Six Major Challenges in LED Display Row Driving

Six Major Challenges in LED Display Row Driving

The display quality of LED displays has always been closely tied to constant-current driver ICs—issues such as ghosting, dead-pixel crosses, low-gray color shift, first-scan dimness, and high-contrast coupling—while row drivers, as devices that merely fulfill scanning requirements, have long received relatively little attention. With the development of fine-pitch displays, LED displays are also placing higher demands on row drivers, evolving from simple P-MOSFET row switching to higher-integration, more powerful multifunction row drivers. The design and selection of row drivers also face six major challenges:

upper ghosting elimination, LED reverse voltage, short-circuit caterpillar, open-circuit cross, excessively high LED VF, and high-contrast coupling. Upper ghosting: when a scanning screen switches, because the PMOS switch turns on and off and the charge on the row parasitic capacitance Cr takes time to discharge, at the instant the next row scan VLED and OUT turn on, the unreleased charge from the previous row scan VLED has a conduction path. When Row(n) turns on, the row parasitic capacitance Cr charges to the VCC potential. When switching to Row(n+1), a potential difference forms between Cr and OUT, and the charge discharges through the LED beads, producing faint LED illumination.

Ghosting phenomenon, illustration of ghosting

Therefore, at the moment of row switching, the charge on Cr needs to be discharged in advance. Usually, row driver ICs with integrated blanking functionality quickly discharge the charge on the parasitic capacitance Cr through an added pull-down circuit during switching. The lower the pull-down potential—that is, the blanking voltage VH—is set, the faster the charge on the parasitic capacitance is discharged, and the better the upper-ghosting elimination effect. Usually, VH < VCC − 1 V is sufficient to eliminate upper ghosting.

LED bead reverse voltage (reverse voltage).

The reverse impulse voltage on LED beads greatly affects their service life, and dead pixels caused by reverse voltage have long been a pain point for LED displays, especially fine-pitch ones. When the output channel is turned off, due to the freewheeling effect of parasitic inductance, the parasitic capacitance at the channel is continuously charged, forming a high-voltage spike. At this time, together with the row driver output, it forms a reverse voltage applied across the LED beads. Therefore, the blanking voltage of the row driver also affects the reverse voltage on the LED beads. When the voltage at the constant-current output channel is fixed, the higher the blanking voltage of the row driver, the lower the reverse voltage on the LED beads. Typically, the nominal reverse voltage of LED beads is 5 V; actual testing by manufacturers shows that a reverse voltage below 1.4 V can greatly reduce dead pixels caused by reverse voltage. Therefore, to address the LED reverse-voltage issue, the blanking voltage must not be too low and generally should not be lower than VCC − 2 V.

Short-circuit caterpillar.

When an LED is short-circuited, an entire column stays lit, commonly referred to as a short-circuit caterpillar. When an LED bead in the middle is short-circuited, the LED beads in the same column form a path as shown in the figure below when that row is scanned. If the voltage difference between VLED and point A is greater than the turn-on voltage of the LED beads, a column of constantly lit caterpillar will form.

Schematic diagram showing the causes of short circuits involving caterpillars

The biggest difference between the short-circuit caterpillar and the open-circuit cross is that the short-circuit caterpillar will appear as long as the screen is in the scanning state, regardless of whether the LED beads are displaying an image, whereas the open-circuit caterpillar will present the open-circuit cross problem only when the lit LED bead is turned on. Usually, the row driver blanking voltage is raised so that the voltage difference is less than the LED forward voltage VF, that is, VLED − VH < VF. Typically, the forward voltage (VF) of red LED beads is 1.6–2.4 V, and the forward voltage VF of green and blue LED beads is 2.4–3.4 V. Testing has shown that red LED beads can be lit at 1.4 V, so taking red LED beads as an example, therefore when VH > VCC − 1.4 V, the short-circuit caterpillar problem can be completely solved. When VCC − 2 V < VH < VCC − 1.4 V, only one red LED below the short-circuit point is weakly lit.

Open-circuit cross: 

In a scanning display, when an LED bead is open-circuited and that point is lit, the voltage of channel OUT1 is pulled down below 0.5 V. If the blanking voltage VH of the scanning row potential is 3.5 V, a conduction path will be formed for the LED beads in that column, creating an open-circuit caterpillar.

Schematic diagram showing the causes of an open-circuit cross-connection

When an LED bead is open-circuited, the voltage of channel OUT1 is pulled down below 0.5 V or even to 0 V, and through parasitic capacitors C1 and C2, it affects the column parasitic capacitance Cr. When the potential of Cr is pulled low, the LEDs in the same row as the open-circuited LED bead become faintly lit. Lowering the blanking voltage of the blanking row driver can effectively solve the open-circuit cross problem—that is, blanking voltage VH < 1.4 V. Some row drivers in the industry also use an adjustable blanking voltage, lowering it below 1.4 V to solve the open-circuit cross; however, this increases the LED reverse voltage and accelerates LED bead damage as well as the short-circuit caterpillar issue.

LED bead VF value too high.

Column constant-on caused by an excessively high LED bead VF value is also a problem that troubles customers in application. Typically, the nominal forward voltage VF of green LEDs is 2.4–3.4 V, while under normal circumstances a voltage difference of 1.8 V between the anode and cathode of a green LED is enough to light it. If the blanking voltage VH of the row driver is too high, it will cause the column to stay constantly lit.

Taking an LED bead with forward voltage VF1 = 3.4 V as an example, when scanning reaches this LED bead, VOUT and VLED1 are turned on at the same time. The channel terminal voltage is: VOUT = VLED1 − VF1. The voltage across the other LED beads in the same column on other rows is: V△ = VH − VOUT = VH − VLED1 + VF1. If V△ > 1.8 V, the column may stay constantly lit—that is, VH − VLED1 + VF1 > 1.8 V. Here VLED = VCC (with the row driver voltage drop ignored), so VH > VCC − 1.6 V is not conducive to solving the column constant-on problem caused by an excessively high LED bead VF value.

High-contrast coupling.

High-contrast coupling refers to the phenomenon in which, against a low-brightness background, a high-brightness image is superimposed, and the region where the low-brightness image and the high-brightness image share the same row shows color shift and dimming, as shown in Figure 7, where the dashed line indicates the superimposed high-brightness image. The high-contrast coupling phenomenon is caused by mutual interference between column channels through the row driver. Lowering the row driver blanking voltage can improve and alleviate high-contrast coupling to a certain extent. By designing the row driver blanking voltage as a clamped voltage—that is, maintaining it at a certain level after discharge ends—high-contrast coupling can be alleviated and improved to a certain extent. However, this design method will bring about problems such as dim short-circuited columns, reddish low-gray display, and excessively high LED bead VF values. From the perspective of the row driver, high-contrast coupling can be improved by lowering the blanking voltage, but this will lead to excessively high LED reverse voltage and the short-circuit caterpillar problem.

Selection of row driver blanking voltage. In summary, the selection of the row driver blanking voltage faces the challenges of the six major issues above, and the trade-offs are mutually constrained; the blanking voltage cannot be too high or too low. Usually, the open-circuit cross is eliminated by detection in the constant-current driver, because an excessively low blanking voltage will reduce the long-term reliability of LED beads. ICN205X series driver ICs all have the function of eliminating the open-circuit cross. The table below summarizes the appropriate range of blanking voltage under various conditions.

Therefore, when comprehensively addressing various application issues, a blanking voltage of 3V to 3.4V (with VCC=5V) is a relatively reasonable choice. Chipone provides users with a variety of row drivers with integrated blanking, which can meet the design requirements of various scanning modules and reasonably solve a variety of application problems.