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Effective techniques for powering passive OLED displays in handhelds

Blaze Display Technology Co., Ltd. | Updated: Oct 28, 2015

【R&D Department of Blaze Display】Techniques for reducing power consumption and dissipation in portable and mobile devices using organic LED displays.

Organic light-emitting diodes are an emerging technology that is set to revolutionize displays, offering a number of advantages over LCDs. These include ease of manufacture, faster response time, wider viewing angles, lower power consumption and brighter/ higher-contrast images. Moreover, OLEDs are self-emissive, requiring no backlight. This not only saves power but creates 1mm-thick displays.

Similar to LCDs, OLED displays come in both passive-matrix and active-matrix configurations. With passive matrix, the display is connected as a grid of diodes, each diode comprising an individual OLED pixel. The rows of the grid are lit one at a time using external drive circuitry.

In contrast, active-matrix displays include transistors within the display that enable pixels to be continuously illuminated. However, unlike LCDs, OLEDs are current-driven. This adds to the complexity of active-matrix design, thus boosting the popularity of passive matrix designs. These PMOLEDs are used in various applications including cellphones, car stereos, MP3 players and other consumer products.

Power up 
Because many OLED displays today are used in portable applications, power consumption is extremely important. Any power IC must be designed to operate with the highest efficiencies and to conserve as much power as possible to maximize battery life, especially when the display is not operating.

The power requirement for OLED displays depends on several factors. As the display is current-driven, the peak-current requirement is dependent on the total number of pixels that need to be illuminated at one time and the maximum current with which they can be driven.

Additional current is also consumed by the display drive electronics. The voltage required depends on the forward drop of the diodes, the drop across interconnects within the display (which tends to be quite resistive) and any drop needed in the display drivers (Figure 1, below).


Figure 1. The voltage required depends on the forward drop of the diodes, across interconnects within the display and in the display drivers


In such application, the maximum voltage needed is given by

Vin=Vdiode + Idiode * (Rcol + Rrow) + Vcd + Vrd   (Equation 1),

where Vdiode is forward drop of the diode, Idiode is current in the diode, Rcol is the resistance of the column connection, Rrow is the resistance of the row metal, Vcd is the overhead needed in the column driver, and Vrd is the overhead needed in the row driver.

In a typical application, Vin will be around 20V. The peak current is given by

Idiode * Xpixels + Icd + Ird                                        (Equation 2),

where Idiode is the current in diode, Xpixels is the number of pixels lit at one time, Icd is the supply current to the column driver, and Ird is the supply current to the row driver.

Saving power 
In portable equipment with LCDs, backlight is turned off after some period of inactivity. A few seconds later, display is completely turned off. In contrast, with OLED displays, there are no backlights, so the display typically dims after a period of inactivity and then turns off some time later. Equation 1 above shows that if the current is reduced in the display, the maximum voltage needed is also reduced.

In a typical application where the supply voltage is constant, this extra voltage will be dropped across the column driver, leading to extra power dissipation and wasted energy. By reducing the supply voltage, this energy is no longer dissipated in the column driver and system efficiency is improved.

Several devices are now coming to market specifically for powering PMOLED displays in portable applications. The ideal device for this application must feature a very efficient boost converter capable of operating from the battery voltages used in portable applications or from pre-regulated supplies within the device.

Features like output load disconnect and low standby current are also important to reduce the drain on the battery when the display is not illuminated. The ideal device also requires few external components and small package size to minimize the solution size in today$$$s compact handheld devices.

Boost converter
The boost used should operate from 2.4V to 5.5V supplies. This covers the full Li-ion input range and also provides operation from pre-regulated 3V or 5V rails. Output voltages required in this type of application can range from 12V to 25V.

The optimal power IC design will also integrate the boost FET and Schottky diode, reducing the need for external components. A 1.2A FET generally supports output voltages up to 28V with efficiencies up to 90 percent.

For optimum operation of the boost circuit, correct component selection is very important. The main components to be considered are the inductor and output capacitor, which affect the stability of the boost control loop. Some boost converters use external compensation that also requires correct selection of the compensation components. An alternative is the internal compensation network.

Such designs require the inductor and capacitor values to be within a certain range, and tables in the datasheet are usually provided to aid in component selection. The inductor value also affects the inductor size. The use of a device designed to work with inductors as low as 3.3 ¼H for small component size is highly recommended. However, low inductor values may cause the device to operate in discontinuous mode, which may lead to higher output ripple.

Preferably, a value should be chosen to maintain continuous mode of operation. The inductor chosen must also be selected to handle the peak and average currents needed by the application. These values are given by the following equations:

where IL is the peak-to-peak inductor current ripple in A, L is the inductor value in H, and Fosc is the switching frequency.

The output capacitor should be chosen to maintain stable operation of the boost loop. Higher values of output capacitor provide lower ripple of the output voltage. Selection is made based on the trade-off of ripple and component count/cost.

The input side capacitor is used to provide the isolate current with the input supply from the switching currents through the resistor. For this application, values in the 10-15 ¼F range are recommended.

Dual-output voltage
As described above, significant power savings can be achieved by reducing the output voltage when the OLED is operated in dim mode. The best choice of power IC for OLED power should thus include the circuitry to perform this function.

This is implemented using two separate feedback paths that can be selected by the use of a simple logic input. This enables very simple support of the Bright- Dim-Off power-saving technique used with PMOLEDs.

The output voltage is set using a potential divider connected from the output pin, to the feedback reference pin. This feedback voltage is compared to an internally set reference to control the output voltage. The accuracy of the output voltage depends on the accuracy of the feedback reference and the resistor values used in the feedback network.

A typical feedback voltage is set to 1.15V, ±2 percent. When the select pin (sel) is set low, the FB0 feedback pin is compared to the reference and the FB1 pin is grounded to provide a feedback ground reference. When sel is high, FB1 is used as a reference and FB0 set to GND. The output voltage is calculated as

Vout = (R1 + R2)/R2 * Vfb when sel = 0, and 
Vout = (R1 + R3)/R3 * Vfb when sel = 1.

Fault detection
It is also useful to integrate a number of protection circuits to ensure that both the IC and external components are protected. These features should include the following:

1) Undervoltage lockout. Ensures the device only operates when the input voltage is above the minimum required for correct operation;

2) Overcurrent protection. Monitors the switching currents and limits these to the maximum allowed with the device;

3) Overvoltage lockout. Stops the device operating if the output voltage exceeds the maximum allowed for the device;

4) Overtemperature protection. Generates a shut-down when die temperature exceeds a preset maximum.

In portable equipment, clock noise and crosstalk are major concerns. The ability to synchronize a switching device to an external clock enables the product designer to reduce these issues by locking all clocks to a single frequency.

For applications where this is not a concern, the power should also be capable of self clocking. High clocking frequencies in the 1MHz range typically offer the best efficiencies while offering small component size.

An effective IC will be able to self clock at 1MHz, and can be easily synchronized to an external clock between 600kHz and 1.4MHz just by connecting that clock to a sync input pin.

Soft-start control
When a power IC first starts operating, the current needed to charge the capacitor in the system can produce significant input- current requirements. If this current is too high, the battery voltage can drop, leading to devices in the system entering reset or providing erratic operation.

To overcome this, a soft-start scheme is used to limit the current at start up. The current capability of the IC is slowly raised until full current capability is reached. Such schemes are typically used in many of today$$$s boost converters.

To improve battery life further, an integrated disconnect switch on the input side of the boost circuit offers a major advantage. When the device is disabled this switch opens to disconnect the OLED display, the drivers and the feedback networks so that no leakage currents can flow. In this power-down mode, the internal IC power consumption should also be reduced to a minimum.



Figure 2. The complex control schemes used in the ISL97702 are an example of a good power IC


When the device is enabled, and the load connected to the input, a DC path is generated from input to output and a large current spike can flow as that output capacitance is charged.

A disconnect switch also should feature soft start mode where the current is limited while the output capacitance is charged, further enhancing the soft-start schemes found in other DC/DC converters.

OLED displays are just one of the new technologies that are generating requirements for specific power ICs and increased functionality. Many new ICs are being designed to meet these challenges. The ISL97702 is one example of these types of products, providing soft-start control, input voltage disconnect and other features ideal for the application. The complex control schemes used represents an example of the kind of stateof- the-art power IC. A typical circuit using such a device is shown in Figure 2, above.



Figure 3. Soft-start operation of the ISL97702.


Figure 3 above shows the soft-start operation of the ISL97702. In section A, the current through the disconnect switch is limited to reduce in-rush current as the load capacitors are charged. In section B, the boost converter starts with the current limit set to 25 percent. In section C, the current limit is set to 50 percent. In section D current limit is 75 percent. In section E, current limit is set to 100 percent.


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