The current HBLED typically have a rated current of 300mA to 700mA. As the envelope is being pushed by the light output devices require more than one ampere are appearing on the market. In all the LEDs, the current-voltage relationship and the binning approach is used by manufacturers, a constant current source for accurate control of light output is used. Choosing the right constant current regulator depends on the operating voltage of the load and source, desired efficiency and costand the size of the plant. A high resistance in series with the LED would be the easiest. Since a strength alone can not be used for current output voltages of origin and evolution of the nonlinear VI characteristics of an LED, a closed-loop system that can adjust the resistance changes in the base. In any case, the energy is not used by the LED in the form of heat through the linear regulator, which discharges to an inefficient system. In most applications HBLED offer better efficiency switching regulator in a wideVoltage range.
HBLED lights try to replace incandescent bulbs and fluorescent tubes have a better efficiency and quality of light and low cost. An integrated switching regulators are used in applications requiring illumination must be a minimum of external components and have a good current regulation. While the control loop can take different forms, all work the same way of moving small amounts of energy from the source to the load. L 'The conversion efficiency is little dependence on the input voltage. However, the choice depends on the topology conversion voltage. A buck topology allows the source voltage exceeds the voltage of the load and is generally used for LEDs.
Programmable Logic Controller
The most important control in each buck regulator is the hysteresis controller. This block controls the current through the inductor by turning on a switch, if it is below the lower threshold and vice versa. A resistor is a shuntconvenient method for detecting the current and minimized loss of mating with a differential current sense amplifier (CSA), a resistance may be the smallest power. The feedback from the CSA is used by analog circuits of the controller.
In all three topologies, the current flows through the inductor, when the appropriate switch (field-effect transistor or FET) is turned on. When the current exceeds a certain limit, the hysteresis controller comes on every topologyFET. Since the current occurs in the inductor, leads them through the diode until it falls below the lower threshold and the FET is turned on. A system that can move quickly to require smaller magnetic flux inductor to save between alternating cycles.
The topology with the red LED is configured with a low-side sense resistor to the pin source of an NFET.
One problem inherent in this implementation is that electricity can be made only by the inductancewhen the switch is turned on. Once the current reaches the peak and the switch is off, the hysteresis controllers use a timer to turn the switch back. If you have not reached during the off-cycle flow or fall below the threshold is exceeded, must be off-set the time until the cycle is stable at the current ripple requirements. Since this technique has hysteresis, but only on one side of the cycle will not be able to quickly adapt to fast transient source and loadConditions. is a hysteresis control system, detecting the position of both falling and rising edges, the feedback loop has to remain independent in the current path of the state of the switch.
the topology of the blue LEDs used in the element direction shows the path of the current induced in the station of loading and unloading. To achieve this, a high-side switch or P-FET is used. Since the RDS (resistance of the FET current) is higher in the P-FET when the N-FET comparedthere is a loss of efficiency. In addition, the high-side driver and the P-FET is generally appreciated more expensive than a low-side N-FET and a driver for the switching function itself. Finally, the green LED in the topology, use the changed position of the FET and sense resistor. This allows the use of an N-FET to increase efficiency, while the position of the measuring cell allows inductor current to be detected during operation of the hysteresis controller. The work as a system,channel LED driver depends on five elements of a topology that is efficient, robust and meets the requirements of the HBLED to create applications. The same can be blocks for other topologies such as boost, buck-boost used single-ended primary inductance converter (SEPIC), etc.
1) Hysteretic Controller
As described above, is the most important function of the hysteresis controller to regulate the LED current. A reliable hysteretic controller is a SR flip-flop, where the 'input' SET 'active when the current falls below the lower threshold and the reset input is activated when the current increases above the upper threshold. production through the use of digital to analog converter (DAC) of the reference voltage, a hysteresis controller can be made programmable. With resolution on the ability of the DAC, checked the reference values higher and lower to change the position of the ripple current can be defined. The ripple reduction in the channel reducesThe ramp times so that the switching frequency. The drivers that are able to work at higher frequencies (500 kHz to 2 MHz in range) can be used to significantly reducing the cost and size of the magnetism to be done. In addition, the controller would be able to provide a logical AND of signals of other modulation functions and travel.
2) power amplifier
A high side sense amplifier allows the hysteresis controller up and down the ramp of the inductor current sense. These CSAneed to distinguish the significance of the level of tension and carry the same reference voltage as a hysteretic controller. Technology for such a CSA that a differential amplifier, signal converter and a secondary cascade amplifier stage. It works by creating a kingdom iSense low voltage current that is proportional to Vsense on the high side. An additional amplifier with adjustable gain is equivalent to a signal with a reference voltage received from the DAC in the ensuingHysteretic controller. A high-gain setting in the CSA permits the use of low-value sense resistors to minimize power loss. A choice between 20 and 100 with the needs of most designs HBLED. Since the CSA is tracking the flows up and down, it is important that the bandwidth of the sensor larger than the switching frequency. With high bandwidth is not required to choose a lower range of the noise was reduced by the positive differential pinAmplifier.
3)-FET gate driver and
Since the choice of gate-driver and FET are inextricably linked to the maximum possible switching frequency and efficiency of the alloy must be carefully chosen for a compromise between cost, size and power of design. A FET with low RDS reduces conduction losses and reduced gate capacitance reduces switching losses. The driver of the gate, the gate capacitance of the converter switching frequency desired. If the gateDriver is not powerful enough, can the ramp rate is too slow, causing the FET in the linear region are inefficient, and if it is too strong, the FET ring could produce EMI emissions.
4) modulator
The output of the modulator provides the dimming signal controller, hysteresis. A high performance of the modulator generates a constant current to the LEDs, while a low-power refers to zero. The choice of modulation should allow a high resolution, wiringPotential of LEDs. Since the human eye can detect small gradients at low intensity modulation is 8-bit side and significant steps to create a sequence extended fade. A higher resolution of 12 to 16-bit modulator requires a clock rate allows a regular slope. However, the frequency modulator to be high enough for a refresh rate, allowing greater persistence of human vision. For example, when using a modulation of 16-bit to 700 HzModulator has a 700Hz * 65536 CNTs ≈ 45MHz clock. Today, different modulation techniques for driving LEDs. Pulse Width Modulation (PWM), contains the desired amount of dimming as a ratio between the width of the pulse at the wrist. Other modulation techniques such as PRISM (TM) (lighting precise signal modulation) dimming the amount distributed in a pseudo-random for the duration of the pulse. The stochastic modulation signal densityenergy spreads across the spectrum to reduce emissions of quasi-peak.
5) circuit trip
Different scenarios require the driver for the constant current source control loop hysteretic stop. This can be caused by sudden fluctuations of input voltage and temperature gradients on the engine performance and longevity to LED. A trip circuit with a programmable DAC and comparator, pulse logic required to travel to the regulator is logical and produce hystereticHold the function switch. Advances in semiconductor technology enables the integration of these components in rapid decline and low-cost programmable logic controllers. PowerPSoC (TM) family of parts includes hysteretic controller channels that the installation, you can create various topologies HBLED units. By driver coupling integrated with a microprocessor on board, the cost factor and the form of a solution with the added benefits associated with reduction of the reducedEMI emissions.
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