A high brightness white light LED dimming circuit design

With the advent of the energy crisis, efficient lighting technology has received widespread attention. Light Emitting Diode (LED) is a device that uses semiconductor PN junction or similar structure to convert electrical energy into light energy. With its advantages of high efficiency, low power consumption, low voltage drive, and long service life, it has been widely used in many application fields, such as various consumer electronic products - mobile phones, PDAs, LCD TV backlight sources, etc. High-brightness LEDs are an ideal alternative to traditional incandescent lamps because the former has a much higher lifespan and efficiency than the latter, and unlike compact fluorescent bulbs, these LEDs can operate at low temperatures. In order to improve the performance and application scope of LED lighting circuits, this article will introduce a cost-effective high-brightness white light LED (HBLED) dimming method.

For HBLD, the on-state voltage is as high as 3-5 V and the operating current can reach 0.15-3 A under high illumination working conditions. The brightness of LED is basically proportional to the forward current flowing through the LED, so one of the key technologies for LED application is to provide a power supply or drive circuit that is compatible with its characteristics. There are two basic dimming methods for high-brightness LEDs. The first is the PWM (pulse width modulation) dimming method, which turns the LED on and off at different duty cycles of 0% to 100% at certain frequencies greater than 200 Hz. The LED works at full current during the on-state period, and no current flows through the LED during the off-state period, which can ensure color consistency. The second method is to control the amount of current flowing through the LED string, which may cause the voltage of the LED string to drop and cause slight color difference. However, if you observe an incandescent lamp working with the dimmer turned on, you will also see obvious color changes.

High-brightness white light diodes are generally driven by constant current power supplies. Because as the LED gradually gets hotter, its voltage drop will decrease, and if the LED string is powered by a constant voltage power supply, the power supply will often continue to provide too much current, causing the output voltage to increase until the power supply reaches the current limit or the LED fails. The pulse width modulation method is to switch the LED at a higher frequency. The switching frequency exceeds the range that people can generally perceive, giving people the illusion that the LED is always bright. Pulse width modulation is now widely used to adjust the brightness of the LED. In some applications, the dimming ratio can reach 5000:1. Commonly used LED drivers include buck, boost, and buck-boost. LM3402 is a buck regulator derived from a controllable current source. The input voltage range covers the entire automotive application field. The built-in MOS tube can drive up to 5 LEDs. It has a high cost-effectiveness, a wide acceptance field, and a simple and practical circuit. It is a leader among many LED driver ICs.

1 System Structure

1.1 Overall structure

Since the luminous efficiency of a single HBLED cannot fully meet the brightness requirements, multiple LEDs are needed to form an array. An LM3402 drives a string of five high-brightness light-emitting diodes (HBLE-Ds) with a constant current. It is controlled by a microprocessor P89LPC932 with PWM pulse width modulation to achieve stepless adjustment. The current flowing through each HBLED is approximately 120 to 350 mA.

1.2 Human-machine interface

There are 3 buttons (off, brighten and dim) and 4 ordinary LED indicators on the operation panel. Pressing the off button will turn off the high-brightness LED string HBLEDs. Pressing this button again will return to the original brightness display state, and it can also return to the set brightness state after power failure or restart; the brighten and dim buttons are used to change the brightness of HBLEDs, corresponding to 4 indicators, each of which has 2 levels of brightness, so that 8 levels of brightness can be indicated.

1.3 Driving Circuit

The driving circuit is the core of the entire LED dimming circuit, which is mainly composed of a microprocessor P89LPC932 and a LM2402 constant current voltage regulator circuit. LM3402 is a step-down regulator derived from a controllable current source. It can drive a series of high-power, high-brightness light-emitting diode strings and can accept an input voltage range of *2V. When using the pin-compatible LM3402HV, the upper limit of the input voltage can reach 75V. The output voltage of the converter is adjusted as needed to maintain a constant current level through the LED array. As long as the combined feedforward voltage of the HBLEDs does not exceed Vo (MAX), the circuit can keep the regulated current unchanged in any number of LEDs. Figure 1 is a typical application circuit diagram of LM3402, where RSNS is the current setting resistor, the average current IF≈0.2/RSNS, and the RON value is related to the number of LEDs in the light-emitting diode string. When there are more than 5 LEDs, the value can be 300KΩ. After testing, when the constant current nominal value is 250mA (RSNS=0.8 Ω), the current fluctuation is within ±10 mA.

Figure 1 Schematic diagram of typical application circuit of LM3402

The logic of DIM1 is direct, so when the DIM1 port is high, the LM3402 will output a stable current; when the DIM1 port is low, any current output is prohibited. Therefore, inputting a PWM signal to the DIM1 port of LM340 2 can dim the LED array. The maximum logic low level of the PWM signal should be 0.8 V, and the minimum logic high level is 2.2 V. Float the DIM1 port or connect it to a logic high level. Once the input reaches 6 V, the LM3402 starts to operate.

Connecting the OFF port to ground places the LM3402 in a low power shutdown state (90 μA typical). This port should always be left open during normal operation.

P89LPC932 is a low-power single-chip microprocessor produced by Philips. It has a power supply voltage of 3.3 V and can run at low power consumption. It is suitable for many occasions that require high integration and low cost. It can meet many performance requirements. P89LPC932 adopts a high-performance processor structure. The instruction execution time only takes 2 to 4 clock cycles, which is 6 times that of the standard 80C51 device. P89LPC932 integrates many system-level functions, which can greatly reduce the number of components, the area of ​​the circuit board and the cost of the system. It has 2 timers inside and can be used as a PWM generator with 256 timer clock cycles. The electrical schematic diagram of the LED dimming circuit is shown in Figure 2.

Figure 2 Electrical schematic diagram of LED dimming circuit

2 Programming

2.1 Program Structure

The controller program realizes the switch or brightness adjustment according to the input status of the three buttons, and displays the brightness status on the four indicator lights. The brightness status n (PWM duty cycle) can be saved at any time by using the EEPROM unit integrated in the microprocessor itself. The main program flow chart is shown in Figure 3.

Figure 3 Main program flow chart

2.2 PWM generation

The current of the high-brightness white light diode string HBLEDs is mainly regulated by PWM on the DIM port of LM3402. The proportion of the actual current to the set current value depends on the duty cycle of PWM. If the frequency of the PWM signal happens to fall between 200 Hz and 20 kHz, the inductance and output capacitor around the white light LED driver will generate audible noise to the human ear, so the low frequency band below 20 kHz should be avoided during design.

The PWM output of the internal timer TO/T1 of the microprocessor P89LPC932 occupies the same pin as the count input and timer trigger output, and automatically triggers the port output when a timer overflow occurs. This function enables timers 0 and 1 respectively through the control bits ENT0 and ENT1 in the AUXR1 register. When this mode is turned on, the output of the port is logic 1 before the first timer overflows. In order for this mode to take effect, the C/T bit must be cleared to select PCLK as the clock source of the timer. The reference procedure for the timer initialization setting is as follows:

The duty cycle is 256-TH1. The overflow of timer 1 will cause the P1.2 or P0.7 port to flip, so the output frequency is 1/2 of the overflow rate of timer 1.

2.3 Energy saving mode

According to tests, under the same illumination requirements, the power consumption of LED dimming control system is reduced by more than 90% compared with incandescent lamps. Of course, in order to further reduce energy consumption, the exploration of energy-saving methods is still of great significance. Most of the time, HBLEDs may be off. If the control system is in standby or power-off state, the power consumption can be reduced to a minimum; or the OFF port can be grounded, and the LM3402 can be placed in an extremely low-power shutdown state. When the power control register PCONA of the microprocessor P89LPC932 is set to 0xFF, the external functional module is powered off; when the power control register PCON is set to 03H, the microprocessor can be completely powered off. It can only be awakened when an interrupt is triggered, and then the external functional module is powered on, and the microprocessor starts working. The microprocessor is mainly awakened by keyboard interrupts. The reference program for keyboard interrupts is as follows:

3 Conclusion

This article introduces an LED dimming control system based on the constant current drive circuit LM3402. The system uses the microprocessor P89LPC932 PWM to control the output voltage, and the user can set the brightness through the button. Due to the use of a low-power microprocessor and the application of a variety of energy-saving methods, the power consumption of the dimming system is extremely low, and it can be applied to most LED lighting energy-saving transformation occasions, which just meets the development needs of the low-carbon economy. With the continuous improvement of LED luminous efficiency, the improvement of packaging technology, the continuous increase in service life, and the reduction of production costs, coupled with the improvement of drive circuit performance, HBLED has a very broad promotion prospect in the lighting market. At present, this technology has been put into mass production and has achieved good social benefits.