At present, large-scale color LED display has become the mainstream product of high-definition large-screen flat panel display devices. The utility model relates to a large-sized flat panel display device which is formed by splicing a display unit composed of a light-emitting diode and a display driving integrated circuit chip thereof. The integrated circuit driving chip in the display unit is mainly used for receiving the digital signal of the back-end control system, and driving the front end. The screen light-emitting diode is turned on to realize information display. Therefore, the performance of the driver chip plays a key role in the display quality of the LED display . In recent years, with the rapid development of LED display technology, special-purpose chips have become the mainstream driver chips for large-scale color LED displays, but there are still some key problems that need to be solved. The most important one is multi-bit constant current drive display. technology. Accurate multi-bit constant current drive determines the uniformity, consistency and commercial value of large color LED display displays.
1, chip system design
The system structure of the chip is shown in Figure 1. The circuit system mainly includes modules such as bandgap reference, constant current reference, high precision current amplifier and logic control. The bandgap reference module generates a high-precision low offset reference voltage, and the constant current reference module generates a constant reference current by using the reference voltage and the external resistor. The high-precision current amplifier of each channel completes the amplification of the reference current, and the logic control module completes the serialization. Conversion and enable control for each channel.
Figure 1 Chip internal structure block diagram
2, circuit design and simulation
2.1 Bandgap reference module
In the bandgap reference module, the offset voltage and low frequency noise are present due to the fact that the operational amplifier is not completely symmetrical. At the same time, the random error caused by the transistor mismatch has a great influence on the accuracy of the reference source. Therefore, for the temperature stability, noise immunity and accuracy of the bandgap reference module, this paper designs the bandgap reference module structure as shown in Figure 2, starting and biasing circuit, bandgap reference source body circuit, oscillator , RC low-pass filter and current mirror and other circuits. The startup circuit helps the circuit leave zero when the module is powered up; the bias circuit provides the proper stable bias for the oscillator and op amp. Here, the power-independent biasing technique is used to design the startup and bias circuits to improve the power supply rejection ratio and voltage regulation rate, and to improve the accuracy of the bandgap reference module. The bandgap reference voltage source main circuit is composed of operational amplifier, chopping modulation circuit and demodulation circuit. It should be pointed out that this paper eliminates the input offset voltage of the op amp by adopting chopping modulation technology and effectively suppresses device noise. The oscillator generates a complementary square wave signal for on-off control of the MOS switch tube in the chopping modulation and demodulation circuit. Here, a ring oscillator composed of an inverter is used, and the square wave of the inverter is used for shaping, thereby ensuring The output quality of the signal while reducing the chip area. The op amp output is connected to an RC low-pass filter to further eliminate the effects of noise. The current mirror provides bias current for other circuit modules. The method of directly biasing the MOS current source by the output voltage of the bandgap reference voltage source improves the temperature stability and reduces the interference level of the transmission bias voltage.
Figure 2 Circuit structure diagram of the bandgap reference module
The bandgap reference module of the above design was scanned from -40 °C to 80 °C using an Hspice simulator. The results show that when the power supply voltage is VDD=5.0 V, the maximum deviation of the reference voltage with temperature is 2.2 mV and the temperature coefficient reaches 14.7 PPM/°C when the five different process angles are changed.
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