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Optoelectronics/Displays??

Changing your walls: The world of digital photo frames

Posted: 29 Oct 2007 ?? ?Print Version ?Bookmark and Share

Keywords:digital photo frames? changing walls? digital entertainment?

By Jeffrey Kwon
National Semiconductor Corp.

It may not be obvious yet but digital entertainment may end up on your walls, replacing the conventional picture hanging in your home. Driven by systems that enable taking digital pictures with downloading and displaying content easily and following the growth of digital and wireless phone cameras, the digital photo frame market is starting to expand rapidly. With continuing lower prices of LCD displays and the growing expectation of a visual media experience and you can see the potential.

Entertainment is becoming more video based and with a large amount of digital content ready to be displayed, this is more evolutionary than revolutionary. (As a side note, many consumers currently have 500G, "half a TERAbyte!," in the home.) And instead of just storing these digital snapshots of life, people are looking to display them" even publicly (i.e. YouTube). For the home that means not just as screen savers but as standalone units, either on the coffee table or as large wall dispays. The vision of ever-changing personal pictures or scenes is a true reality and quickly becoming mainstream.

And it is not only digital video and pictures but the audio as well. The ability to put your playlists into these same devices will forever change the way entertainment is dispersed around the home. This expansion is seen by a number of new products being released to meet the needs of consumers looking to bring a richer multimedia experience to all areas of the home. To continue expanding these digital capabilities manufacturers must advance the features of components that drive these consumer products. To better understand how these features might be implemented, we'll look at a the inner workings of these digital picture frames and the technologies involved.

Figure 1 shows an example of a digital photo frame block diagram, where many chipsets are used to implement functions such as LCD bias power, LCD BLU, audio features and DC/DC performance (in this example products are from National Semiconductor). The display size of digital photo frames is generally 5.6-, 7- and 8 inches, with 7- and 8 inches being the most popular today. They are mostly adapted for use in advertising areas but the key technical aspects are the same for all. The backlighting implementation is very critical and product specific but can be easily understood.

White LED backlighting solutions
The LCD panel is a major portion of the digital photo frame, using white LEDs for backlighting. Regulators and drivers (LM2733 & LM27313) are applied to the white LED BLU solution for major models and key power areas are bias (LM2622) and video buffering (LMH6683). Audio and other solutions can be applied as shown on Figure 1.

Figure 1

Figure 1. Digital photo frame block diagram

The LED module in Figure 2 is composed of 24 total white LEDs, three in series and eight sets in parallel (landscape). There are many types of LED modules. A 7-inch model uses 3 x 7 array (or 7 x 3 array). Since the required voltage and current depends on these arrays, the selection of LED drivers is important early in the design phase. Figure 2 is an example circuit for driving 8-inch LCD modules.

Figure 2 and Figure 3 show two circuits of white LED backlighting for 8-inch digital photo frame LCDs. Note the differences between the two designs (#2 for landscape, #3 for portrait layouts). The most important fact in designing a circuit for driving LED backlighting is to keep constant current during LED forward bias. Since LEDs are a current driven device, the intensity of light depends on the quantity of current. Keeping constant current is necessary to maintain the intensity of light and improve the LED lifetime. Which layout you use is a critical part of the design solution.

Figure 2

Figure 2. 8-inch LCD module with circuit for 3-by-8 backlight LEDs

Figure 3

Figure 3. 8-by-3 LED module driving circuit for 8-inch LCD

Three steps to design process
Now that some basics are covered, actual design implementation can be discussed. The different array types and component selections all play a major role in circuit layout. It is important for the current to be controlled and to maintain the working range of the specific LEDs chosen. By following these simple steps, backlighting for digital frames can be designed successfully.

1. First of all, find out the spec of white LEDs such as forward drop voltage (VF) and forward current (IF) and define the structure of LED array.

LED drivers can be selected by the array structure and VF & IF of LED. That is, in Figure 2, the spec of W LED for 8-inch LCD BLU is VF = Max 4V, IF = Max 25mA and the array structure is 3-by-8.

Total voltage between two LED nodes is followed.

Total VF = VF X serial LED number = 4V X 3 = 12V

Total IF = IF X parallel line number = 25mA X 8 = 200mA

Thus the LED driver has to be selected to drive over 12V, 200mA. A step-up DC/DC solution is required to make 12V because as shown in Figure 2, the circuit gets constant 5V voltage with DC/DC converter from a 12V wall adapter. In case of step-up converter, the available capacity has to be considered by internal FET capacity in terms of input max current and output max voltage. In Figure 2, the required FET capacity is as follows:

Output power is POUT = VOUT - IOUT = 12 - 200mA = 2.4W, so that the required input power is PIN = 1.2 - POUT = 1.2 * 2.4W = 2.88W (assumed 80 percent of efficiency). In this case, the required input current is IIN = PIN VIN = 2.88W 5V = 576mA because VIN is 5V. As a result, internal switch (FET) has to withstand over 12V, 576mA.

For the case of 8 x 3 LED array, the results are as follows.

Total VF = VF X serial LED number = 4V X 8 = 32V

Total IF = IF X parallel line number = 25mA X 3 = 75mA

The internal switch (FET) has to withstand over 32V, 75mA. Considering the key performances of LM2733 & LM27313, the results are determined as follows:

As we can see from the table, LM27313 is not suitable for 8 by 3 arrays because max available voltage of switch is up to 30V. As shown in these examples, the LM2733 is suitable for 8-by-3 array and LM27313 can be applied to 3-by-8.

2. Design the constant current resistor (RCC) to drive LED constantly.

As we mentioned before, it is very important to keep the current constantly for LED. In case of 3-by-8 array, the required current is approximately 210mA, so that the resistor RCC for constant current in Figure 2 is as follows.

Rcc = FB voltage Total IF = 1.23V 200mA = 6.15

As a result, 6 is applied.

In case of 8 by 3 array, the required current is approximately 90mA, so the resistor RCC for constant current in Figure 3 is as follows.

Rcc = FB voltage Total IF= 1.23V 75mA = 16.4

17 is applied for this case.

As we can see in Figure 2 and Figure 3, FB voltage is maintained constantly with 1.23V of feedback reference voltage for error amplifier, so that current through Rcc is always kept constantly.

3. Activate shutdown function for dimming control

Dimming control is the function to control the intensity of the display light by the customer. Currently there are two kinds of dimming control methods. One is to control conductive current through the LED directly. Another is to control the on-time of the LEDs by using the power to switch the LEDs on/off (known as PWM or Pulse Width Modulation). Currently, PWM is preferred because the circuit of the first method is complicated and reduces the lifetime of LEDs due to the continuous current.

As shown in Figure 2 and Figure 3, LEDs can be turned on/off using PWM on the SHDN pin of either LM2733 & LM27313.The lighting intensity of LEDs can be controlled accurately by adjusting the duty cycle of the pulse. Remember that the frequency of pulse has to be over 20kHz. Lower frequencies than 20 kHz can generate audible noise by the oscillation at multilayer ceramic capacitor which is applied to output of circuit.

4. Need overvoltage protection (OVP)

Most of LED modules are connected to the main board through a connector. If the output of LM2733 & LM27313 would be opened by mistake or damage to the LED module, the output voltage will rise up infinitely because there is no error signal for negative amplifier input.. It is a fatal situation, which destroys the output diode or control chip. To solve this problem, an OVP circuit is added as in Figure 4.

Figure 4

Figure 4. 8-by-3 LED module driving circuit with OVP function

In Figure 4, the resistors R1 and R2 generate a constant output voltage that does not rise up infinitely by feeding output voltage to the error amplifier pin (FB). The added circuits can protect the device from excessive output voltage. Note the value of R1 is relatively much larger than the value of R2 and Rcc in parallel, and the role of D2 is to avoid R2 and Rcc aligning in parallel when the LED module is not connected and protects if the output voltage goes over.

Therefore, this circuit can operate as constant voltage mode when LED module is either connected or not. The output voltage in constant voltage mode with R1 & R2 has to be set higher than the total VF of LED module. For example, in case of 3-by-8 array, the total VF is approximately 12V, so that the output voltage with R1 & R2 is set as follows:

VOUT = 1.23(R1/R2 + 1)

From this equation, set R2 with 10 k. If VOUT is set to 15V, the result is:

R1 = 112k

Due to VF of D2, the voltage on two nodes of Rcc becomes 1.23+VF. Thus Rcc is getting larger as follows:

Rcc = (1.23V + VF) Icc = (1.23V+ 0.4V)200mA = 8.15

If we calculate power consumption in Rcc, the result is:

P = I - R = 0.18 - 8 = 0.32W

Since the power consumption is higher than 0.32W, it is recommended to apply a 1W resistor (1W) or normal resistors in parallel for safety.

Now that the circuit values for the backlighting is dealt with, the system supply rails are addressed for the LCD.

DC/DC for powering the LCD panel
In a typical LCD panel display for digital photo frames, the input power voltage is around 5.0V. Several different voltages are required to maximize the panel performance and generally step up converters are needed. In this example, National has the best power solution, the M2622, which is a de-facto power solution standard of middle sized LCD modules. Generating the following voltages with LM2622 from +5.0V input is fairly straight forward.

? +8.0V for Column Driver Analog Supply
? +23.0V for Row Driver "on" Voltage
? '8.0V for Row Driver "off" Voltage

The circuit in Figure 5 shows how the LM2622 can be configured to provide outputs of 8V, '8V, and 23V, which is convenient for biasing TFT displays.

Figure 5

Figure 5. Typical bias circuit of LCD power for 8-inch LCD

1. +8.0V Main Analog Voltage for Column Driver

Normally, the analog supply for the column drivers is between +7.5V and +10.0V. As in Figure 5, the analog output voltage is controlled by the resistor-divider of Rfb1 and Rfb2. As the voltage of the FB pin is internally fixed at +1.26V, the resistor values of 40.2k for Rfb1 and 7.5k for Rfb2 can be recommended in order to make +8.0V.

RFB1 = RFB2 (VOUT -1.26V)/1.26V

2. +23 V for Row Driver "On" Voltage

This 23V supply is used for the row drivers to gate the flat panel display. In Figure 5, only several components are needed to generate +23V power supply. In general, this configuration provides 3 times the column driver output voltage as the row driver "on" voltage. This is a simple and cost effective method to generate 23V supply from the LM2622 using capacitor charge pumps.

3. -8.0V for Row Driver "Off" Voltage

To generate -8V supply that is used by the row drivers as the off voltage for the TFT gate, the LM2622 can be used in conjunction with a diode inverter circuit as shown in Figure 5.

For more information, please refer to the LM2622 datasheets.

Audio and video solutions
These last two areas have a whole range of solutions based on the needs of the product. The digital photo frame typically needs video buffering or a digital interface - it depends on the type of panel input. National also provides video amps such as LMH6643, LMH6683 and LMH6601.

For digital panel interface applications, a customer can use several types of solutions. By using SerDes solutions, it can reduce EMI and help speed the system build up with previously verified solution sets.

Audio is one of important functions in digital photo frames because it can provide great sound. In particular, a built-in 3D with approximately 2W with 3D effect.function enables good sound quality in a small area by using LM49270 (a class D amp from National Semiconductor).

About the author
Jeffrey Kwon
, field application engineer at National Semiconductor, is responsible for National's power management business in Korea. He holds a bachelor's degree in Power Electronics from Kwang Woon University. He can be reached at Jeffrey.Kwon@nsc.com.




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