Design of LED dimming circuit based on LM3402

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 LED is an ideal alternative to traditional incandescent lamps, because the former has a much higher life and efficiency than the latter, and unlike compact fluorescent bulbs, these LEDs can work 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 to 5 V under high illumination working conditions, and the operating current can reach 0.15 to 3 A. The brightness of the LED is basically proportional to the size of 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 from 0% to 100% at some frequency greater than 200 Hz. The LED operates at full current during the on period, and no current flows through the LED during the off 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 differences. 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 heats up, 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. Pulse width modulation is to switch LEDs at a higher frequency. The switching frequency is beyond the range that people can generally perceive, giving people the illusion that the LED is always on. Pulse width modulation is now widely used to adjust the brightness of LEDs. In some applications, the dimming ratio can reach 5,000:1. 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. One LM3402 drives a string of five high-brightness light-emitting diodes (HBLE-Ds) with constant current. It is controlled by a microprocessor P89LPC932 with PWM pulse width modulation, which can achieve stepless adjustment. The current flowing through each HBLED is about 120-350 mA.
1.2 Human-machine interface
There are three buttons (off, brighten and dim buttons) and four ordinary light-emitting diode indicators on the operation panel. Pressing the off button will turn off the high-brightness light-emitting diode string HBLEDs. Pressing this button again will return to the original brightness display state. 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 two levels of bright and dark indication, 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. The LM3402 is a step-down regulator derived from a controllable current source, which can drive a series of high-power, high-brightness light-emitting diodes and can accept an input voltage range of 6 to 42V. 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 feed-forward voltage of the HBLEDs does not exceed Vo(MAX), the circuit can maintain the regulated current 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=0.8Ω), the current fluctuation is within ±10 mA.

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, the LED array can be dimmed by inputting a PWM signal to the DIM1 port of the LM340 2. The maximum logic low level of the PWM signal should be 0.8 V and the minimum logic high level should be 2.2 V. The DIM1 port is suspended or connected to a logic high level. Once the input reaches 6 V, the LM3402 starts to operate.
The OFF port is grounded to put the LM3402 in a low-power shutdown state (typical value is 90μA). During normal operation, this port should always remain in an open circuit state.
P89LPC932 is a low-power single-chip microprocessor produced by Philips. The power supply voltage is 3.3 V. It can operate with low power consumption and is suitable for many occasions requiring high integration and low cost. It can meet many performance requirements. P89LPC932 uses a high-performance processor structure, and 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.

2 Program Design
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.

2.2 PWM generation
The current of the high-brightness white light diode string HBLEDs is mainly PWM-regulated by the DIM port of the LM3402. The proportion of the actual current to the set current value depends on the duty cycle of the PWM. If the frequency of the PWM signal falls exactly 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 when designing.
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 the timer overflows. 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 for the timer. The reference procedure for the timer initialization setting is as follows:

duty cycle = 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 the test, under the same illumination requirements, the power consumption of the LED dimming control system is reduced by more than 90% compared with the incandescent lamp. 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 the minimum; or the OFF port is 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 function 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 function module is powered on, and the microprocessor starts working. The microprocessor is mainly awakened by keyboard interrupt. The reference program of keyboard interrupt is as follows:

3 Conclusion
This paper 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 low-power microprocessors and the application of a variety of energy-saving methods, the power consumption of the dimming system is extremely low, which can be applied to most LED lighting energy-saving transformation occasions, just in line with 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.