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In recent years, the appeal of traditional paper photo frames has faded. With the popularity of digital cameras and mobile phones, the digital photo frame market is on the rise. The digital photo frame has built-in speakers and headphones, which can make a moving picture through MP3 function, so we can easily predict that a considerable proportion of traditional photo frames will be replaced by digital photo frames. In addition, once the price of digital photo frames begins to fall, its popularity will soon come.It is for these reasons that many companies have introduced the latest digital photo frame models. This article introduces National Semiconductor's new products for digital photo frame applications and highlights which products are most effective in each application.
FIG. 1 is an example of a functional block diagram of a digital photo frame. As shown, many of National's products have been used in LCD bias power supplies, LCD BLU (backlight modules), audio and DC/DC modules. Typically, digital photo frame displays are 5.6-inch, 7-inch, and 8-inch, with 7-inch and 8-inch being mainstream products. Larger size digital photo frames are designed specifically for the advertising industry. This article will focus on small-size digital photo frame solutions.
| Figure 1: Schematic diagram of the digital photo frame function module. |
The LCD panel is a major component of the digital photo frame with white LEDs for backlighting. Currently, the LM2733 and LM27313 have been widely used in white LED BLU solutions. The LM2622 is used for LCD biasing and the LMH6683 is used for video buffering. As shown in Figure 1, audio and other solutions are also available for digital photo frame products.
White LED backlight solution
Figures 2 and 3 show a white LED backlight circuit for an 8-inch digital photo frame LCD. When designing an LED backlight drive circuit, the most important issue is to maintain a constant current when the LED is forward biased. Since LEDs are current-driven devices, the intensity of the light depends on the amount of conduction current. In order to ensure the light intensity and increase the life of the LED, it is necessary to maintain a constant current.
| Figure 2: 3 x 8 array LED module drive circuit in an 8-inch LCD. |
The LED module in Figure 2 consists of a total of 24 white LEDs, which are connected in series in groups of 3 and are connected in parallel in 8 columns. There are many types of LED modules on the market. The 7-inch model uses a 3 x 7 array or a 7 x 3 array. The 8-inch models are available in 3 x 8 or 8 x 3 arrays. Since the required voltage and current depend on these arrays, it is important to select the right LED driver at the beginning of the design. 2 is an example of an LED module driving circuit in an 8-inch LCD.
Figure 3: 8 x 3 array LED module driver circuit for an 8-inch LCD.
1. Determine the specifications of the white LED and define the structure of the LED array.
The LED driver can be selected by the array structure of the LEDs and the forward voltage drop and forward current of the LEDs. As shown in Figure 2, the specifications of the white LED for the 8-inch LCD BLU are: V F (maximum) = 4V, I F (maximum) = 25mA, and the array structure is 3 x 8. The total voltage between the two LED nodes is as follows:
Total V F = V F × number of series LEDs = 12V
Total I F =I F ×number of parallel lines=25mA×8=200mA
Therefore, the drive capability of the selected LED driver must exceed 12V, 200mA. As shown in Figure 2, a step-up DC/DC solution is required to provide 12V. This circuit uses a DC/DC converter to obtain a constant 5V from a 12V wall adapter. If a boost converter is used, the internal FET capacity needs to take into account the available capacity, ie the maximum input current and the maximum output voltage. In the circuit of Figure 2, the required FET capacity can be calculated by the following method.
The output power is:
P OUT =V OUT ×I OUT =12×200mA=2.4W
Therefore the required input power is:
P IN =1.2×P OUT =1.2×2.4W=2.88W
Assuming a conversion efficiency of 80%, since VIN is 5V, the required input current is I IN = P IN ÷ V IN = 2.88W ÷ 5V = 576mA. Therefore, the internal switch (FET) must support a voltage above 12V and a current exceeding 576mA. For an 8 x 3 LED array, the result is:
Total V F = V F × number of LEDs in series = 32V
Total I F =I F ×number of parallel lines=25mA×8=75mA
The FET must be more than 32V, 75mA. If you consider the key performance of the LM2733 and LM27313, you can get the results listed in Table 1: From the table we can see that the LM27313 is not suitable for 8 × 3 arrays, because the maximum available voltage of the switch is 30V. Figures 2 and 3 show that the LM2733 is suitable for 8x3 arrays and the LM27313 is suitable for 3x8 arrays.
2. Design a constant current resistor (R CC ) to continuously drive the LED
As mentioned earlier, it is important to keep the current of the LED constant. In a 3 x 8 array, the required current is approximately 210 mA. The resistance of the constant current resistor in Figure 2 can be calculated by the following formula:
R CC = FB voltage ÷ total I F = 1.23V ÷ 200mA = 6.15Ω
Therefore, a 6Ω resistor is used. In the 8 × 3 array, the required current is about 90 mA, so the resistance value of the constant current resistor in Figure 3 can be calculated by the following formula:
R CC = FB voltage ÷ total I F = 1.23V ÷ 75mA = 16.4 Ω
In this case, a 17Ω resistor is used. It can be seen from Fig. 2 and Fig. 3 that the FB voltage is kept constant by the feedback reference voltage of the error amplifier 1.23V, so the current through R CC can always be kept constant.
3. Active shutdown function for dimming control
Dimming control refers to controlling the light intensity of the display according to the preferences of the customer. There are currently two methods of dimming control. One is to control the conduction current directly through the LED, and the other is to control the on-time of the LED by turning on or off the LED by controlling the power on/off. The second method is currently more popular because the circuit of the first method is more complicated, and always being powered on will shorten the working life of the LED. As shown in Figures 2 and 3, the LED switches can be controlled by turning the LM2733 and LM273133 on and off, while the switching control of the LM2733 and LM273133 can be achieved by applying pulse width modulation (PWM) on the SHDN pin. The illumination intensity of the LED can be precisely controlled by adjusting the pulse duty cycle. It is important to remember that the pulse frequency must exceed 20 kHz. A frequency below 20 kHz causes the multilayer ceramic capacitor at the output of the circuit to oscillate and produce audible noise.
4. Overvoltage protection (OVP) required
Most LED modules are connected to the motherboard through a connector. If the outputs of the LM2733 and LM27313 in Figures 2 and 3 are open due to malfunction or damage to the LED module, the output voltage will rise unrestricted due to the absence of a signal at the negative input of the error amplifier. This will have devastating consequences, damaging the LM2733 and LM27313 or the output diode. To solve this problem, an overvoltage protection circuit can be added (as shown in Figure 4).
Figure 4: 8×3 LED module driver circuit with overvoltage protection.
In Figure 4, resistors R1 and R2 ensure that the constant output voltage does not rise indefinitely by feeding the output voltage to the error amplifier pin (FB). The added circuitry protects the device from excessive output voltages. The role of D2 is to prevent R2 and R CC from forming a parallel connection when the LED module is not connected, and to prevent output overvoltage, because the resistance of R1 is much larger than the parallel value of R2 and R CC .
Therefore, when the LED module is not connected, the circuit operates in a constant voltage mode; when the LED module is connected, it operates in a constant current mode. In constant voltage mode with R1 and R2, the output voltage must be set higher than the total V F of the LED module. That is, in a 3 x 8 array, the total V F value is about 12V, then the output voltages of R1 and R2 can be calculated by the following equation:
V OUT =1.23 (R1/R2+1)
In this equation, we need to set R2 to 10kΩ. If VOUT is set to 15V, the following result is obtained: R1 = 1212 kΩ. Considering that the voltage at the two nodes V F and R CC of D2 becomes 1.23 + V F . Therefore, R CC will increase, as follows:
R CC =(1.23V+V F )÷I CC =(1.23V+0.4V)÷200mA=8.15Ω
If you calculate the power consumption of R CC , you can get the following results:
P=I 2 ×R=0.18 2 ×8=0.32W
Since R CC consumes more than 0.32W, Wattage resistors (1W) or parallel standard resistors are recommended to improve circuit safety.
DC/DC conversion for LCD panel power supplies
In a typical LCD flat panel display for a digital photo frame, the input supply voltage is approximately 5.0V. A number of different voltages must be provided to allow the display panel to achieve optimum performance through the input voltage, typically using a boost converter. National Semiconductor has the best power solution, the LM2622. The LM2622 is the de facto standard for medium size LCD module power solutions. We can see how the LM2622 achieves the following different voltages at +5.0V input voltage:
+8.0V column driver analog supply voltage;
+23.0V row driver turn-on voltage;
-8.0V line driver shutdown voltage.
The circuit in Figure 5 shows how the LM2622 should be configured to provide output voltages of 8V, -8V, and 23V, which is very convenient for biasing TFT displays.
| Figure 5: Typical bias circuit for an 8-inch LCD power supply. |
1. +8.0V column driver main analog voltage
Typically, the column driver's analog supply voltage is between +7.5V and +10.0V. As shown in Figure 5, the analog output voltage is controlled by resistor dividers Rfb1 and Rfb2. Since the voltage on the FB pin is internally set to +1.26V, to achieve the +8.0V output voltage shown in the red dotted coil, it is recommended to set the resistance values ​​of R FB1 and R FB2 to 40.2kΩ and 7.5kΩ, respectively:
R FB1 =R FB2 ×(V OUT -1.26V)/1.26V
2. +23V row driver turn-on voltage
This 23V supply voltage is used in the row driver to turn on the gate of the flat panel display. As shown in Figure 5, a supply voltage of +23V can be achieved with only a few components. In general, this configuration can provide three times the voltage of the column driver output voltage for use as an open row driver. The method is simple and economical, and a 23V supply voltage can be realized on the LM2622 by carrying a capacitor charge pump.
3. -8.0V row driver turn-off voltage
To provide the row driver with an -8V supply voltage that is used to turn off the TFT gate, the LM2622 can be used in conjunction with a diode inverter circuit, as shown in Figure 5.
Video solution
Digital photo frames are inseparable from video buffers or digital interfaces. The exact solution depends on the type of tablet input. National Semiconductor offers a variety of solutions to meet customers' needs for a wide range of analog solutions, as well as a variety of video amplifiers such as the LMH6643, LMH6683 and LMH6601.
For digital flat panel applications, customers can use National's various solutions. These solutions reduce electromagnetic interference (EMI) and help customers easily build systems with SerDes solutions.
Audio solution
Audio is one of the important features of digital photo frames, because digital photo frames can not only display photos, but also achieve excellent audio playback. National Semiconductor offers a variety of audio solutions, and it's worth noting that built-in 3D capabilities enable high-quality sound in a small range. The LM49270 is a typical product for this application, which can provide a 3D effect of about 2W.
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