Powering LED Lighting & Other Illumination Devices
282
Hua (Walker) Bai
60V, synchronous step-down high current LED driver
its predecessor, the LT3743. It can be applied in a number of other applications thanks to its three additional regulation loops:
1) An output voltage regulation loop enables constant output voltage operation. This can be used to provide open LED protection or charging termination for a battery charger.
2) A second current regulation loop can be used to set an input current limit.
3) An input voltage regulation loop can be used for maximum power tracking (MPPT) in solar-powered applications.
48V input to 35V output, 10A LED driver optimized for efficiency
Figure 282.1 shows a design that delivers 350W output power to drive up to seven LEDs in series from a 48V source. At this high power level, dissipated power is a major concern, so
Introduction
The meaning of the term “high power LED” is rapidly evolv- ing. Although a 350mA LED could easily earn the stamp of
“high power” a few years ago, it could not hold a candle to the 20A LED or the 40A laser diodes of today. High power LEDs are now used in DLP projectors, surgical equipment, stage lighting, automotive lighting and other applications tra- ditionally served by high intensity bulbs. To meet the light output requirements of these applications, high power LEDs are often used in series. The problem is that several series- connected LEDs require a high voltage LED driver circuit.
LED driver design is further complicated by applications that require fast LED current response to PWM dimming signals.
The LT3763 is a 60V synchronous, step-down DC/DC con- troller designed to accurately regulate LED current at up to 20A with fast PWM dimming. It is a higher voltage version of
Figure 282.1 • 48V Input to 35V Output, 10A Figure 282.2 • Efficiency of the 48V Input to 35V Output Circuit Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00282-9
high efficiency is critical. Each 1% of efficiency improvement reduces the loss by 3.5W—significant if the total power loss budget is less than 7W. This circuit is optimized to operate with 98.2% efficiency at full load—Figure 282.2 shows the efficiency reaching 98% when LED current is above 3A and peaking at 98.4% at ∼6A.
At high voltage, the switching losses of the MOSFETs and the inductor outweigh conduction losses. The switching fre- quency is set to 200kHz to minimize switching losses while maintaining small solution size. Running at full load, this cir- cuit’s hot spot occurs at the top MOSFETs, which settles at less than 50°C temperature rise—a very comfortable range for the MOSFETs.
36V input to 20V output, 10A LED driver with fastest PWM dimming
PWM LED dimming is the standard dimming method for high power, high performance lighting applications. Fast LED current response to a PWM signal is important in image- producing applications, such as DLP projectors. Figure 282.3 shows the LT3763 in an application optimized for fast LED PWM dimming.
To achieve fast LED current response to the PWM signal, the LT3763 includes many innovative features. For a given input voltage, the smaller the inductance, the faster the induc- tor current ramps up, which translates to faster LED current response. This circuit takes only a few microseconds to reach full LED current from zero current when a PWM dimming
signal is turned on. Figure 282.4 shows the performance in the PWM dimming application. Efficiency is 97% at full load.
Solar-powered battery charger
The LT3763 can also regulate its input voltage by adjusting its output current. This is useful for applications that must track peak input power such as in a solar-powered battery charger.
Every solar panel has a point of maximum output power that depends on panel illumination, voltage and output cur- rent of the panel. In general, peak power is achieved by main- taining the panel voltage in a small range by reducing output current when needed to prevent the panel voltage from mov- ing out of this range. This is called maximum power point tracking (MPPT).
The LT3763’s input voltage regulation loop keeps the panel voltage in maximum power point range by adjusting out- put current. The constant current, constant voltage (CCCV) operation and C/10 function make the part a natural fit for battery charger applications.
Conclusion
The LT3763 is a 60V, synchronous, high current step-down LED driver controller that can be used to drive the latest high power LEDs, with fast PWM dimming response if needed.
The LT3763 is not limited to LED driver applications, due to its three additional voltage and current regulation loops and a number of other powerful features.
283
Keith Szolusha
60V buck-boost controller drives high power LEDs, charges batteries and regulates voltage with up to 98.5%
efficiency at 100W and higher
LED driver that runs from 15V to 58V input with up to 98.5% efficiency.
The 4-switch synchronous topology drives high power LEDs with minimal switch power loss (and minimal temper- ature rise). Unlike other topologies, the LT3791 buck-boost can be shorted from LED+ to both LED− and GND and, as a feature, can be programmed to latch off or keep trying to turn back on if the short is removed. Diagnostic output flags report both short-circuit and open-LED conditions.
The solution in Figure 283.1 features up to 100:1 PWM dimming at 100Hz for accurate brightness adjustment with- out color shift and analog LED dimming when a PWM oscilla- tor is not present (Figure 283.2).
Introduction
The LT3791 is a 4-switch synchronous buck-boost DC/DC converter that regulates both constant-current and constant- voltage at up to 98.5% efficiency with a single inductor. It can deliver hundreds of watts and features a 60V input and out- put rating, making it ideal for driving high power LED strings and charging high voltage batteries when both step-up and step-down conversion is needed. It can also be used as a con- stant-voltage buck-boost regulator with current limiting and monitoring for both input and output.
Buck-boost controller drives 100W LED string for airplane and truck lights
Airplanes and big trucks with 24V batteries need powerful, efficient and robust headlights and spotlights. Figure 283.1 shows a 33.3V, 3A (nine Luminus SSR-90 LEDs) buck-boost
Figure 283.1 • 15–58VIN to 33.3V 3A LED Driver with up to 98.5% Efficiency Figure 283.2 • Efficiency of Figure 283.1
36V, 2.5A SLA battery charger
The buck-boost converter shown in Figure 283.3 charges a 36V 12Ah SLA battery at 44V with 2.5A DC from a 9V to 58V input.
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00283-0
Specially integrated C/10 current sensing and battery volt- age detection drops the battery voltage from its charging volt- age (44V) to its float voltage (41V) when the battery is near full charge. The OPENLED flag is used to change the state of the charger from charge to float. When the battery voltage drops far enough, voltage feedback returns the charger to its charge state.
The LT3791 can be tailored to charge a range of battery chemistries and capacities from a variety of input sources regardless of the voltage relationship between them. An exter- nal microcontroller can be programmed and used to create a maximum power point tracking device to charge the battery from a solar panel. The output diagnostics and dimming input pins make this a simple interface for high power solar panel applications design.
120W, 6V to 55V voltage regulator
The LT3791 can also be used in high power constant-volt- age buck-boost applications. The FB pin doubles as both the main voltage feedback for the buck-boost, and the overvolt- age protection detection and regulation when used as an LED driver.
Figure 283.1 can be easily turned into a 100W voltage regula- tor with VOUT between 6V and 55V by placing a 100k resistor between the SS and VREF pins to defeat soft-start reset during a fault.
The voltage regulator is short-circuit proof. The SHORTLED flag reports when there is a short circuit on the output. The input current monitor can still be used in constant- voltage regulation to protect any system that is input-current limited. RLED sets the output current limit for short circuit, but can be removed to provide a PGOOD flag using OPENLED by shorting ISP to ISN.
Conclusion
The LT3791 synchronous buck-boost controller delivers 100W and higher power at up to 98.5% efficiency to a num- ber of different loads. The wide, 4.7V to 60V input and 0V to 60V output voltage range make it both powerful and versa- tile, and its short-circuit capability makes this a robust choice for many applications in potentially hazardous environments.
This seemingly limitless IC can be used in applications where a typical buck or boost converter cannot because of the cross- over of input and output voltage ranges.
284
Wei Gu
Offline LED lighting simplified: high power factor, isolated LED driver
needs no opto-isolators and is TRIAC dimmer compatible
current sensing scheme delivers a well-regulated output cur- rent to the secondary side without using an optocoupler. Its unique bleeder circuit makes the LED driver compatible with TRIAC dimmers without additional components. Open- and shorted-LED protection ensures long term reliability.
No-opto operation
Figure 284.1 shows a complete LED driver solution. The LT3799 senses the output current from the primary side switch current waveform. For a flyback converter operating in boundary mode, the equation for the output current is:
IOUT=0.5ãIPKãNã(1−D)
Introduction
As environmental concerns over traditional lighting increase and the price of LEDs decreases, high power LEDs are fast becoming a popular lighting solution for offline applications.
In order to meet the requirements of offline lighting—such as high power factor, high efficiency, isolation and TRIAC dimmer compatibility—prior LED drivers used many external discrete components, resulting in cumbersome solutions. The LT3799 solves complexity, space and performance problems by integrating all the required functions for offline LED lighting.
The LT3799 controls an isolated flyback converter in criti- cal conduction (boundary) mode, suitable for LED applica- tions requiring 4W to over 100W of LED power. Its novel
Figure 284.1 • TRIAC Dimmable 20W Offline LED Driver Using the LT3799 Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00284-2
IPK is the peak switch current, N is the primary to second- ary turns ratio and D is the duty cycle. The IC regulates the output current by adjusting the peak switch current and the duty cycle through a novel feedback control. Unlike other primary side sensing methods that need to know input power and output voltage information, this new scheme provides much better output current regulation since the accuracy is barely affected by transformer winding resistance, switch RDS(ON), output diode forward voltage drop and LED cable voltage drop.
High power factor, low harmonics
By forcing the line current to follow the applied sine-wave voltage, the LT3799 achieves high power factor and complies with IEC61000-3-2, Class C lighting equipment Harmonics Requirement. A power factor of one is achieved if the current drawn is proportional to the input voltage. The LT3799 mod- ulates the peak switch current with a scaled version of the input voltage. This technique provides power factors of 0.97 or greater. A low bandwidth feedback loop keeps the output current regulated without distorting the input current.
TRIAC dimmer compatible
When the TRIAC dimmer is in the off state, it’s not com- pletely off. There is considerable leakage current flowing through its internal filter to the LED driver. This current charges up the input capacitor of the LED driver, causing random switching and LED flicker. Prior solutions added a bleeder circuit, including a large, expensive high voltage MOSFET. The LT3799 eliminates the need for this MOSFET or any other extra components by utilizing the transformer primary winding and the main switch as the bleeder circuit.
As shown in Figure 284.2, the MOSFET gate signal is high and the MOSFET is on when the TRIAC is off, bleeding off the leakage current and keeping the input voltage at 0V. As soon as the TRIAC turns on, the MOSFET seamlessly changes back into a normal power delivery device.
Open- and shorted-LED protection
The LED voltage is constantly monitored through the trans- former third winding. The third winding voltage is pro- portional to the output voltage when the main switch is off and the output diode is conducting current. In the event of
threshold as the third winding cannot provide enough power to the IC. The IC then enters its start-up sequence as shown in Figure 284.4.
CTRL pins and analog dimming
The LT3799’s output can be adjusted through multiple CTRL pins. For example, the output current would follow a DC control voltage applied to any CTRL pin for analog dimming.
Overtemperature protection and line brownout protection can also be easily implemented using these CTRL pins.
Conclusion
The LT3799 is a complete offline LED driver solution featur- ing standard TRIAC dimming, active PFC and well-regulated LED current with no optocoupler. This high performance and feature-rich IC greatly simplifies and shrinks offline LED driver solutions.
Figure 284.2 • MOSFET Gate Signal and VIN
Figure 284.3 • Output Open-Circuit Event
285
Daniel Chen
Reduce the cost and complexity of medium LCD LED backlights with a single inductor LED driver for 60 LEDs
Typical application
Figure 285.1 shows the LT3598’s six channels driving 60 LEDs, with each string programmed at 20mA. The CTRL pin and PWM pin provide analog and digital dimming, respec- tively. True Color PWM dimming delivers constant LED color with a 3000:1 dimming ratio. Figure 285.2 shows the typical
±0.5% current matching between strings, which yields the uniform light distribution that is so important in large back- light applications.
Need more current?
For applications that demand more than 30mA per string, multiple channels of the LT3598 can be easily combined for higher LED current. Figure 285.3 shows a configuration that drives two strings at up to 90mA per string. The 1000:1 PWM dimming waveform at 125°C junction temperature (worst case) is shown in Figure 285.4.
Introduction
One inductor, one IC, one string of LEDs. This is the con- ventional way to build a boost LED driver for LCD display backlights. Although this is a perfectly acceptable solution for small LCD displays that only require a few strings, in larger displays the number of control ICs and inductors multiplies quickly, as do the expense and PCB real estate requirements.
This is a major hurdle in the race to replace CCFLs with robust, spectrally superior LEDs in medium sized, bright displays.
A better driver is needed to bring the cost and complexity of LED backlights in line with CCFLs. The LT3598 answers the call by driving six strings of ten LEDs at up to 30mA per string.
It also has a built-in power switch to save space and design time. Efficiency is optimized via an adaptive feedback loop that monitors all LED pin voltages to provide an output voltage just high enough to light all LED strings. The LED current is regulated even when VIN is greater than VOUT. The LED current can be derated based on programmed LED temperature through an NTC resistor divider or by programming die junction temperature.
Figure 285.1 • LED Driver for 60 × 20mA LEDs Figure 285.2 • Current Matching for Figure 285.1 Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00285-4
TSET pin for thermal protection
The TSET pin voltage can be programmed to limit the internal junction temperature of the LT3598. Once this temperature is reached, the LED current will linearly decrease if the junc- tion temperature keeps increasing, as shown in Figure 285.5.
This thermal regulation feature provides important protection at high ambient temperatures, and allows a given application to be optimized for typical, instead of worst-case, ambient temperatures.
Channel disable capability
Unused LED pins can be tied to VOUT to disable them, so no current flows into the disabled channels. Fault detection ignores any channels tied to VOUT. Figure 285.6 shows an application with two disabled channels that yields efficiency as high as 90%.
Conclusion
LT3598 is a versatile LED driver with a built-in power switch for multiple LED strings. High PWM dimming is possible even with its robust fault detection. Furthermore, a voltage loop regulates the output voltage when all LED strings are open.
Figure 285.3 • LED Driver for Two Strings of 90mA LEDs
Figure 285.4 • 1000:1 PWM Dimming for Figure 285.3 at 125°C
Figure 285.5 • TSET Function Reduces LED Current at High Temperatures
286
Keith Szolusha
100V controller drives high power LED strings from just about
any input
mode, buck mode, SEPIC, flyback and other topologies. Its high power capability provides potentially hundreds of watts of LED power over a wide input voltage range. Its 100V float- ing LED current sense inputs provide accurate LED current sensing. Excellent PWM dimming architecture produces high dimming ratios.
A number of features protect the LEDs and surround- ing components. Shutdown and undervoltage lockout, when combined with analog dimming derived from the input, pro- vide the standard ON/OFF feature as well as a reduced LED current should the battery voltage drop to unacceptably low levels. Analog dimming is accurate and can be combined with PWM dimming for a wide range of brightness control. Soft- start prevents spiking inrush currents. The OPENLED pin informs of open or missing LEDs and the SYNC (LT3756-1) pin can be used to sync switching to an external clock. The FB voltage loop limits the max VOUT to protect the converter in the case of open LEDs.
Introduction
Strings of high power solid-state LEDs are replacing tradi- tional lighting technologies in large area and high lumens light sources because of their high quality light output, unmatched durability, relatively low lifetime cost, constant-color dimming and energy efficiency. The list of applications grows daily, including LCD backlights and projection, industrial and archi- tectural lighting, automotive lights, streetlights, billboards and stadium lights.
As the list expands, so does the range of VIN for the LED drivers. LED drivers must be able to handle wide ranging inputs, including transient voltages of automotive batteries, a wide range of other batteries and wall wart voltages. For LED lighting manufacturers, applying a different LED driver for each application means stocking, testing and designing with a number of controllers. It would be better to use just one that can be applied to many solutions.
The LT3756 high voltage LED driver features a unique topo- logical versatility that allows it to be used in boost, buck-boost
Figure 286.1 • A 125W, 83V at 1.5A, 97% Efficient Boost LED
Driver for Stadium Lighting Figure 286.2 • An 80VIN Buck Mode LED Driver with PWM Dimming for Single or Double LEDs
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00286-6
The 16-pin IC is available in a tiny QFN (3mm × 3mm) and an MSE package, both thermally enhanced. For lower input voltage requirements, the 40VIN, 75VOUT LT3755 LED controller is a similar option.
Boost
Lighting systems for stadiums, spotlights and billboards require huge strings of LEDs running at high power. The LT3756 controller drives up to 100V LED strings. The 125W LED driver in Figure 286.1 has a 40V–60V input.
The high power gate driver switches two 100V MOSFETs at 250kHz. This switching frequency minimizes the size of the discrete components while maintaining high 97% effi- ciency, producing a less-than-50°C discrete component tem- perature rise—more manageable than the heat produced by the 125W LEDs.
Even if PWM dimming is not required, the PWMOUT MOSFET is useful for LED disconnect during shutdown. It prevents current from running through the string of LEDs.
If the LED string is removed, the FB constant-voltage loop takes over and regulates the output at 95V. Without overvolt- age protection, the LED sense resistor would see zero current and the output cap voltage would go over 100V, exceeding several max ratings. While in OVP OPENLED goes low.
Buck mode
When VIN is higher than VLED, the LT3756 can serve equally well as a buck mode LED driver. The buck mode LED driver in Figure 286.2 operates with a wide 10V-to-80V input range to drive one or two LEDs at 1A.
PWM dimming requires a level-shift from the PWMOUT pin to the high-side LED string. The max PWM dimming ratio increases with higher switching frequency, lower PWM dimming frequency, higher VIN and lower LED power. In this case, a 100:1 dimming ratio is possible with a 100Hz dim- ming frequency and a 48V input. Although higher switching frequency is possible, the duty cycle has its limits. Generous minimum on-time and minimum off-time restrictions require a frequency on the lower end of its range (150kHz) to meet both the harsh high-VIN-to-low-VLED (80VIN to one 3.5V LED) and low-VIN-dropout requirements (10VIN to 7VLED).
OVP of the buck mode LED driver has a level shift as well.
Without the level-shifted OVP network tied to FB, an open LED string would result in the output capacitor charging up to V . Although the buck mode components will survive
Buck-boost mode
A common LED driver requirement is that the ranges of both the LED string voltage and the input voltage are wide and overlapping. In fact, some designers prefer to use the same LED driver circuit for several different battery sources and several different LED strings. Such a versatile configuration trades some efficiency, component cost, and board space for design simplicity, and time-to-market.
The buck-boost mode driver in Figure 286.3 uses a single inductor. It accepts inputs from 9V to 36V to drive 10V–50V LED strings at 400mA.
The inductor current is the sum of the input current and the LED string current; the peak inductor current is equal to the peak switching current. Below 9V input, CTRL analog dimming scales back the LED current to keep the inductor current under control. UVLO turns off the LEDs below 6VIN. COUT, DI and MI can see voltages as high as 95V here.
Conclusion
The LT3756 controller is a versatile high power LED driver.
It has all the features required for large (and small) strings of high power LEDs. Its high voltage rating, optimized LED driver architecture, high performance PWM dimming, host of protection features and accurate high side current sensing make the LT3756 a single-IC choice for a variety of lighting systems.