Design Of Dimming LED Driver Circuit With Over-temperature Protection Function

Compared with traditional light sources,Light-Emitting Diode(LED)has the advantages of high electro-optical conversion efficiency,soft light,good dimming and long life.Thus,it is widely used in daily lighting,traffic indication,liquid crystal display,etc.The quality of LED lamps mainly depends on the performance of the driver chip.Therefore,improving the performance indicators of the driver chip has become one of the core issues in this field.And research on high-performance driver chips has attracted widespread attention.There are many problems of the existing LED driver chip,including that(1)drive circuit is resistant to power fluctuations and noise performance and the driver chip uses a capacitor or a passive RC low-pass filter to suppress fluctuations and noise of the power supply.But the effect is not satisfactory.(2)The over-temperature protection circuit structure is relatively large,mainly detecting the voltage VBE through the positive-phase input terminal of the amplifier or the hysteresis comparator,performing a difference with the reference voltage of the inverting input terminal,and generating an over-temperature shutdown signal after being amplified.However,the amplifier and the hysteresis comparator circuit use a large number of components,which makes the entire driver chip easy to overheat.(3)For the dimming module,the existing method has a narrow dimming range.And the pulse frequency is low,thus stroboscopic phenomenon is easy to occur.In this paper,we design a dimmable LED driver chip with function of over-temperature protection.Based on some problems existing in the driver chip,it has been improved and optimized.The specific research contents include that(1)the pre-adjustment circuit is designed,which generates the bias current and voltage almost independent of the power supply through the negative feedback of the branch,and then uses the clamping action of the amplifier to make the voltages of the two input terminals substantially equal,and one end is used as the output of the pre-conditioning circuit,thereby improving the ability of circuit to resist power fluctuations and noise.(2)To avoid damage to the driver chip due to excessive temperature,we monitor the operating temperature of the chip in real time by using a current with a temperature coefficient.The positive and negative temperature coefficient currents are compared to generate an over temperature control signal,thereby simplifying the circuit complexity and reducing the heat generation of the overall circuit.(3)For the need of the pulse signal generated by the dimming circuit,a pulse generating circuit with adjustable frequency and duty cycle is designed.And it is integrated on the driver chip to improve the stability of the driver chip and achieve a fast response to the dimming process.The design of this circuit is based on the 0.5?m CSMC process using Spectre simulation tools to verify the simulation of each module and the overall circuit.The power supply voltage of the driving circuit is 8V,the outputs of driving circuit is three-channel driving current,the current value of each channel is 350 mA and the suppression ratio of driving current is-102 dB in the low frequency band;when the power supply voltage fluctuates within the range of 6.5~9.5V,the linear adjustment rate of the drive current is 1.3%.