The rapid development of LED lighting applications is replacing almost all the traditional forms of lighting applications.With this acceleration, LED driver power requirements have also increased, if not at the expense of efficiency, then the greater the current, the current detection accuracy to maintain the more The LED driver must maintain the current sense accuracy while quickly providing current to multiple independent LED loads and can be connected in parallel and accurately current sharing.
Some high-power LEDs have unique mechanical and electrical considerations, and their anodes are electrically connected to thermally-conductive backsheets. In a traditional LED driver with a buck regulator configuration, thermal management is achieved by cooling the chassis, The positive cable connection creates an electromechanical design problem.The rear panel must have good thermal conductivity to the heatsink, but must also be electrically isolated from it (if the voltage on the rear panel is different from that of the chassis.) As the LED manufacturer changes The production process or package is very difficult, so the LED driver itself must meet the design challenge.
One option is to use a four-switch forward buck-boost LED driver, but the additional switching MOSFET adds system complexity and cost. The negative output buck-boost topology uses only one set of switching power MOSFETs and allows positive Directly (electrically) connected to the heat sink, eliminating the need for an electrical isolator on the heatsink and simplifying the mechanical design of the system.
To meet high performance requirements, the LT3744 can be configured as a synchronous buck or negative output buck-boost controller that drives LED loads with more than 20A continuous current. The LT3744's power inputs accept voltages from 3.3V to 36V. When used as a buck converter, the device regulates LED current from 0V up to supply voltage. When used as a negative output buck-boost converter, the LT3744 accurately adjusts the LED from 0V up to -20V Current.
Accuracy of analog current regulation is 3% over full scale and accuracy is better than ± 30% at 1/20 scale. The LT3744 has three independent analog and digital control inputs and three compensation and Gate Drive Output for a Variety of LED Configurations The LT3744 can be configured as either a buck or a minus output buck-boost controller by separating the inductor current sense and LED current sense. For system design all input signals Based on the board ground (SGND, signal ground), there is no need for a complex discrete level shifter.
In a negative output Buck-Boost configuration, the overall forward voltage of the LED can be higher than the input supply voltage, allowing high-voltage LED strings to be driven from a low-voltage supply.When dissipating component power dissipation is required for PCB power density considerations, The LT3744 can also be easily paralleled to drive large LED pulse load currents or DC load currents.
High accuracy current detection
The LT3744 uses a high-accuracy current-regulating error amplifier that enables accurate analog dimming of 1/20 of the total current control range.In applications where the overall digital PWM dimming range is limited, or in applications that require very large dimming ranges For example, the LT3744 is capable of 1250: 1 PWM dimming at 100Hz PWM dimming frequency and 1MHz switching frequency and can also be combined with a 20: 1 analog dimming to provide an overall dimming range Expand to 25000: 1.
Figure 1 shows the production uniformity of the LT3744 offset voltage as a function of temperature when the analog control input is 0V, where the typical number of devices is 380. With a low offset of the error amplifier, the control loop operates on a 1/20 scale simulation Dimming achieves ± 10% typical accuracy Figure 2 shows the settling voltage across multiple LED current sense pins when the control input is equal to 1.5 V. The accuracy of the full scale range is better than ± 3%, which is equivalent to ± 1.8mV accuracy at 60mV full scale adjustment.
NUMBER OF UNITS: Number of Devices 380 TYPICAL UNITS: 380 Devices Typically REGULATED VLED_ISP - VLED_ISN VOLTAGE: Stable VLED_ISP - VLED_ISN Voltage
Flicker-free performance
One of the most important metrics for measuring LED driver performance is the recovery rate of the LED current during PWM dimming, in which the performance of the driver has a significant impact on the quality of the final product during the first few switching cycles following the rising edge of the PWM-on signal The LT3744 uses proprietary PWM, compensation, and clock synchronization techniques to provide flicker-free performance even when driving LEDs up to 20A.
Figure 3 shows the recovery of the LED current over a 5-minute period with a 20A supply of 12V power to the red LED with a switching frequency of 550kHz, an inductor of 1μH, a PWM dimming frequency of 100Hz and an on-time of 10μs 1000: 1 dimming ratio.) The figure shows about 30,000 dimming cycles with no jitter in the switching waveforms, and each recovery switch cycle is the same.
10V / DIV: 10V5-MINUTE PERSISTENCE per grid: 5 minutes remaining
High-speed dimming between 3 different stable currents
In projection systems, allowing the light source to turn on faster reduces the timing constraints, and the reduced timing constraints also increase the image update rate, which provides higher resolution images and reduces the rainbow effect of fast moving white objects. The LT3744 can transition between different output current states in less than 3 switching mids.
The LT3744 has three stable current states so color mixing system designers can determine the color temperature of each LED. High color accuracy can be achieved through color mixing to correct LED color inaccuracies and eliminate the various Bias. The LT3743 has both low and high current conditions. The LT3744 has three current states, so all three color (RGB) LEDs can be mixed with their respective light outputs to independently correct the color of the LEDs.
Figure 4 shows a 24V input / 20A output single LED driver that provides three different stable currents, which are determined by the analog voltage on CTRL and the digital state of the PWM pin. Note that since RS is used only for limiting Inductor peak current and provide absolute overcurrent protection, then the accuracy of this resistor does not have to be high, which reduces the system cost.
20A MAXIMUM: maximum 20ABLUE: blue light
The PWM dimming between the three different current states is shown in Figure 5 and Figure 6. The PWM signals are sequentially turned on and off in Figure 5. PWM3 has the highest priority and PWM1 is the lowest.This allows a single input signal to be fast Conversion to change the output current as shown in Figure 6, PWM input signal can have any length of time between the intervals.
A complete RGB LED solution for pico projectors or smartphone projectors
In miniature projection systems or smart phone projection systems, it is important to reduce the footprint and cost of the overall solution, in which PCB space is extremely limited and the total volume of the driver solution, including the component height, must be minimized. Driving all three LEDs with only one LED driver can significantly reduce the space required, allowing the use of larger batteries or larger power LEDs to extend battery life and improve projection system throughput.
The LT3744 combines both switched-capacitor technology and floating-gate driver to form a complete RGB solution with a single LED driver. The LT3744 provides a unique gate driver for the PWM output pin. The driver's negative rail floats on On the VFNEG pin, all the off-state switch gates can be pulled down to a negative voltage, which ensures that the switch in series with the output capacitor will not turn on under any conditions. This driver allows 15V Pressure difference.
Each LED can be turned on sequentially, with a certain time delay between each other, or in accordance with any mode provided to the PWM digital input. In addition, with three independent analog control inputs, each LED can be different Stable current operation. When the LT3744 is configured as a negative output buck-boost converter, a single Li-Ion battery can drive three independent LED strings with only a single controller. Figure 7 shows a dedicated RGB miniature projection Designed for 3.3V / 5A Negative Output, 3-Color, Buck-Boost LED Driver.
Two LT3744 LED drivers connected in parallel to form a 324W dual LED driver
An important limiting factor in any high-power / high-current controller design is the PCB's power density, which is limited to about 50W / cm2 to prevent the power path components from rising too high. In situations where multiple converters can be paralleled to share the load when the power required by one LED load exceeds the limit that can be provided by a single driver (still within power density limits).
A high-efficiency, high-current LED driver controller with a new power MOSFET can provide approximately 200W (solution size is approximately 4cm2) and can limit the temperature of all power path components to below 80ºC. For LED loads above 200W and As an aside, the LT3744 can be paralleled to limit the temperature rise of any component. All compensated outputs should be connected in parallel to allow current sharing between converters.
Figure 8 shows a 324W converter consisting of two ADI DC2339A demo boards in parallel, where each parallel controller generates 27A and generates 54A in total at 6 V. By applying the corresponding The compensated outputs are connected together and both controllers operate in unison to provide smooth, good start-up and accurate DC regulation.
Figure 9 shows the LED current startup for each board. Note that the steady-state current provided by each board is the same throughout the startup. Figure 10 shows that when the DC is stable and no PWM dimming , Excellent current sharing is achieved between the two application boards (the waveforms are directly on top of each other.) Figure 11 shows that at 100% duty cycle, the temperature rises to about 55ºC above board ambient temperature. L1 is the inductor, Q1 and Q3 are switching power FETs, R5 is the inductor current sense resistor, R32 is the LED current sense resistor, and U1 is the LT3744.
CHANNEL: Channel 10ms / DIV: 10ms per cell
In this application, PWM dimming can be performed on two independent LED strings at full 54 A. With PWM dimming, Figure 12 shows that the LED current is perfectly split between the two drivers During the test, the rise time of LED current from 0A to 54A was 6.6μs. The electrical connections from each driver output to the LED must be carefully balanced to avoid adding inductance to either path, which would reduce the effective rise time.
Figure 13 shows the temperature rise for each demo board with an LED current of 54 A at 50% PWM dimming. To minimize the inductance from each demo board to the LED, the parallel LED driver board is mounted directly Top of each other A more optimal layout is to have two drives mounted on a single circuit board with the layout of each drive mirroring one another across their shared connection to the LEDs Whenever a design is from the LED Drive to the high-current LED's conduction path, you should pay close attention to the overall inductance. Since the inductance is a function of the length of the wire, then the longer wire, LED current recovery time longer, no matter how fast the drive.
The two LT3744s are paralleled to form a negative output buck-boost 120W LED driver
Negative output buck-boost applications, like non-negative output converters, have the same thermal issues and added design challenges for increased inductor current. For low input voltage and high LED voltage, the inductor For example, if the input is 3.3V, the output drives a green LED, and the LED has a forward voltage of 6V at 20A, the inductor peak current is 70 A. The inductor used in this design The saturation current should be at least 20% higher, so in this case it should be higher than 80A.
Since this current flows through the switching MOSFET, the MOSFET rating must be greater than 80 A. By paralleling two LT3744 negative output buck-boost converters, the peak switching current is reduced by half, reducing the need for power path Component requirements.
In a negative output buck-boost topology, inductor current is provided to the load only when the synchronous FET is on. If two parallel converters are allowed to operate at their free-running frequency then There is a significant beat frequency due to the slight difference in switching frequency.To avoid this problem, each converter uses the same RT value, but these converters are synchronized with an external clock. In the application, the converter is designed to operate at an unsynchronized frequency of 300kHz with a synchronous clock of 350kHz.
Figure 15 shows the rise in component temperature for a 30A current supply to an LED in a parallel negative output buck-boost application.
in conclusion
The LT3744 features high current regulation accuracy, floating PWM gate drivers and input signal level translation to drive LEDs in a wide variety of applications. The LT3744 can be used as a single driver in RGB projection systems for significant Reduce the total space required for solution, which makes it possible to use smart phones light output large video projection.
By using three current-regulation states, the LT3744 allows system designers the freedom to determine LED colors to produce more accurate color video images. The LT3744 generates a negative voltage by directly adjusting the LED current and level-shifting all signals Drives multiple LED strings with a low-voltage battery-powered system with a simple two-switch solution. The LT3744 can be connected very simply in parallel to deliver extremely high currents to LEDs with high current accuracy and current sharing, even over PWM The same is true of dimming.The parallel LT3744 reduces board temperature and inductor current and boosts the supported LED power to hundreds of watts.