1 Band gap reference module
In the bandgap reference module, because the operational amplifier is not completely symmetrical in actual conditions, there are offset voltages and low-frequency noise; at the same time, random errors caused by transistor mismatch have a greater impact on the accuracy of the reference source. Therefore, in view of the temperature stability, noise resistance and accuracy of the bandgap reference module, the structure of the bandgap reference module is designed in this paper. It consists of a start-up and bias circuit, the main circuit of the bandgap reference voltage source, an oscillator, and an RC low-pass filter. And current mirror and other circuits. The start-up circuit helps the circuit to leave the zero point when the module is just powered on; the bias circuit mainly provides proper stable bias for the oscillator and operational amplifier. Here, the power-independent bias technology is used to design the start-up and bias circuit to improve the power supply rejection ratio and voltage regulation rate, and improve the accuracy of the bandgap reference module. The main circuit of the bandgap reference voltage source is composed of an operational amplifier, a chopper modulation circuit and a demodulation circuit. It should be pointed out that this article uses the chopper modulation technology to eliminate the input offset voltage of the operational amplifier and effectively suppress the device noise. The oscillator generates a complementary square wave signal, which is used for the on-off control of the MOS switch in the chopper modulation and demodulation circuit. Here, a ring oscillator composed of an inverter is used, and the square wave is shaped by the inverter to ensure The output quality of the signal reduces the chip area at the same time. The output terminal of the operational amplifier is connected to an RC low-pass filter to further eliminate the influence of noise. The current mirror provides bias current for other circuit modules, and adopts the method of directly biasing the MOS tube current source from the output voltage of the band gap reference voltage source, which improves the temperature stability and reduces the interference degree of the wiring that transmits the bias voltage. The Hspice simulator is used to scan the temperature of the bandgap reference module designed above from -40°C to 80°C. The results show that, when the power supply voltage VDD=5.0V and changes in five different process angles, the maximum deviation of the reference voltage with temperature changes is 2.2mV, and the temperature coefficient reaches 14.7PPM/℃.
2 Constant current reference module
In this design, the constant current reference module adopts the external precision resistance and the negative feedback mode of the operational amplifier to provide a constant current reference for the high-precision current amplifier. Considering that the high-precision current amplifier works in the on-off state, an improved current mirror, clamped current mirror and follower are added to the design. Among them, the operational amplifier adopts a two-stage structure and undergoes Miller compensation to ensure the stability of the system, and at the same time eliminate the influence caused by the zero point by inserting a resistance method; the improved current mirror is used to reduce the mismatch caused by the channel length modulation effect, and Improve the matching accuracy of output impedance and output drive current; clamp the current mirror can increase the speed of the current mirror, support 25MHz data shift frequency and high-speed current response; the follower isolates the interference of the high-precision current amplifier to the constant current reference module. The simulation results show that under VDD=5.0V and various process angles, the reference current generated by the constant current reference module at -40℃~80℃ is inversely proportional to the external resistance REXT, the magnitude is 1.25V/REXT, and the deviation is in the range of 0.1% within.
3 High-precision current amplifier
The high-precision current amplifier is directly connected to the LED, and the switch of the output drive current is controlled by the logic control module. When the logic control module input changes from valid to invalid, the pull-up network and pull-down network are used to turn off the op amp and output to achieve the purpose of quickly turning off the LED. In addition, considering the impact of the capacitance of the high-voltage tube, a discharge circuit is used to eliminate the clutter in the output drive current. The AC characteristics of the op amp circuit are scanned by the Hspice simulator. The results show that the OP's open loop gain is 99dB～103dB, the unity gain bandwidth is 1.7MHz～2MHz, and the phase margin is 62°～70°.
4 logic control module
The logic control module is used for receiving, latching, serial-to-parallel conversion and enabling control of external display data, and combined with pulse width modulation, outputs 16-bit LED logic control signals to realize on-off control and gray-scale control of the LED display. In the logic control module of this article, the SDO pin and OE pin are specially designed so that the external display data can be serially input through the SDO pin to support the data shift clock frequency up to 25MHz, and realize the image display on the color LED display. Fast refresh; use pulse width modulation to control the enable OE pin to achieve dynamic control of the grayscale and brightness of the color LED display; add Schmitt trigger to each input pin for shaping to eliminate the presence of capacitance to ground And the longer transmission line and the influence on the rising and falling edges of the waveform.
