LED display and life expectancy, largely depends on the performance of the driver chip merits. LED screen factory in the LED display driver chip into the introduction, the more attention to the display, such as low gray whether the show is consistent, whether the transition is evenly gradient, oblique whether there are dark spots, open short circuit, etc., these test items will Show to end customers.
The results of the test, is the most important step, but also the first step.
The driver chip has a more in-depth understanding, can make our LED display performance and life of a more clear assessment. Now, more and more screen factory engineers, in the drive chip import, began to focus on the chip itself, the working environment and work. For example, the consistency of the drive current, the consistency of the REXT voltage, the chip VDD spike, the waveform of the drive port, and so on.
LED display driver port oscillation phenomenon
Drive the chip OUT drive port, directly connected with the LED lights, is the core part of the drive chip. In view of the oscillation to the LED display brings a lot of harm, this paper will focus on some of the driver chip port oscillation problem to do a brief introduction, and analyze the causes and improve the way.
Test Methods
Power supply for 5V40A power supply with single module. Module normal light, with the oscilloscope probe, the ground connection module ground connection port, the probe contact with the driver chip driver port.
Test Results
A mainstream market driver chip in the interference of the larger modules (such as P10 static, etc.), there has been a port oscillation phenomenon.
The danger of oscillation
Port oscillation, brought a lot of harm:
The display is poor
Chip constant current feedback loop, in a stable working condition, in order to achieve the desired constant current effect. Port oscillation, the chip can not achieve constant current. It should be noted that such easy to oscillate the chip, in the low ash, there will still be oscillation occurs, seriously affecting the low ash effect. On the consistency of the requirements of the full color screen, the screen factory spent a constant current chip money, even with a constant pressure chip even worse results.
LED lamp life greatly reduced
From the oscillation waveform shows that the negative LED lamp has been in the voltage difference of 2V, frequency of about 20MHz sine wave to work. Larger power spikes, transient high current, continuous high frequency of the switch will reduce the LED lamp life.
Power supply fluctuations, reduce power life
If the 5V40A power supply is full load operation, that is, 40A current load, then the power supply is concerned, the current change will be amps current at about 20MHz continuous oscillation. DC power output capacitor, in the high frequency of high current fluctuations, the equivalent resistance to heat, reduce life expectancy, resulting in a larger module spikes. Damage is not just the power supply, but the module on all the components and driver chip.
EMI exceeded
Continuous frequency of 20MHz sine wave, each drive port current changes in the 0mA to 40mA or so, the power is the ampere-class current in about 20MHz continuous oscillation. This will lead to the entire box, at 20MHz frequency point on the dBuV value superposition, and ultimately lead to EMI at the frequency of easy to exceed the standard.
Module capacitance is abnormal
Capacitive sound is often caused by capacitive heat damage. Similarly, in the power capacitor, the port oscillation will be more likely to cause damage to the capacitor and issued abnormal sound.
Causes of Oscillation and Preventive Measures
Cause of oscillation
Take the constant current source driver chip application as an example to explain the OUT port oscillation phenomenon.
The inductance L0, L1 ... LN is the parasitic inductance on the system power supply trace, and the capacitance C0 is the trace parasitic capacitance.
When the system power supply VIN is powered on, the OUT port voltage drops more than the LED lamp voltage drop, the OUT port starts to output current, the parasitic inductance L0 ~ LN on the current changes, so the system power line voltage ripple.
In the figure above, the output current of the OUT port is turned on, and the voltage surge is generated on the parasitic inductance L of the port. At the same time, there is voltage ripple in the power supply VIN, so the voltage ripple appears on the OUT port of the chip.
LED display on all the LED lights are lit at the same time, that is, all the driver on the screen to open the output current, in this instant OUT port voltage ripple Source: 1) chip OUT port parasitic inductance OUT port voltage ripple; 2) The parasitic inductance on each power supply or ground causes the voltage ripple on the system power supply VIN to be transmitted in series to the chip OUT port.
In addition, the increase in the driver port response time approach, will result in low ash display poor problem.
Precautions
From the above analysis, we can see that the measures to avoid port oscillation are:
1, drive chip stability: replace the stability of the better driver chip, from the source to avoid the emergence of oscillation.
2, the module alignment optimization, reduce the parasitic inductance: as far as possible the use of ground, power line shop copper way, but limited by the board space constraints, to improve the space is not.
3, the module capacitor increases: will bring the cost increase.
Conclusion
As some of the market driver chip internal stability is poor, in the board parasitic inductance under the influence of the driver port oscillation problem. This will lead to poor display, shorten the LED lights and power life, higher EMI, capacitance and other issues.
The use of high stability of the driver chip to avoid port oscillation problems.