LCDs Enjoy Flexible Power-Up Sequencing

March 1, 2005
A programmable power-management IC will meet the power-up requirements for multiple-vendor liquid-crystal displays.

For many systems, liquid-crystal displays (LCDs) are a large part of the bill of materials (BOM). In some cases, more than 80% of the entire system cost is that of the LCD panel (e.g., high-definition televisions or HDTVs). LCDs also are a major factor in a consumer's perceived quality perspective. If the LCD begins to fail, the end customer will associate the system and its manufacturer with poor quality. Clearly, safeguarding the LCD component is a critical aspect of these system designs.

Typically, LCDs are comprised of a liquid-crystal fluid. This liquid is sandwiched between two polarizing layers of glass or plastic, which are separated into cells (pixels). For color LCDs, each pixel is divided into three sub-cells: red, blue, and green. When a voltage is applied across these LCD sub-cells, a certain amount of light passes through the LCD cell from the backlight or reflective surface. The amount of light depends upon the voltage applied and the polarization of the liquid-crystal fluid.

To apply voltage across a particular sub-cell, LCDs use thin-film transistors (TFT) or electrodes in a matrix of rows and columns on the glass substrate. These TFTs or electrodes also are used to control the amount of voltage that's applied to the crystal fluid. The crystal in the sub-cell twists according to the amount of voltage that's applied to the electrodes. It therefore allows light to pass from the backlight through to the polarized glass. Typically, 256 levels-of-brightness (8-b) voltage can be applied to the sub-cell. That voltage amount will twist the crystal in up to 256 positions: from open to closed and from bright to dark. Note that 24-b color is three strings of 8-b brightness for the red, blue, and green sub-cells.

In order for the LCD to operate reliably, the liquid-crystal fluid must be protected from the DC voltages (typically negative) that cause breakdown. If too much DC current inadvertently flows through the LCD fluid, the fluid will become damaged. It will then electrochemically decompose and break down over time as the fluid changes state. This change in state becomes noticeable as the color of the liquid-crystal pixel changes. Given further damage, gas bubbles will form inside the liquid-crystal cell. These gas bubbles will result in permanent damage and a noticeable degradation of display quality. Pixel damage cannot be repaired. A damaged wireless LCD kiosk, for example, will require replacement.

In order to prevent any DC current from damaging the liquid-crystal fluid, some LCDs have an "M" clock. When it's activated at power up, this clock produces an AC current wave on the electrodes and into the fluid cells before the display "comes to life." It thereby protects the cells from breakdown. Many LCDs have built-in controllers that require DC logic signals upon power up.

Care must be taken in the order and timing of how power is applied to the LCD. The Vcc supply voltage is usually turned on first. Within 10 to 20 ms, all logic and data signals are then applied. If the data signals come in either before or too long after Vcc power up, latch-up or damage to the LCD cells can occur. Similarly, at power down, removal of Vcc is usually delayed until the logic and data signals have been turned off.

All LCDs have power-up sequencing requirements of some type. The key to power-up sequencing requirements is to make sure that the LCD fluid is protected from DC voltages without the AC wave being set up. It also must be kept within the requirements that were provided by the LCD vendor.

This article examines a solution that provides proper power-supply sequencing to an LCD panel. In addition, this solution prevents the LCD panel from getting damaged due to a faulty power supply and faulty controller. This solution can be easily customized across multiple types of LCD displays.

A basic power-up sequence for a popular 18-in. TFT LCD—a size that might be used in a wireless kiosk—is shown in FIGURE 1. For this particular LCD, Vcc needs to be powered up before data comes in (no longer than 10 ms). In addition, the ramp rate of Vcc must not be longer than 60 ms. Vcc cannot go below 11.4 V for more than 20 ms. If Vcc goes below 9.6 V, a proper power-down sequence should occur. For this design to support LCDs from multiple vendors, multiple power-up circuits will most likely be required. Support for multiple LCD vendors' timing requirements can be complex, as the values that are shown for t3, t2, and Vcc can vary.

Single-system support for multiple LCD vendors can be extremely beneficial. After all, multi-vendor competition often leads to price reductions of 10% or more. Plus, the design supports multiple vendors. If an LCD vendor decides to obsolete a particular LCD, the design will easily accommodate the switch to another vendor. For example, in an application like remote database access over a wireless kiosk or inventory monitoring system, being first to market might require choosing a particular LCD vendor. Subsequently, however, changing LCD vendors may be required to reduce cost.

So far, our focus has been on power-up sequencing requirements for the LCD. The assumption has been that the power supply is operating correctly. Now, consider a case in which the power supply itself becomes faulty. In this scenario, the liquid-crystal display needs protection from the damage that could be caused by the faulty power supply. Such protection is provided by ensuring that the unwanted DC current is not pumped into the LCD fluid. The end customer will find a minor repair for a faulty power supply more tolerable—and less damaging to the manufacturer's reputation—than a complete liquid-crystal-display replacement.

In order to protect the LCD from a faulty power supply, a circuit is needed that senses the correct operation of the power supply. Only then will such a circuit supply DC to the LCD controller in the proper sequence for a particular LCD.

The power-up circuitry that's required to protect the LCD from unwanted DC at power-up and faulty power supplies can be complex. This circuitry demands several analog and digital components. Note that the system design also will require power-up and faulty power-supply protection of some type. It would be preferable to be able to re-use some or all of this circuitry for additional portions of the system. In a wireless medical monitoring system, for example, some additional power-up circuitry may be desired to monitor that the radio is powered on after a soft reset.

Many chips, which are common in systems, also have power-up sequencing and timing requirements (ASICs, DSPs, ASSPs, FPGAs, microprocessors, etc.). And many of these chips comprise much of the overall system cost. Coordinating the power up of the I/O and core supply voltages and protecting these chips in conjunction with the LCD would be yet another important aspect of the system design.

To further complicate matters, some of these chips might require their highest voltage to be turned on prior to their lowest voltage or vice versa. They also may require the reverse for power-down. In most cases, however, protecting two different chips or LCDs would require a completely different power-up circuit. This task also would demand a different design and more components.

Traditionally, designers have used standard off-the-shelf power-management ICs to individually address the power-supply sequencing, monitoring for faulty power supplies, and power-supply management of the controller board. Such power-management ICs include supervisors, reset generators, sequencers, charge pumps, etc. Yet this approach often results in a solution that requires multiple ICs (in some cases from different vendors). In addition, this solution is inflexible because the complete system-level management functions have to be realized by hardwiring individual power-management ICs together. When the LCD panel has to be changed to address obsolescence or cost reduction, this aspect makes the task a time-consuming exercise. Plus, the panel change will often require a total redesign of power management.

In contrast, the use of programmable power-management ICs for this function requires fewer ICs to implement the complete function. It also eases the modification of the power-management function to accommodate different power-supply sequencing, monitoring, and timing functions.

FIGURE 2 shows the power-supply arrangement for the LCD panel as well as its control circuitry. To control its turn-on/off ramp rate, the LCD panel is powered by the +12-V supply through a MOSFET. The supply voltage is monitored for four different voltage-threshold levels in order to meet its sequencing and monitoring specifications. As described earlier, the power supply-sequencing algorithm also factors in the presence of clock/data at the input of the display. This part of the specification, which is controlled by a low-voltage differential signaling (LVDS) buffer, is met by monitoring the data presence through a low-pass filter. There are three power supplies—3.3 V, 2.5 V, and 1.5 V—generated locally in the system. These power supplies also are controlled and monitored to meet the power-management requirements of other logic-control circuitry.

The design is straightforward for the LCD power-up/power-down sequencing and monitoring. The signals that are used for the device employ the following convention: signal name (device pin). The low-pass filter averages the LVDS data and generates the LVDS present signal. The threshold is set at 80 mV to detect the presence of data with very few excursions to logical 1.

The 80-mV precision feature of the power-manager IC allows the detection of the present LVDS signal. LVDS_EN allows LVDS data into the LCD through an LVDS buffer. 12V_OK checks for a faulty power supply (above 10.8 V). LCD_Vcc_on connects Vcc to the LCD through a p-channel MOSFET. Meanwhile, LCD_12V6 monitors Vcc>12.6V, which would result in a shutdown sequence. LCD_11V4 monitors Vcc>11.4V, which is the low point of proper Vcc operation. If the power supply falls below this level, a 20-ms timer is started. A shutdown procedure is initiated if either the timer expires or the voltage drops below 9.6 V.

LCD_10V8 monitors Vcc>10.8 V, which is the threshold to begin the timer between Vcc and LVDS Data. LCD_9V6 monitors Vcc>Vt, requiring a proper shutdown if Vcc dips below Vt. The remaining signals are used for 3.3-, 2.5-, and 1.5-V bricks to power additional logic circuitry. These power supplies are turned on starting with 1.5 V and followed by 2.5 V and 3.3 V tracking each other. In case of a faulty (12V_OK) signal, these supplies are turned off with 2.5 V and 3.3 V first followed by 1.5 V.

The power-management designs that use the programmable power-manager IC are implemented with the software tool, PAC-Designer. All of the following are implemented using software: the monitoring voltage threshold; current to control the ramp rate through the MOSFET; logic algorithm that controls the sequence of events including the power-supply sequencing of the controller board; LCD panel; monitoring of power-supply voltages; and corrective action under power-supply faults.

Obviously, the power-management algorithm is entirely implemented in software. As a result, changing the design to accommodate the power-supply-management requirements for a different LCD panel can be achieved simply by changing the software code. That code is implemented on the programmable power-manager IC device. This flexibility would be useful for field repairs. If a wireless LCD kiosk becomes damaged in the field, the technician can replace the LCD with whatever LCD panel is in stock. He or she simply has to change the software code in the programmable power-manager device.

Power-management solutions for LCD panels from multiple vendors can vary greatly. Designing a standardized power-management solution to accommodate LCD panels from multiple vendors usually results in a solution that is both expensive and requires many off-the-shelf power-management devices.

Programmable power-management devices, such as the Power1208P1, offer a convenient solution for power management and sequencing for LCDs from multiple vendors. In addition, such devices can be used to offload additional power-up tasks like logic circuitry. Because power-up management can be integrated into one programmable design platform, the benefit to the designer is clear: The device can be re-programmed during manufacturing to support multiple display vendors.


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