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A high brightness white LED dimming circuit

In Electronic Infomation Category: A | on April 18,2011

With the arrival of the energy crisis, efficient lighting technology has been widespread concern. Light-emitting diode LED (Light Emitting Ddiode) is the use of semiconductor PN junction or a similar structure to convert electrical energy into light energy device, with its high efficiency, low power, low voltage drive, long service life, etc., have been applied in many fields universal applications, such as various types of consumer electronic products - mobile phone, PDA, LCD TV backlighting and TPS767D301PWP datasheet and so on. Conventional high-brightness LED is an ideal alternative to incandescent lamps, because the life and TPS767D301PWP price and efficiency of the former is much higher than the latter, and TPS767D301PWP suppliers and the Unlike compact fluorescent light bulbs, the LED can work at low temperatures. LED lighting circuits to improve performance and scope, this article presents a cost advantage of high brightness white LED (HBLED) dimming methods.

For HBLD terms of working conditions in high illumination turn-on voltage of up to 3 ~ 5 V, operating current of up to 0.15 ~ 3 A. LEDs brightness and the flow through the LED forward current is basically proportional to the size, so the LED is one of the key technology applications and their features are adapted to provide power or drive circuit. There are two basic high-brightness LED dimming methods. The first is the PWM (pulse width modulation) dimming methods, that is larger than some of the frequencies 200 Hz to 0% to 100% of the different duty cycle to turn on and off LED. LED turn-over during the current work, and turn-off period no current flows through the LED, you can ensure color consistency. The second method is to control the amount of current flowing through the LED strings, which may lead LED string voltage drop, and cause a slight color difference. But if you look dimmer incandescent work to open the case, will see significant color change.

Generally use high brightness white LED constant current source driver. Because as the LED gradually becomes hot, the voltage drop will decrease, and if the LED strings, then by the constant voltage power supply, power supply tends to continue to provide too much current, the output voltage increases until the power supply to current limit or LED failure. Pulse width modulation is the higher frequency switching LED, switching frequency can be detected beyond the scope of ordinary people, giving the illusion of a bright LED total, now commonly used in PWM mode LED brightness adjustment, in some applications , 000:1 dimming ratio of up to 5, commonly used with step-down LED drivers (Buck), Step-Up (Boost), buck (Buck ~ Boost), etc. 3. LM3402 is a controlled current source derived from the buck regulator, input voltage range covers the entire automotive applications, built-in MOS tube can drive up to 5 LED, cost-effective, and to accept a wider area, the line of simple and practical , is one of many outstanding middle LED driver IC.

1 System Architecture

1.1 overall structure of the

Luminous efficiency as a single HBLED not fully meet the brightness requirements, therefore, need to use multiple array of LED components, an LM3402 on the five high-brightness light-emitting diodes string (HBLE-Ds) to constant current driver, received 1 The PWM pulse width microprocessor P89LPC932 regulation control, stepless adjustment, the current flowing through each HBLEDs about 120 ~ 350 mA.

1.2 man-machine interface

Operator panel has three buttons (off, brighten and dim buttons) and four ordinary light-emitting diode lights. Press the Close button will turn off high-brightness light-emitting diode string HBLEDs, press the button again, you can display the status back to the original brightness, power down or restart the brightness settings can also be returned to the state; brighten and dim buttons for change HBLEDs brightness, corresponding to four lights, each light is bright and dark two directions, thus indicating 8 block brightness.

1.3 Driver

Drive circuit is the LED dimming circuit of the core, mainly composed of a microprocessor P89LPC932 and the LM2402 constant-current regulator circuit. LM3402 is a current source derived from the controlled buck regulator capable of driving high-power series, high-brightness light-emitting diode string acceptable range * 2V input voltage. When using a pin-compatible LM3402HV, the maximum input voltage up to 75V. The converter as needed to adjust the output voltage to maintain constant current through the LED array level. As long as the combination of HBLEDs feed-forward voltage does not exceed Vo (MAX), the circuit can maintain any number of LED in the regulation of the current unchanged. Figure 1 shows the LM3402 typical application circuit diagram, which RSNS for the current setting resistor, the average current IF 0.2/RSNS, RON value and the light-emitting diode in the LED string related to the number, when more than 5 LED values ??300K, tested , constant current when the nominal value of 250mA (RSNS = 0.8 ), current fluctuations within the 10 mA.


Figure 1 LM3402 typical application circuit diagram

DIM1 logic is straightforward, so when DIM1 port is high, LM3402 will output a stable current; when DIM1 Department is low, and no current output. So the DIM1 LM340 2 PWM signal input port can be dimming on the LED array, PWM signal is logic low should be the maximum 0.8 V, the minimum logic high as 2.2 V. Will DIM1 port connected to the suspension or logic high, once the input reaches 6 V, LM3402 began operation.

The OFF port ground, which will be placed in a LM3402 low-power shutdown state (typically 90A). During normal operation, the port should always be kept open.

P89LPC932 manufactured by Philips low-power single-chip microprocessor, power supply voltage of 3.3 V, low power consumption can run many of the requirements for high integration, low cost applications. Meet a wide range of performance requirements. P89LPC932 uses a high performance processor architecture that executes instructions in clock cycles 2 to 4 and 6 times the standard 80C51 devices. P89LPC932 incorporates many system-level functions, this would greatly reduce the number of components, circuit board area and system cost, which has two internal timers can be used as a clock cycle of 256 timer PWM generator use. LED dimming circuit electrical schematic diagram shown in Figure 2.


Figure 2 LED dimming circuit electrical diagram

2 Program

2.1 Program Structure

3 button controller program based on the input state of switch or brightness adjustment, and brightness of light on the state of the four displayed. Using its own integrated EEPROM microprocessor unit, may at any time save the brightness state n (PWM duty cycle). Main program flow chart shown in Figure 3.


Figure 3, the main program flow diagram

2.2 PWM place

High brightness white LED string HBLEDs current DIM mainly through the port of LM3402 PWM regulator, set the current value of the actual current account depends on the proportion of the duty cycle of PWM duty cycle. If the PWM signal frequency falls exactly between 200 Hz ~ 20 kHz, white LED drive around the inductor and output capacitor will have a human ear can hear the noise, so the design should avoid the use of these low frequency 20 kHz. P89LPC932 MCU internal timer TO/T1

the PWM timer output and trigger input and output count occupy the same pins, a timer overflow occurs automatically toggle a port output. This feature AUXR1 register control bits are enabled ENT0 and ENT1 Timer 0 and 1. The model is open, the first timer overflow before the port output to a logic 1. In order to effect this mode, you must clear the C / T bit to select the PCLK as the clock source. Set reference timer initialization procedure is as follows:


Which the duty cycle duty cycle = 256-TH1, Timer 1 overflow will occur P1.2 or P0.7 port flip, so the output frequency of Timer 1 overflow rate of 1 / 2.

2.3 power saving mode

After testing, the same illumination required in the circumstances, the use of LED dimming control system power consumption reduced by 90% or more than the incandescent lamp, of course, to further reduce energy consumption, energy saving methods of inquiry are still of great significance. Most of the time, HBLEDs may be in the off state, if the control system is in standby or power-down state, power consumption can be reduced to a minimum; or OFF port ground, the LM3402 can also be placed in a very low low-power shutdown state. P89LPC932 power the microprocessor control register is set to 0xFF PCONA, the external function module power-down; the Power Control register PCON is set to 03H, it can completely power-down state of the microprocessor is only triggered when the interrupt can be wake up, then power to the external function modules, the microprocessor to work. Primarily through the keyboard interrupt wake-up the microprocessor, the keyboard interrupt reference procedures are as follows:


3 Conclusion

This paper, a constant current drive circuit based on the LM3402 LED dimming control system, the system microprocessor P89LPC932 PWM control output voltage, the user can set the brightness button. As a result of low-power microprocessor, and apply a variety of energy-saving methods, making the dimming system, low power consumption, can be applied to the majority of energy-saving LED lighting situations, just meet the development needs of low-carbon economy. With the continuous improvement of LED luminescence efficiency, packaging technology improvements, increased service life and lower production costs, coupled with the drive circuit performance improvement, HBLED lighting market promotion prospects. At present, the technology has been put into mass production, and achieved good social benefits.

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