OLED (Organic Lighting-Emitting Diode) is made by organic materials that emit light when current is applied. Each pixel can emit light, so OLED does not have a problem of light from backlight leaking through the display. The basic structure of OLED pixel is shown in Figure 1-1. Scan line is used for pixel selection to turn on T1 to allow data to be written to T2. Vg from Data line can control the gray scale. Data storage capacitor keeps Vg constant during frame time. In general, driving OLED requires bipolar voltage rails like ELVDD(Positive), ELVSS(Negative). Once T2 is turned on, current can be driven from ELVDD(Positive) to ELVSS(Negative). The luminous brightness is adjusted by changing the voltage across OLED.

Figure 1-1 Basic Structure of OLED Pixel

Simply the ground can be used for ELVSS rather than negative output voltage. But recently, the panel makers use negative output voltage for ELVSS to minimize flickering issue. Most applications using OLED panel such as monitors support VRR (Variable Refresh Rate) feature to synchronize the display refresh rate with the video input frame rate. VRR can eliminate the stuttering or tearing of the image and enable smooth display of the source. However, as the refresh rate is changed within VRR range such as 30Hz – 140Hz, the charging speed of data line can be impacted. This changes the charging level of the data storage capacitor, which can cause the current of OLED to be changed accordingly. VRR causes the luminance to vary depending on the frequency (refresh rate) even though the same luminance is the target. This phenomenon has been considered as the flickering issue by users. To prevent and minimize this flickering issue, ELVDD voltage level or internal compensation circuit of OLED can be tuned. But it is sometimes limited and complex. So this becomes popular to simply use negative output voltage for ELVSS to minimize the flickering issue. The voltage level is decided by the characteristics of OLED and internal compensation circuit. Therefore, the requirement of panel makers is very important.

The power supply for ELVDD and ELVSS need to have sufficient current capability to drive OLED pixels. Higher current capability is required if OLED panel size is bigger which can contain more pixels. This means negative output power also need to cover high current capability as much positive output power can do. Therefore inverting buck-boost for negative voltage become popular than charge pump design which has limited current capability. Also IBB design can help designer achieve lower BOM cost and small PCB size design.

For the standard buck converter, the inductor is connected to V_OUT and the switch pin (SW). To change a standard buck converter to an inverting buck-boost, reassign the buck converter V_OUT to system ground, and the old buck system ground to - V_OUT. The input capacitor needs to be reconnected to the new system ground, and a new bypass capacitor, C_IO, is needed between V_IN and -V_IN.

The positive input and the feedback resistors remains the same as in the buck converter. To adjust the output of the inverting buck-boost, calculate the feedback resistor values as if the feedback resistor was a buck converter. The schematics in Figure 2-1 show the changes that have to be made when configuring the standard buck converter as an inverting buck-boost converter. This inverting topology allows the output voltage to be inverted and always lower than the ground.

Figure 2-1 Converting From Buck to Inverting Buck-Boost Topology

The circuit operation is different in the inverting buck-boost topology than in the buck topology. Figure 2-2 shows that the output voltage terminals are reversed, though the components are wired the same as a buck converter. As Figure 2-3 shows, during the ON-time of the control MOSFET, the inductor is charged with current while the output capacitor supplies the load current. The inductor does not provide current to the load during this time.

During the OFF-time of the control MOSFET and the ON-time of the synchronous MOSFET shown in Figure 2-4, the inductor provides current to the load and the output capacitor. These changes affect many parameters, which the following subsections describe in further detail.

Figure 2-2 Inverting Buck-Boost Configuration

Figure 2-3 ON-Time of IBB Configuration

Figure 2-4 OFF-Time of IBB Configuration

This application note uses TI’s reference design PMP23333. PMP23333 design used LM61495 synchronous buck regulator, with internal top and bottom FETs, which is configured as a synchronous inverting buck-boost converter. The LM61495 is regulator that provide either fixed or adjustable output voltage that can be set from 1V to 95% of expected input voltage. LM61495 can operate under a wide input voltage range from 3V to 36V and have transient tolerance up to 42V which can give proper design flexibility to designers.

Figure 3-1 PMP23333 (Top) Board Image

Figure 3-2 PMP23333 (Bottom) Board Image

This design generates an output of –8V, capable of delivering 2.7A continuous (4A peak) of current to the load, from a +12V, ±10% input. The design covers up to 32W power rating. So PMP23333 can be a great start point of design to cover up to 49 inch OLED panel for monitor, small TV applications. As described in Section 1, -V_OUT can be decided by OLED characteristics. -V­_OUT value can be set by configuration of feedback resistors as shown in Figure 3-3, PMP23333 schematic. If designer requires higher power rating for bigger OLED panel, controller design needs to be considered due to current, thermal limitations, and so on.