In recent years the traditional paper-based photo frame has become more attractive dissipated, with digital cameras and the popularity of camera phones, digital photo frame market is ascendant. Digital Photo Frame with built-in speakers and headphones, MP3 functionality can be realized through touching the screen, thus we can expect a considerable proportion of the traditional photo frame will be replaced by digital photo frame.  In addition, once the Digital Photo Frame prices start down, the popular trend will soon come.
 It is precisely for these reasons, many companies have introduced the latest Digital Photo Frame model. This article will introduce the National Semiconductor for Digital Photo Frame application of new products and focus on verification in each application in which products are most effective.

 Figure 1 is the Digital Photo Frame module map example.  As shown, many of National's products have been used in LCD bias supply, LCD BLU (backlight module), audio and DC / DC module.  Digital Photo Frame usually display the size of 5.6 inches, 7 inches and 8 inches, 7 inches and 8 inches is the mainstream. Larger size of the digital photo frame is specifically designed for the advertising industry.  This paper will focus on small digital photo frame programme.

 Figure 1: Digital Photo Frame module diagram

 Digital Photo Frame LCD panel is the main component, with a backlight for illumination of the white LED. At present, LM2733 and LM27313 has been used extensively for white LED BLU solutions.  LM2622 be used for LCD bias, LMH6683 were used for video buffer.  As shown in Figure 1, audio and other solutions can also be used for Digital Photo Frame products.

 White LED backlighting solutions
 Figure 2 and Figure 3 shows the 8-inch Digital Photo Frame for the white LED LCD backlight circuit.  In the design of LED backlighting drive circuit, the most important issue is the LED forward to maintain constant current bias.  As is the current drive-LED devices, light intensity depends on the conduction current size. To ensure the LED light intensity and increase the life expectancy, we must maintain a constant current. 

 Figure 2: 8-inch LCD in the 3 × 8 array LED drive circuit module

 Figure 2 in the LED modules from a total of 24 white LED components, each of these LED a group of three series together, divided into eight parallel connection.  On the market have multiple types of LED modules.  8-inch models were used 3 × 8 or 8 × 3 array. As the required voltage and current depend on these arrays, so early in the design choose a suitable LED driver is very important. Figure 2 is the 8-inch LCD module in the LED drive circuit of an example.

 Figure 3: LCD array of 8 × 3 LED drive circuit module

 1. Determine the specifications and white LED definition LED array structure.

 By the LED array structure and the LED forward voltage drop and to select LED forward current drive.  As shown in Figure 2, 8-inch LCD BLU for the white LED to the specifications: VF (Max) = 4V, IF (Max) = 25mA, for the array of 3 × 8.  LED between the two nodes of the total voltage as follows:

 Total VF = VF × series of LED = 12V
 Total IF = IF × number of parallel lines = 25mA × 8 = 200mA

Therefore the selected LED driver's ability to drive more than As shown in Figure 2, takes a step-up DC / DC solutions to provide 12V voltage, the circuit of DC / DC converters from the 12V wall adapter was 5V constant voltage. The circuit in Figure 2, FET capacity required by the following method.

 Output:

POUT=VOUT×IOUT=12×200mA=2.4W POUT = VOUT × IOUT = 12 × 200mA = 2.4W

 Therefore the necessary input power:

PIN=1.2×POUT=1.2×2.4W=2.88W PIN = 1.2 × POUT = 1.2 × 2.4W = 2.88W

 Assumptions conversion efficiency of 80 per cent, due to VIN for 5V, the input current requirements for the IIN = PIN ÷ VIN = 2.88W ÷ 5V = 576mA.  Therefore, the internal switch (FET) to support the voltage above 12V and more than 576mA of current.  8 × 3 for the LED array, the results are:

VF = VF × series of LED = 32V

 Total IF = IF × number of parallel lines = 25mA × 8 = 75mA

If we consider the key LM2733 and LM27313 performance, can be drawn on the results listed in Table 1: From the table we can see, LM27313 not suitable for 8 × 3 of the array, the maximum available because switching voltage 30V.  Figure 2 and Figure 3 shows, LM2733 apply to the 8 × 3 array, LM27313 is applicable to 3 × 8 array.

 2. Design constant current resistance (RCC) continued to drive LED

 As noted above, to maintain constant LED current is very important.  3 × 8 in the array, the current requirements of about 210mA.  Figure 2 in the constant resistance resistance can be calculated by the following formula:

 RCC = FB voltage ÷ total IF = 1.23V ÷ 200mA = 6.15 Ω
 8 × 3 in the array, the current requirements of about 90mA, therefore Figure 3 in constant resistance to the resistance value calculated the following formula:

RCC=FB 电压 ÷ 总 IF=1.23V÷75mA=16.4Ω RCC = FB voltage ÷ total IF = 1.23V ÷ 75mA = 16.4 Ω

 In such circumstances the use of 17 Ω resistor.  From Figure 2 and 3 we can see that through the error amplifier feedback reference voltage 1.23V FB maintain constant voltage, through the RCC can always maintain the current constant.
 3. Dimming control for the active shutdown

 Dimming control is based on customer preferences control display light intensity.  There are two dimming control methods.  Is a direct current through the LED control conduction, and the other is by controlling the power on / off to on-or off-LED, LED thus control the on-time. he second method is more popular now because the circuit first method more complicated, and will always be in power status LED will shorten the working life. Figure 2 and Figure 3 shows, through on / off and LM2733 LED LM273133 to control the switch, and the LM2733 and LM273133 the switch can be controlled through the SHDN pin on the application of pulse width modulation (PWM) can be realized.  LED lighting intensity can be adjusted to achieve precise control pulse duty cycle.  To remember is that the pulse frequency must be more than 20kHz.  Lower than the frequency of 20kHz output of the circuit will multi-layer ceramic capacitors have a wave oscillation, resulting in audible noise.
 4. Need to over-voltage protection (OVP)

 Most LED modules through a connector and connected to the motherboard.  If Figure 2 and 3 of the LM2733 and LM27313 output due to misuse or damage to LED modules and pave the way, then because of the error amplifier no negative input signal, the output voltage will be increased indefinitely. This will have devastating consequences, damage LM2733 and LM27313 or output diodes.  To solve this problem, can add a over-voltage protection circuitry (shown in Figure 4).

Figure 4: carrying over-voltage protection function of 8 × 3 LED drive circuit module

 In Figure 4, resistors R1 and R2 through to the error amplifier pin (FB) feeds the output voltage to ensure that the constant increase in output voltage will not be unlimited. The increase in circuit protection devices can not be too high voltage output by the damage.  D2's role is to avoid R2 RCC and not connected in a parallel LED module, and to prevent over-voltage output, because the resistance R1 and R2 RCC parallel relative value is much greater.

。 Therefore, in the absence of access to LED modules, the circuit to constant pressure mode when the access LED modules, while constant current mode of operation.  In the R1 and R2 by the constant pressure mode, the output voltage must be set LED modules than the total value of VF higher. That is, in the 3 × 8 array, the total value of about VF 12V, then the R1 and R2 output voltage can be calculated by the following equation:

VOUT=1.23(R1/R2+1) VOUT = 1.23 (R1/R2 +1)

 In this equation, we need to R2 set to 10k Ω.If VOUT set to 15V, available the following results: R1 = 112k Ω.  Taking into account the D2 VF, RCC two nodes on the voltage into a 1.23 + VF.  Therefore, RCC will be increased, as follows:

RCC=(1.23V+VF)÷ICC=(1.23V+0.4V)÷200mA=8.15Ω RCC = (1.23V + VF) ÷ ICC = (1.23V +0.4 V) ÷ 200mA = 8.15 Ω

 If the calculation of the RCC power, can be obtained the following results:

P=I2×R=0.182×8=0.32W P = I2 × R = 0.182 × 8 = 0.32W

 As the RCC power than 0.32W, recommended Wattage resistor (1W) or parallel standard resistor circuit to enhance security.

 LCD flat-panel power for the DC / DC converters

 Digital Photo Frame used in a typical LCD flat panel displays, input supply voltage is 5.0V.。 Must provide a number of different voltage, so that the display panel through the input voltage to achieve optimal performance, under normal circumstances needed to boost converter. National Semiconductor has the best power solution LM2622.  LM2622 is a middle-size LCD module power solution of the de facto standard. We can see how to use LM2622 is +5.0 V input voltage under the following different voltage
 +8.0 V drive out analog supply voltage;

 +23.0 V line drive on-voltage;

 - 8.0V to drive off voltage.
 The circuit in Figure 5 shows how to configure the LM2622, to provide 8V,-8V and 23V output voltage, the bias in terms of TFT display is very convenient.

 Figure 5: 8-inch LCD power of the typical bias circuit

 1. +8.0 V drive out the main analog voltage
 Under normal conditions, drive out the analog supply voltage between +7.5 V and +10.0 V between. As shown in Figure 5, the analog output voltage from the resistor divider Rfb1 and Rfb2 control. As the voltage on FB pin was nominated as the +1.26 V, for the realization of virtual red coil shown in the +8.0 V output voltage, it is proposed to RFB1 and RFB2 resistance values were set to 40.2k Ω and 7.5k Ω:

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

2. +23 V voltage line drive to open

 The 23V supply voltage is used to drive to open the gate flat-panel displays.  As shown in Figure 5, only a number of components to achieve +23 V supply voltage. Generally speaking, this configuration can provide three times the output voltage drive out the voltage used to open the line drive.  The method is simple and very economic, carrying through the capacitor charge pump, the LM2622 can achieve 23V supply voltage.

 3.-8.0V to drive off voltage

 For the line drive to provide for the shutdown of the TFT gate-8V supply voltage can be LM2622 frequency and diode circuits used in conjunction, as shown in Figure 5.

 Video solutions

 Digital Photo Frame can not be separated from buffer or digital video interface.  Specific solutions depend on the type of flat-panel input. National Semiconductor to offer various solutions to meet customer simulation solutions for various needs, but also able to provide all kinds of video amplifiers, such as the LMH6643, LMH6683 and LMH6601, and so on.

 For the digital flat panel interface applications, the customer may be National's all kinds of solutions.  These solutions can reduce electromagnetic interference (EMI), and to help customers using SerDes solutions easily build systems.
 Audio Solutions
 Audio Digital Photo Frame is one of the important function, not only because of the digital photo frame can display photos, but also to achieve excellent audio players. National Semiconductor offers a variety of audio solutions, in particular, it is worth mentioning that the built-in 3D features a small area to achieve high-quality audio. LM49270 is a typical application of such products, it can provide about 2W the 3D effect.