package. Both packages are thermally enhanced with exposed metal ground pads on the bottom of the package. Accurate Current Monitoring and Matching Each of the four LT3476 current monitor thresholds is trimmed to within 2.5% at the full scale of 105mV. The LT3003 drives three separate strings of LEDs at up to 350mA/string with 3% accurate current matching. Both measures result in uniform LED brightness and intensity. LT3476 Delivers 100W in Buck Mode In today’s large LCD TVs with LED backlights, the power requirement for driving the LEDs can be a couple hundred watts. Figure 1 shows a circuit for a high power LED driver. It is configured as a buck mode converter, delivering 100W to the LEDs from a 33V supply at 96% efficiency. Two of these circuits are enough to drive all the LEDs for a 32-inch LCD TV. For simplicity’s sake only channel 1 is discussed here. All four LT3476 channels are independent and function in the same way. When the internal power switch turns on, the SW1 pin is grounded. Wide Range of Operating Frequencies to Match any Application The LT3476 frequency is adjustable between 200kHz and 2MHz, allowing the user to trade off between the efficiency and the solution size. The voltage crossing the inductor L1 is PVIN – VLED1, where the VLED1 is the voltage drop on the LED string at the given current. As a result, the inductor L1 current ramps up linearly and energy builds up. When the power switch is off, the inductor sees VLED1. The energy in the inductor is discharged and transferred to the LEDs through the catch diode D1. The capacitor C5 filters out the inductor current ripple. The LED current is the average of the inductor current. Figure 2 shows the efficiency as a function of the LED current. To change the maximum LED current, adjust the R1 value or the resistor divider values at the VADJ1 pin. The VADJ pins can be used for white balance calibration. At 100Hz PWM frequency, the PWM control of this circuit allows 1000:1 dimming as shown in Figure 3. Figure 4 shows that the PWM dimming ratio has a good linear relationship to the average LED current. Faster switch on/off time is possible if a PFET disconnect circuit with a level shifter is in series Applications continued on page 33 VIN 8V TO 16V D1 DFLS140 L1 4.7µH 13 14 1 10 9 3 7 8 5 R5 1.02M 1% 17 SW SW FBN 6 18 N/C 19 N/C 20 N/C VIN IADJ1 IADJ2 SHDN ISP2 LT3477 R6 45.3k 1% 11 R4 0.3Ω 1% FBP ISN2 VREF VC GND GND 15 21 D2 1N4148W R1 10k C4 4.7µF 16 ISP1 ISN1 PWM VMAX SUMIDA CDRH5D16-4R7 C1 1µF 25V Q1 2N7002 C2 R2 22nF 0Ω SS 4 RT 12 2 R3 6.81k 6-8 LEDs/STRING C3 0.033µF 6 7 8 3 2 LED3 LED2 LED1 VMAX 4 LT3003 VIN PWM OT1 OT2 GND 11 1 SHDN VEE 10 VMAX VIN 5 9 C3 1µF 25V Figure 7. The LT3476 and LT3003 in boost mode Linear Technology Magazine • December 2006 DESIGN IDEAS L Better than Buck-Only or Boost-Only Solutions To avoid the cost or real estate requirements of traditional SEPIC or cascaded boost-buck topologies, some designers opt for buck-only or boostonly solutions. For example, in two AA alkaline cell applications such as MP3 players, 2.5V often serves as the main rail since it drives both the flash memory and the main processor I/O. In such applications, some designers use a synchronous boost converter to save cost and space. The problem is that the boost converter is very inefficient while the battery voltage is above 2.5V because a boost converter incurs both the losses inherent in an LDO and the switching losses an LDO doesn’t have. Figure 4 shows that the boost converter operates inefficiently for 28% of the battery runtime (the portion of the battery life when the battery’s voltage declines from a fully charged 3V to 1.8V). An LTC3530 solution results in significantly longer battery runtimes compared to these solo boost or buck solutions. Conclusion Linear Technology’s synchronous buck-boost converter simplifies the design of lithium-ion or 2-AA-cell powered handheld devices that require up LT3476, continued from page with a LED string. With this PFET disconnect circuit, the switch off time is less than 2µs. Boost Circuit for Automobile Lighting It is straightforward to use the LT3476 for a boost application given the fact the main power switch is tied to the ground. Figure 5 shows a boost circuit for applications such as automotive exterior and interior lighting. This circuit provides 350mA to eight Luxeon LEDs per channel from a car battery. The efficiency is over 92% with a 16V input. Triple the Number of LED Strings with the LT3003 Each LT3476 channel can be configured to drive three parallel LED strings by adding the LT3003. In such a configuration, each LED string uses one third of the output current of the channel. The LT3003 easily operates in boost mode, or in buck mode with an architecture that allows the power ground (VEE) to move with the output capacitor voltage. Figure 6 shows LT3476 channel 1 plus a LT3003 circuit in buck mode. The stringto-string current matching is 5%, important to maintaining uniform LED brightness between the strings. Figure 7 shows a LT3477 and a LT3003 circuit in boost mode. The VMAX of the Linear Technology Magazine • December 2006 Figure 8. Recommended parts placement and layout LT3003 should be tied to the highest voltage in a circuit. In the buck mode, it is PVIN. In the boost mode, it is the cathode of D1. Layout Considerations For proper operation and minimum EMI, care must be taken during the PCB layout. Figure 8 shows the recommended components placement for LT3476 in buck mode for a 4-layer board. The schematic is shown in Figure 1. In a buck circuit, the loop formed by the input capacitors (C2 and C3), the SW pins and the catch diodes (D1, D2, D3 and D4) should be as small as possible because of the present of high di/dt pulsing current in this loop. The second layer should be an unbroken ground plane. The SW nodes should be as small as possible. From each sense resistor, the traces to 600mA output. Programmable softstart and switching frequency, as well as external compensation, make the LTC3530 a flexible and compact solution. The buck-boost topology helps a designer extend battery runtime while the automatic Burst Mode operation further maximizes the runtime in applications with widely varying load requirements. L for the latest information on LTC products, visit www.linear.com to the CAP pin and to the LED pin should be a Kelvin trace pair. Those traces should be in the third layer for best shielding. The fourth layer should be another ground plane. If long wires are used to connect a power supply to PVIN of the LT3476, an aluminum-electrolytic capacitor should be used to reduce input ringing which could break down the LT3476 internal switch. See Linear Technology Application Note 88 for more information. To ensure reliable operation, good thermal designs for both the LT3476 and the LT3003 are essential. The exposed pads on the bottom of the packages must be evenly soldered to the ground plane on the PCB so that the exposed pads act as heat sinks. Unevenly soldered IC package degrades the heat sinking capability dramatically. To keep the thermal resistance low, the ground plane should be extended as much as possible. For the LT3476, on the top layer, ground can be extended out from the pins 19, 20, 21, 30, 31 and 32. This also allows tight loop components placement mentioned above. The second and fourth layers should be reserved for the ground plane. Thermal vias under and near the IC package helps transfer the heat from the IC to the ground plane and from inner layers to outer layers. L 33