The display quality of LED displays has always been closely related to the constant current driver chip, addressing issues such as ghosting, dead pixel crosshairs, low grayscale color shift, dark first scan, and high contrast coupling. Horizontal drive, as a simple scanning requirement, has traditionally received less attention. With the development of smaller pitch LED displays, higher demands are being placed on horizontal drives, evolving from simple P-MOSFETs for horizontal switching to more integrated and powerful multi-functional horizontal drivers. The design and selection of horizontal drivers also face six major challenges: ghosting elimination, reverse voltage of LED chips, short-circuit issues, open-circuit crosshairs, excessively high VF values of LED chips, and high contrast coupling.
Ghost Shadow
When switching between scanning screens, due to the time required for the PMOS transistor switches to turn on and off, and for the charge to dissipate on the parasitic capacitance Cr of the row lines, the undischarged charge of the VLED from the previous row scan has a conductive path at the instant the VLED and OUT of the next row scan are turned on. When Row(n) is turned on, the parasitic capacitance Cr of the row is charged to the VCC potential. When switching to Row(n+1), a potential difference is formed between Cr and OUT, and the charge is discharged through the LED, producing a dim LED light.
Therefore, the charge on the Cr capacitor needs to be discharged in advance at the line break time. Usually, the horizontal output transistor with integrated blanking function uses a pull-down circuit to quickly discharge the charge on the parasitic capacitance Cr during switching. The lower the pull-down potential, i.e., the blanking voltage VH, is set, the faster the charge on the parasitic capacitance is discharged, and the better the effect of eliminating upper ghosting is. Usually, VH < VCC - 1V is sufficient to eliminate upper ghosting.
LED reverse voltage
The reverse surge voltage of LED chips significantly impacts their lifespan, and pixel defects caused by reverse voltage have always been a major concern for LED displays, especially those with small-pitch displays.
When the output channel is off, the parasitic inductance's freewheeling current continuously charges the parasitic capacitance at the channel, creating a high voltage spike. This spike, combined with the horizontal output transistor (HIP), forms a reverse voltage across the LED chip. Therefore, the HIP's blanking voltage also affects the LED chip's reverse voltage. With a fixed voltage at the constant current output channel, a higher HIP blanking voltage results in a lower reverse voltage for the LED chip. While LED chips typically have a nominal reverse voltage of 5V, manufacturer testing has shown that a reverse voltage below 1.4V can significantly reduce pixel defects caused by reverse voltage. Therefore, the blanking voltage should not be too low to address LED chip reverse voltage issues, generally not lower than VCC-2V.
Short-circuit caterpillar
When an LED is short-circuited, a row of constantly lit LEDs will appear, commonly known as a short-circuit caterpillar. When the middle LED is short-circuited, the LEDs in the same row will form a path as shown in the diagram below when scanning that row. If the voltage difference between VLED and point A is greater than the LED's illumination value, a row of constantly lit caterpillars will be formed.
The biggest difference between a short-circuit caterpillar and an open-circuit cross is that a short-circuit caterpillar will appear as long as the screen is in scanning mode, regardless of whether the LED beads are displaying an image, while an open-circuit caterpillar only shows the open-circuit cross problem when the open-circuit LED bead is lit. This is usually resolved by increasing the horizontal output transistor's blanking voltage so that the voltage difference is less than the LED's forward voltage VF, i.e., VLED - VH < VF. Typically, the forward voltage VF for red LED beads is 1.6~2.4V, and for green and blue LED beads it is 2.4~3.4V. Testing showed that a red LED bead can be lit with 1.4V; therefore, taking a red LED bead as an example, when VH > VCC - 1.4V, the short-circuit caterpillar problem is completely solved. When VCC - 2V < VH < VCC - 1.4V, only one red LED below the short-circuit point is weakly lit.
Opening Cross
When an open-circuit LED appears on the scanning screen and that point is illuminated, the voltage of channel OUT1 is pulled down to below 0.5V. If the blanking voltage VH of the scanning row potential is 3.5V, a conductive path will be formed for that row of LEDs, creating an open-circuit "caterpillar" effect.
When an LED is open-circuited, the voltage of channel OUT1 is pulled down to below 0.5V or even 0V. This affects the column parasitic capacitance Cr through parasitic capacitances C1 and C2. When the potential of Cr is pulled low, the LEDs in the same row as the open-circuited LED will dim.
Lowering the blanking voltage of the horizontal output transistor (output transistor) can effectively solve the open-circuit cross problem, i.e., the blanking voltage VH < 1.4V. Some output transistors in the industry also use adjustable blanking voltages to lower the blanking voltage below 1.4V to solve the open-circuit cross problem, but this will increase the reverse voltage of the LED, accelerate LED damage, and cause short circuits.
The VF value of the LED is too high.
The issue of columns remaining constantly lit due to excessively high VF values in LEDs is another problem that plagues users. Typically, the nominal forward voltage VF of a green LED is 2.4~3.4V. Normally, a voltage difference of 1.8V between the anode and cathode of the green LED is sufficient to light it. However, an excessively high blanking voltage VH of the horizontal output transistor will cause the column to remain constantly lit.
Taking an LED with a forward voltage VF1 = 3.4V as a column, when scanning reaches the next LED, VOUT and VLED1 turn on simultaneously. The channel terminal voltage is: VOUT = VLED1 - VF1. The voltages across the other LEDs in that column are: VΔ = VH - VOUT = VH - VLED1 + VF1. If VΔ > 1.8V, it may cause the column to remain constantly lit, i.e., VH - VLED1 + VF1 > 1.8V, where VLED = VCC (ignoring the horizontal output transistor voltage drop). Therefore, VH > VCC - 1.6V is not conducive to solving the problem of columns remaining constantly lit due to excessively high VF values in LEDs.
High contrast coupling
High contrast coupling refers to the phenomenon where a bright image is superimposed on a low-brightness background, causing color shift and darkening in the area where the low-brightness and bright-brightness images are parallel, as shown by the dotted line in the image above, which represents the superimposed bright image. This high contrast coupling is caused by interference between column channels through the horizontal output transistors. It can be mitigated to some extent by designing a clamping voltage, maintaining it at a certain level after discharge, thereby lowering the horizontal output transistor's blanking voltage. However, this design method introduces problems such as short-circuit column darkening, low-gray areas appearing reddish, and excessively high VF values for the LEDs. Improving high contrast coupling from the horizontal drive perspective can be achieved by lowering the blanking voltage, but this results in excessively high reverse voltage for the LEDs and the "caterpillar" short-circuit problem.
Selection of horizontal output blanking voltage
In summary, selecting the blanking voltage for the horizontal output transistor (HIP) faces challenges related to the six issues mentioned above, each with its own specific difficulties. The blanking voltage cannot be too high or too low. Typically, the open-circuit crosshair is cleared by constant current drive detection, as an excessively low blanking voltage reduces the long-term reliability of the LED. The table below summarizes the suitable range of blanking voltage under various conditions.
Therefore, considering various application issues, a blanking voltage of 3V~3.4V (VCC=5V) is a reasonable choice. This can meet the design requirements of various scanning modules and thus reasonably solve multiple application problems.
