| Many portable electronic products as notebook PC, mobile phone, PDA, digital camera, etc. are growing popularity rapidly, since the 21st century. White LED is widely used as its best backlighting. LED has many advantage such as small size, low power consumption, long life, high reliability and fast response speed, color and brightness control, on LED do not containing mercury and lead in line with the requirements of energy-saving and environmental protection. However, LED need high turn-on voltage and majority electronic products need 3-6 the backlight white LED, its need 10V to 20V. The vast majority battery capacity of portable electronic products are between 1.5V to 6V, so it requires a special power management chip to drive LED.In this paper, we design a white LED driver chip of using Boost DC-DC converter-type structure, PSM Modulation and Voltage-mode control, adjustable load current, working on DCM. The chip can drive the LED with constant-current, meet the brightness requirements to ensure that individual LED brightness uniformity and color consistency; input voltage range is 2.7V-5.5V, meeting the practical application of battery-powered; there is over-voltage protection and soft-start function; the chip integrated I2C bus interface protocol, has two way of brightness control type: the digital and the I2C bus.I2C (Inter Integrated Circuit) bus is a simple bi-directional serial communication bus line system opened out by PHILIPS last century. Using Verilog HDL hardware description language, I2C bus interface circuit for a behavior-level design using Mentor's ModelSim simulation software for Verilog simulation procedures, simulation results show that the Verilog program designed with the design function and timing is expected to achieve Driver IC I2C bus in the correct connection. Use Synopsys's Design Compiler synthesis tool in the integrated interface program. At the same time, the use of Core Electronics Co., Ltd. Hangzhou Culture of GW48-PK2 EDA / SOPC development board experiments and models of Altera's FPGA chips EP1C3T144C8 on the I2C bus interface procedures to verify the physical level, through the LED lights and the oscilloscope observed, the design of I2C bus interface circuit chip Verilog program in full compliance with work requirements.There are three types of DC-DC converter topology: Buck, Boost and Buck-Boost type, through LED, power tube, inductor and capacitor combination of the different connections. The designed driver IC means control of the circuit switching part, and integrated power control in the chip, according to chip-driven power control mode and the shape of pulse generator, DC-DC converter is divided into modulation: pulse width modulation (PWM), pulse frequency modulation (PFM) and pulse skipping modulation (PSM) in three; different based on feedback signals, DC-DC voltage converter is divided into two kinds of voltage patterns and current-mode control. Chips were divided into nine parts: I2C module, OSC module, PSM module, Selector module, Bias Enable module, Band Gap Module, Regulator Module, Voltage COMP module, Driver module. PSM module is the core of the circuit by the feedback control loop in Regulator module and Voltage COMP module, it will be provided 8MHz oscillator into a square wave pulse required to provide to the Driver Power Tube module with digital control methods.When an external control signal is given, the bias can be given low-level module to enable the signal to start the work of the entire chip. After the start of the drive circuit output voltage stay low, LED negative feedback voltage VFB extreme zero, the feedback loop of the Regulator module and Voltage COMP module output voltage in high-level, PSM module is setting, and the output duty cycle of 25% of the pulse signal. Boost converter in accordance with the principle of step-up, gradually increase the output voltage when the output voltage rise is enough, the white LED at both ends of the voltage to exceed its threshold voltage and the LED turn-on, PSM module in the adding model, duty cycle pulse signal output gradually increase to 62.5% and maintain. Because of increased duty cycle, Boost converter output voltage high rapidly, spur track white LED reach pre-set luminous intensity, when the chip will be in a state regulator. When LED meets the request, the output voltage continues to raise, the feedback voltage VFB is also increased, Voltage COMP module output for the low electrical level inversion, counter in PWM module work on subtract mode. Pulse reduce the signal duty cycle, while output Voltage COMP is low, so the output down pulse, the power switch at turn-off state, by the principle of Boost, by the inductor to the output capacitor and load LED power supply, inductor current will gradually reduced to zero, and then from the output capacitor to provide power to the load, with the capacitance of the discharge, resulting in decreased output voltage, feedback voltage also reduce, thus also making Voltage COMP output level for the high re-turned, PSM output pulse increased duty cycle, the power switch turn-on over again, inductance charg up constantly, provide electrical energy to capacitor and duty, begin to increasing the output voltage.The four different way of control digital brightness are two different digital signal combinations: Regulator module provides four different reference voltages to control the LED current flowing through 20mA, 10mA, 5mA and 0mA, then get different brightness. Brightness I2C bus control, bus interface circuit of the cycle have a duty cycle control signal, so that in the high electrical level the LED current is 20mA, low at 0mA, LED switch is repeatedly, and brightness are depend on the time ratio of on / off, to achieve control of LED brightness.The chip using CHRT 0.35um 2P4M BiCMOS process design and the simulation using Cadence software. Design and simulate of the sub module of the circuit first, the simulation shows that the PSM module by the control signal correctly respond to the roles, add, subtract, set mode could conversion accurately, timing and functionality in line with the design requirements; Band Gap module simulation results as follows: the benchmark voltage Vref = 1.22V, temperature from -40oC to 100oC, the greatest change in output voltage is 4mV, 10Hz, 1kHz and 100kHz, for the respective PSRR value -54dB, -54dB and -49.7dB, the electrical source voltage from 2.7V to 5.5V, the output voltage of bandgap voltage reference changes 5mV; Regulator and Voltage COMP modules based on feedback signal to the output of the control signal correctly. Then the overall chip circuit simulation results showed that: when the input voltage is 2.7V-5.5V, the chip can drive 2-6 white LED, start the process successfully, and steady. It can reach the set current, easy to achieve brightness control; when change the input supply voltage, the output voltage and current remained stable to adapt to the actual situation of battery-powered; in steady-state the inductor in discontinuous current mode, the maximum output ripple voltage is about 30mV, changes in load current of about 0.1mA; off-chip current is very small, prolong the battery life effectively, drive different load efficiency is 72% to 85%, relatively higher than before. Finally, using the database technology to map out the territory of the entire chip, it's not only the functions related to the integrated circuit is correct, but also greatly affect the performance, cost and power consumption of IC.In this paper, the design of the driver chip meet the design objectives, with a high conversion efficiency, operating temperature, input electrical source and wide load range, LED brightness control is easy to carry out, it's suitable for portable electronic production white backlight LED driver circuit. |
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