Powering Passive OLED Displays in Portable Devices

Organic light-emitting diode displays (OLEDs) are an emerging technology that is set to revolutionize the display industry. OLEDs use organic materials that emit light when an electric current flows through them, and they offer many advantages over current LCD technology. One advantage is that they are easier to manufacture, ultimately resulting in lower-cost displays. Performance advantages include faster response times, wider viewing angles, lower power consumption, and brighter/higher contrast images. The core advantage is that OLEDs use a self-luminous technology, so they do not require a backlight. This not only saves power, but also enables displays that are only 1mm thick.

Similar to LCD displays, OLED displays are available in both passive matrix and active matrix configurations. In the passive matrix case, the display is connected as a grid of diodes, with each diode forming an individual OLED pixel. An external driver circuit can be used to illuminate the grid one row at a time. In contrast, active matrix displays contain transistors that allow pixels to be illuminated continuously. However, unlike LCDs, OLEDs use a current-driven matrix mode, which increases the complexity of active matrix design, so the majority of OLEDs are currently passive matrix products (PMOLEDs). These PMOLEDs can be found in a variety of devices, including cell phones, car stereos, MP3 players, and other consumer products.

Power supply for OLED displays

Because many OLED displays are now being used in portable applications, power consumption is particularly important. Any power chip must operate at maximum efficiency to save as much power as possible and extend battery life, especially when the display is not in use.

The power requirements of an OLED display depend on many factors. Since the display is current driven, the peak current requirement depends on the total number of pixels lit at the same time and the maximum current driving them. The display driver circuitry also consumes some current. The voltage requirement depends on the forward voltage drop of the diode, the voltage drop of the display's internal interconnect (which is often resistive), and any voltage drops required by the display driver (see Figure 1).

In this case, the maximum voltage required is given by:

Where: Vdiode is the forward voltage drop of the diode; Idiode is the current flowing through the diode; Rcol is the resistance of the column connection; Rrow is the resistance of the row metal; VCD is the overhead required for the column driver; VRD is the overhead required for the row driver; In a typical application, VIN is approximately 20V.

The peak current is equal to:

Where: Idiode is the current flowing through the diode; Xpixels is the number of pixels lit at one time; ICD is the current supplied to the column driver; IRD is the current supplied to the row driver.

Energy saving for portable displays

For portable devices with LCD displays, it is common to turn off the backlight if not used for a period of time, and then completely power down the display after a few seconds. OLED displays do not have backlights, so the screen will usually be dark after a period of inactivity, and then powered down after a period of time. As can be seen from Equation 1, if the current of the display is reduced, the maximum voltage required will also be reduced. In a typical application with a constant supply voltage, this additional voltage will be dropped on the column driver, resulting in additional power consumption and energy waste. By reducing the supply voltage, this energy is no longer consumed by the column driver, and the system efficiency is improved.

Figure 1: OLED display driver

OLED power chip

New devices are now available on the market specifically for powering PMOLED displays in portable devices. The ideal power device for this application should have a very efficient boost converter that can operate from the battery voltage in portable applications or from a pre-regulated supply in the device. Features such as output load disconnect and low standby current are important to reduce battery drain when the display is not illuminated. The ideal device also requires few external components and a small package size to minimize the form factor of today's compact handheld devices.

Boost Converter

The boost converter used should be able to operate from 2.4V to 5.5V. This range covers the full input range of the Li-Ion battery and should also be able to operate from a pre-rectified 3V or 5V rail. The output voltage range required for this type of application is 12~25V. The most optimized power chip design will also integrate the boost FET and Schottky diode, thus reducing the need for external components. 1.2A FETs generally support output voltages up to 28V with efficiencies of up to 90%.

In order to make the boost circuit work in the best condition, it is very important to choose the right components. The main components that need to be considered are the inductor and the output capacitor, because they will affect the stability of the boost control loop. The external compensation circuit used by some boost converters also requires the reasonable selection of compensation components. Another method is to use an internal compensation network. This design requires the inductor and capacitor values ​​to be within a certain range, and the design manual usually provides a table to help select the device. The inductor value will affect the inductor size. In order to achieve a smaller device size, it is recommended to use a device that can work with a small inductor of 3.3uH. However, a low inductance value may cause the device to operate in discontinuous mode, thereby increasing the output ripple. Therefore, it is best to choose an inductance value that can maintain continuous operation mode. The selected inductor must also be able to withstand the peak and average currents required by the application. These values ​​are obtained by the following formulas:

Where: ΔIL is the peak-to-peak value of the inductor current ripple, in A; L is the inductance value, in H; FOSC is the switching frequency.

The principle of output capacitor selection is to ensure stable operation of the boost loop. The higher the output capacitor capacity, the smaller the output voltage ripple. The specific selection needs to make a compromise between ripple and component count/cost. The input capacitor is used to isolate the input current from the switch current passing through the resistor. In this application, a capacitor in the range of 10~15uF is recommended.

Dual output voltage options

As mentioned above, when the OLED is operating in dark mode, it is possible to significantly save power by reducing the output voltage. Therefore, the best power chip selected for OLED power supply should include circuits that can provide this function. This function can be achieved using two independent feedback paths, and these two feedback paths can be selected through simple logic inputs. Therefore, this method can simply implement the bright → dark → joint power technology used in PMOLED.

The output voltage is set by a voltage divider connected between the output pin and the feedback reference pin. The feedback voltage is compared with an internally set reference voltage and used to control the output voltage. The accuracy of the output voltage depends on the accuracy of the feedback reference voltage and the resistor values ​​used in the feedback network.

The typical feedback voltage is 1.15V ± 2%. When the select pin (SEL) is set low, the FB0 feedback pin is compared with the reference voltage and the FB1 pin is grounded to provide the feedback reference. When SEL is high, FB1 is used as the reference voltage and FB0 is grounded. The output voltage can be calculated using Equation 5 and Equation 6:

Fault Detection

In order to protect the IC and external components, it is also important to integrate numerous protection circuits. These functions should include:

1. Undervoltage lockout, ensuring that the device only works when the input voltage is greater than the minimum required voltage;

2. Overcurrent protection, monitoring the switch current and limiting the current to below the maximum current value allowed by the device;

3. Overvoltage lockout: when the output voltage exceeds the maximum value allowed by the device, the device stops working;

4. Over-temperature protection, shuts down the device when the die temperature exceeds the preset maximum value.

Clock synchronization

In portable devices, clock noise and crosstalk will become major concerns. Synchronizing switching devices to an external clock, thereby locking all clocks to a single frequency, can help product designers reduce these issues. For devices where clocking is not an issue, the power supply should also be able to synchronize itself. High-frequency clocks in the 1MHz range provide the best efficiency and also help reduce device size. An efficient IC should be able to self-synchronize to a 1MHz clock and also be able to easily synchronize to an external clock between 600kHz and 1.4MHz by connecting that clock to the sync input pin.

Soft-start control and input voltage disconnect

When the power chip just starts working, the current needs to charge the capacitors in the system, resulting in a significant input current demand. If this current is too high, the battery voltage will drop, causing the devices in the system to enter a reset state or produce erroneous operation. To overcome this shortcoming, a soft start mechanism should be used to limit the current at startup. At this time, the current of the IC increases slowly until it reaches the full current load. This mechanism is common in many boost converters today.

To further improve battery life, an integrated disconnect switch at the input of the boost circuit is very useful. When the device is not in operation, this switch will open to disconnect the OLED display, driver and feedback network, so that there is no leakage current. In this power-off mode, the power consumption of the internal IC is reduced to a minimum.

When the device is working, the load is connected to the input, and a DC path is established from the input to the output, which will form a large current spike when the input capacitor is charged. The disconnect switch should also provide a soft-start mode to limit the current when the output capacitor is charged, thereby further strengthening the soft-start mechanism commonly found in other DC/DC converters.

OLED devices that meet functional requirements

OLED displays are just one of many new technologies that are requiring special power ICs and added functionality. Many new ICs are being developed to meet these challenges. Intelsil's ISL97702 is just one example of these types of products, with soft-start control, input voltage disconnect, and other features suitable for this application. The complex control mechanism used in the ISL97702 represents an excellent example of the current state-of-the-art power ICs that fully meet the requirements of compact handheld devices to power OLEDs. A typical circuit for such a device is shown in Figure 2.

Figure 2: Typical ISL97702 circuit

Figure 3 shows the soft-start operation process of the ISL97702.

Figure 3: ISL97702 soft start

In section A, the inrush current when charging the load capacitor is reduced by limiting the current flowing through the disconnect switch; in section B, the boost converter starts with a current limit of 25%; in section C, the current limit is set at 50%; in section D, the current limit is 75%; and in section E, the current limit is 100%.