The display effect and service life of LED display will depend greatly on the performance of the drive chip. LED screen factories are using LED display to drive the chip’s import, more attention to the display effect. For example, whether the low ash is consistent, whether the gradient is evenly distributed, whether the oblique scan has a dark bright spot, and the short circuit, etc., these test items will be displayed to the end customers.
Show the effect of the test
A more in-depth understanding of the driver chip can give us a clearer assessment of the performance and longevity of LED displays. Now, more and more screen factory engineers are beginning to pay attention to the working environment and working mode of the chip as they drive the chip import. For example, the consistency of the driving current, the consistency of the REXT voltage, the chip VDD peak, the waveform of the driver port, and so on.
LED display driver port oscillation phenomenon
The OUT drive port that drives the chip is directly connected to the LED light and is the core part of the drive chip. In view of the many harms caused by the oscillation to LED display, this paper presents a brief introduction to the port oscillation problems in some driving chips, and analyzes the reasons and ways of improvement.
The test method
Power supply is 5V40A power supply, with single module. The module is normally lit, using an oscilloscope probe, grounding terminal connecting module ground connection, probe contact driver chip drive port.
The port oscillation phenomenon occurs when a mainstream driving chip in the market (such as P10 static, etc.) is interfering with larger modules.
Hazard of oscillation
1. Poor display effect
The constant current feedback loop inside the chip, in stable working condition, can achieve the ideal constant current effect. The port oscillates and the chip cannot reach the constant current. More to the point, such oscillating chips will continue to oscillate at low ash, which will severely affect the effect of low ash. In the case of the high consistency requirement, the screen factory spent the money on the constant current chip, even worse than the constant pressure chip.
2. The life of LED lamp is greatly reduced
It can be seen from the oscillation waveform that the negative pole of the LED lamp has been working under the class sine wave of 2V and the frequency 20MHz. Large power peak, transient large current, continuous extremely high frequency switch will reduce the life of LED lights.
3. The power supply fluctuates greatly and reduces the power life
If the 5V40A power supply is full load, that is 40A current load, then for the power supply, the current change will be the ampere-level current oscillating continuously around 20MHz. Dc power output terminal capacitance, in the high frequency of high current fluctuation, equivalent internal resistance heating, reduced life, resulting in larger die peak. The damage is not just power, but all components and drive chips on the module.
4 EMI paint
The constant frequency 20MHz of the class sine wave, the current change of each driver port is around 0mA to 40mA, and the power supply is the ampere-level current oscillating continuously around 20MHz. This will lead to the whole box, which will be superimposed on the dBuV value at the frequency point of 20MHz, which will eventually lead EMI to exceed the standard at this frequency point.
5 module capacitor noise
The capacitance noise is often caused by the heat loss of the capacitor. Similarly to the power capacitor, the port oscillations are more likely to cause the capacitance to damage and make a noise.
Causes of oscillation and preventive measures
Reason of oscillation:
System power on the VIN, the OUT port after the LED lamp voltage drop the voltage drop, the OUT port to output current, the current on the parasitic inductance L0 ~ LN changes, so the system go line voltage ripple.
The output current of the OUT port is turned on, the port parasitic inductance L produces a voltage surge, and there is a voltage ripple in the power supply, so the chip OUT port appears to have a large voltage ripple.
All the LED lights on the LED display are illuminated simultaneously, that is, all the driving ports on the screen open the output current, and the voltage ripple source of the OUT port is at this moment:
1) chip OUT port parasitic inductance generates OUT port voltage ripple;
2) the parasitic inductance on the power supply or ground line causes the voltage ripple on the system power to be transmitted to the chip OUT port in series.
In addition, the increase of the driver port response time will cause poor grey display effects.
Measures to prevent
From the above analysis, the measures to avoid the port oscillation are:
1. Stability of the drive chip: replace the stable and better drive chip and avoid the oscillation from the source.
2. Optimize the line of module, reduce the parasitic inductance: use the ground wire as far as possible, and the power line is copper, but limited by the space limitation on the plate, there is not much room for improvement.
3, the capacitance of module is increased: it will bring cost increase.
Due to the poor internal stability of some driving chips on the market, the problem of driving port oscillation is affected by the parasitic inductance on the plate. This can result in poor display effect, shorten LED lamp and power life, higher EMI, capacitor noise, etc. Therefore, it is possible to avoid the problem of port oscillation by adopting a more stable driving chip.