Realizing high-quality LED display using fast response of driver chip

Nowadays, LED display screens are increasingly used in finance, securities, sports, traffic information, advertising, etc., and because of the decline in LED costs and the increase in brightness in recent years, LED display screens have the advantages of low power consumption, long life, wide viewing angle, and fast response speed. Moreover, they can be customized to the corresponding size according to different locations and needs, and have quickly emerged in the market as a new generation of media darlings, and their conditions are unmatched by other large display devices. This article will further explain how to achieve high-definition LED display screens without changing the circuit design and using the fast response advantage of the driver chip.

Overall speed improvement - higher refresh rate and frame rate

LEDs are driven by the current flowing through them, and the pulse width can control the brightness and grayscale of the LEDs. Simply put, if the system design is not taken into consideration, the refresh rate is determined by the addressing time (Tacc) and the speed of the current flowing through the LEDs. In addition to system support, the increase in frame rate requires faster addressing time, and the addressing time has a strong positive correlation with the transmission clock (DCLK) and the addressing number.

For example, there is a full-color outdoor display whose addressing number is 768. If different clocks are used, the overall addressing time will also be different.

Working clock is 10Mhz -> 768X0.1us = 76.8us

Working clock is 30Mhz-> 768X0.033us = 25.6us

The speed at which the current flows through the LED determines the refresh rate of the LED display. For example, if the addressing number of an LED display is 768, the operating clock is 30Mhz, the grayscale adjustment is 8 bits, the brightness adjustment is 2 bits, and the interval time of each sub-field is 4us; the pulse width of the traditional driver chip is 250ns, while the pulse width of the SnapDrive driver chip is 50ns. There is a significant difference in the refresh rate that the two can achieve.

A. Traditional driver chip (pulse width is 250ns)

The weights are arranged as 1/64, 1/32, 1/16, 1/8, 1/4, 1/2, 1, 2, 4, 8, 16, 32

Tfr=25.6usx[6+63]+5x4us = 1786.4us

Fr = 559.7Hz

B.SnapDrive driver chip (pulse width is 50ns)

The weights are arranged as 1/512, 1/256, 1/128, 1/64, 1/32, 1/16, 1/8, 1/4, 1/2, 1, 2, 4

Tfr=25.6usx[9+7]+8x4us=441.6us

Fr=2264.5Hz

Display grayscale enhancement

The OE response time of the common traditional driver chip in the market is about 250ns. If we take the above example, the highest grayscale is 8 bits; that is, R, G, B each have 256 grayscales. Its colors are 256X256X256 = 166777216, about 16 million colors. If we want to increase the grayscale to 14 bits, that is, 16384X16384X16384=4.39 trillion colors; the refresh rate between the two will also be significantly different

A. Traditional driver chip (pulse width is 250ns)

The weights are arranged as 1/64, 1/32, 1/16, 1/8, 1/4, 1/2, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048

Tfr=25.6usx[6+4095]+5x4us = 105005.6us
Fr = 9.5 Hz

B.SnapDrive driver chip (pulse width is 25ns)

The weights are arranged as 1/1024, 1/512, 1/256, 1/128, 1/64, 1/32, 1/16, 1/8, 1/4, 1/2,1 ,2 ,4 ,8, 16, 32, 64, 128

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Tfr=25.6usx[10+255]+9x4us=6820us

Fr=146.6Hz

Table 1 shows the comprehensive performance of traditional driver chips and SnapDriveTM driver chips

The following are the test conditions and results of the driver chip containing SnapDriveTM technology launched by Taiwan Xunjie Technology. It can be clearly seen from Figures 1 and 3 that the output current of the driver chip is still linear under extremely small OE pulse width, while traditional driver chips cannot provide linear output.

Test conditions:

Vcc=5V, Iout=38.3mA, RL=47Ω, CL=13pF

Distortion reduction

For driver chips with different output current slopes, we use simulation software (HSPICE2007) to obtain different results in terms of distortion rate.

Table 2: Distortion rate comparison table

Simulation conditions: Traditional driver chip: Ton:160ns, Tof:70ns

SnapDriveTM driver chip: Ton:15ns, Tof:15ns

Vin: 5V, Iout=20mA, LED equivalent circuit RL: 52Ω, CL: 10pf

OE pulse width: 250ns

Solve LED heat problems and increase LED life

Figure 5 is a schematic diagram of the current output of 50% duty cycle. If the pulses of the output current are evenly dispersed within the same time period, it will not only not affect the output current and the brightness of the LED, but also avoid the phenomenon of overheating and premature degradation of the LED life caused by long-term lighting of the LED.

Fast response circuit design

Although using a fast-response driver chip can improve the grayscale and refresh rate of the LED display, according to the inductance effect formula & Delta V = L?di/dt, as time t decreases, the instantaneous voltage increases relatively, so surges are more likely to occur. The author lists several circuit design improvements for readers' reference:

ΔV: voltage change

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L: Parasitic inductance in the circuit

di: differential of current

dt: differential with respect to time

There are several points that need special attention in circuit design:

1. The PCB should preferably be 4 layers or more, with the power supply and ground being separate layers; the shorter the traces, the better.

2. Add a large voltage-stabilizing capacitor from VLED and VCC to the ground. It is recommended that CP1 and CP2 be 1000~1500uF.

3. VLED and VCC are separated into different power supplies.

4. An RC circuit can be added to the clock input end to reduce its peak value and reduce the impact on electromagnetic interference; it is recommended that Rt<22Ω and Ct<33pF.

Scanning screen; it is recommended to connect a resistor between the Gate terminal of MOS and 74HC138 to avoid the surge caused by the inductance effect of VLED terminal and the parasitic capacitance of MOS terminal, which may cause the 74HC138 to burn out; it is recommended that Rg<100Ω, Cg<47pF (the capacitor part can be optional).

in conclusion

The fast response (SnapDriveTM) driver chip can not only improve the grayscale display and refresh rate of the entire screen, and reduce the current output distortion rate, but also because the traditional driver chip has a long time to climb and fall the current, its nonlinear output will affect the LED's luminous characteristics (wavelength) when the set current is not reached, which can easily cause the color distortion of the display. However, due to the increase in transmission and operating frequency, designers need to be more careful in circuit design, and the only way is to select high-quality and highly reliable driver chips.

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