BACKGROUND

1. Field

Embodiments relate generally to a pixel circuit. More particularly, embodiments of the invention relate to a pixel circuit including an organic light-emitting element (e.g., an organic light-emitting diode), a storage capacitor, a switching transistor, a driving transistor, an emission control transistor, a compensation transistor, an initialization transistor, etc.

2. Description of the Related Art

Generally, a pixel circuit included in an organic light-emitting display device may include an organic light-emitting element, a storage capacitor, a switching transistor, a driving transistor, an emission control transistor, a compensation transistor, an initialization transistor, etc. Here, when the transistors are low temperature poly silicon (“LTPS”) transistors, a flicker may occur when the organic light-emitting display device is driven at a driving frequency less than a predetermined driving frequency (e.g., less than 30 hertz (Hz)). In other words, because a leakage current flows through the transistors even when the transistors are turned off, a data signal stored in the storage capacitor (i.e., a voltage of a gate terminal of the driving transistor) may be changed by the leakage current when the organic light-emitting display device operates in a low-frequency driving mode, and thus a viewer (or user) may recognize a luminance-change. In particular, when the pixel circuit has a structure (e.g., a structure in which the gate terminal of the driving transistor, one terminal of the storage capacitor, one terminal of the initialization transistor, and one terminal of the compensation transistor are connected at a predetermined node) which sequentially performs an initializing operation, a threshold voltage compensating and data writing operation, and a light-emitting operation, the data signal stored in the storage capacitor (i.e., the voltage of the gate terminal of the driving transistor) may be changed because the leakage current flows through the compensation transistor and the initialization transistor even when the compensation transistor and the initialization transistor are turned off. Thus, a conventional pixel circuit reduces the leakage current flowing through the compensation transistor and the initialization transistor by including the compensation transistor having a dual structure and/or the initialization transistor having a dual structure.

SUMMARY

A conventional pixel circuit has a limit that an effect of reducing the leakage current is slight when an organic light-emitting display device operates in the low-frequency driving mode.

Some embodiments provide a pixel circuit preventing a flicker that a viewer recognizes by minimizing (or reducing) a change in a voltage of a gate terminal of a driving transistor, which is caused by a leakage current flowing through a compensation transistor and an initialization transistor when an organic light-emitting display device operates in a low-frequency driving mode

An embodiment of a pixel circuit may include a main circuit including a driving transistor that includes a gate terminal that is connected to a first node, a first terminal that is connected to a second node, and a second terminal that is connected to a third node and an organic light-emitting element that is connected to the driving transistor between a first power voltage and a second power voltage and controls the organic light-emitting element to emit light by controlling a driving current corresponding to a data signal that is applied via a data line to flow into the organic light-emitting element, and a sub circuit including a first compensation transistor that includes a gate terminal that receives a first gate signal, a first terminal that is connected to the first node, and a second terminal that is connected to a fourth node, a second compensation transistor that includes a gate terminal that receives a second gate signal, a first terminal that is connected to the fourth node, and a second terminal that is connected to the third node, and an initialization transistor that includes a gate terminal that receives an initialization signal, a first terminal that is connected to the first node, and a second terminal that receives an initialization voltage. Here, in a low-frequency driving mode, a driving frequency of the first gate signal may be N hertz (Hz), which is a driving frequency of an organic light-emitting display device, where N is a positive integer, a driving frequency of the initialization signal may be N Hz, a driving frequency of the second gate signal may be M Hz, where M is a positive integer and different from N, the first compensation transistor and the initialization transistor may be turned on during a first time duration in N non-light-emitting periods per second, and the second compensation transistor may be turned on during a second time duration in M non-light-emitting periods per second.

In an embodiment, in the low-frequency driving mode, the driving frequency of the first gate signal and the driving frequency of the initialization signal may be lower than the driving frequency of the second gate signal.

In an embodiment, the first gate signal and the second gate signal may be generated, respectively by respective signal generating circuits that are independent of each other.

In an embodiment, the first time duration may be equal to the second time duration.

In an embodiment, a turn-on voltage level period of the second gate signal may be consistent with a turn-on voltage level period of the first gate signal.

In an embodiment, in a normal non-light-emitting period in which an initializing operation and a threshold voltage compensating and data writing operation are performed, the first compensation transistor and the second compensation transistor may be simultaneously turned on and then off after the initialization transistor is turned on and then off.

In an embodiment, in a hold non-light-emitting period in which the initializing operation and the threshold voltage compensating and data writing operation are not performed, only the second compensation transistor may be turned on and then off.

In an embodiment, the initialization voltage may be changed from a first voltage level to a second voltage level that is higher than the first voltage level at a start point of the hold non-light-emitting period, and the initialization voltage may be reset to the first voltage level at a start point of the normal non-light-emitting period.

In an embodiment, the initialization voltage may be additionally changed to at least one voltage level that is higher than the second voltage level after the initialization voltage is changed to the second voltage level at the start point of the hold non-light-emitting period.

In an embodiment, the first time duration may be longer than the second time duration.

In an embodiment, a turn-on voltage level period of the second gate signal may overlap a turn-on voltage level period of the first gate signal.

In an embodiment, a start point of the turn-on voltage level period of the second gate signal may be consistent with a start point of the turn-on voltage level period of the first gate signal, and an end point of the turn-on voltage level period of the second gate signal may be before an end point of the turn-on voltage level period of the first gate signal.

In an embodiment, a start point of the turn-on voltage level period of the second gate signal may be after a start point of the turn-on voltage level period of the first gate signal, and an end point of the turn-on voltage level period of the second gate signal may be consistent with an end point of the turn-on voltage level period of the first gate signal.

In an embodiment, a start point of the turn-on voltage level period of the second gate signal may be after a start point of the turn-on voltage level period of the first gate signal, and an end point of the turn-on voltage level period of the second gate signal may be before an end point of the turn-on voltage level period of the first gate signal.

In an embodiment, in a normal non-light-emitting period in which an initializing operation and a threshold voltage compensating and data writing operation are performed, the second compensation transistor may be turned on and then off while the first compensation transistor is turned on after the initialization transistor is turned on and then off.

In an embodiment, in a hold non-light-emitting period in which the initializing operation and the threshold voltage compensating and data writing operation are not performed, only the second compensation transistor may be turned on and then off.

In an embodiment, the initialization voltage may be changed from a first voltage level to a second voltage level that is higher than the first voltage level at a start point of the hold non-light-emitting period, and the initialization voltage may be reset to the first voltage level at a start point of the normal non-light-emitting period.

In an embodiment, the initialization voltage may be additionally changed to at least one voltage level that is higher than the second voltage level after the initialization voltage is changed to the second voltage level at the start point of the hold non-light-emitting period.

In an embodiment, the sub circuit may further include a bypass transistor including a gate terminal that receives a bypass signal, a first terminal that receives the initialization voltage, and a second terminal that is connected to an anode of the organic light-emitting element. In addition, in the low-frequency driving mode, a driving frequency of the bypass signal may be N Hz, and the bypass transistor may be turned on during the first time duration in N non-light-emitting periods per second.

In an embodiment, the bypass signal may be a same signal as the initialization signal.

Therefore, a pixel circuit in embodiments may minimize (or reduce) a leakage current flowing through a first compensation transistor and an initialization transistor when an organic light-emitting display device operates in a low-frequency driving mode by having a structure that includes a first compensation transistor and a second compensation transistor that are connected in series between a gate terminal of a driving transistor and one terminal of the driving transistor, where one terminal of the first compensation transistor is connected to the gate terminal of the driving transistor, and one terminal of the second compensation transistor is connected to the one terminal of the driving transistor, by turning on the first compensation transistor and the initialization transistor during a first time duration in N non-light-emitting periods per second, where N is a positive integer, when the organic light-emitting display device operates in the low-frequency driving mode (i.e., a driving frequency of a first gate signal that controls the first compensation transistor and a driving frequency of an initialization signal that controls the initialization transistor may be N Hz, which is a driving frequency of the organic light-emitting display device), and by turning on the second compensation transistor during a second time duration in M non-light-emitting periods per second, where M is an integer greater than N, when the organic light-emitting display device operates in the low-frequency driving mode (i.e., a driving frequency of a second gate signal that controls the second compensation transistor may be M Hz, which is higher than the driving frequency of the organic light-emitting display device). Thus, the pixel circuit may prevent (or reduce) a flicker that a viewer recognizes (i.e., may prevent a change in a voltage of the gate terminal of the driving transistor).

