This is a training module on the Portable LED Backlight
Welcome to the training module on the Portable LED Backlight . This training module will discuss the solution of LED backlights for portable devices
Liquid crystal display, LCD for short, is the most common type of display panels in the current market, especially for portable devices, like mobile phones, PDAs, portable game player, MP3 players etc. But LCD only hold a translucent image, you are not going to see much without a backlight unit to punch through and light it up. Traditionally, LCDs used a cold-cathode fluorescent lamp (CCFL) backlight. The biggest drawback to CCFLs is that over time the backlight dims somewhat. LED backlighting fixes that particular issue, and offers some added benefits. LED backlighting won't dim as it ages; LED backlighting is at its full brightness from the moment switching on. LED backlighting is also more energy efficient, making it ideal for portable devices. Besides LCD backlighting, the same technology probably lights up your digital watch, the buttons on your monitor, and the indicators on your stereo.
According to iSuppli Corp., revenues for light-emitting diodes (LEDs) are climbing rapidly, and driving that growth is the strong demand for LEDs ore as backlights in keypads and mobile-use displays. For example, the volume of LED backlight units utilized in cellphones, still cameras and PDAs has been increasing as they are converted from CFL to LED backlighting. Nearly all digital still cameras, and more than 90% of digital video cameras and PDAs have converted to LED backlight untis. Not only in consumer market, LED backlight units are used in other segments, like medical equipment, automotive electronics, and industrial devices.
As we know, LEDs emit light when biased in the forward direction. For quality performance, a constant current driver is required to compensate for LED voltage drops and changes with temperature. This ensures stable light output. The design of proper driver circuits is most important challenge for LED backlighting. The LED devices may be arranged in series or parallel, which adds on another challenge for the driving circuits design - to maintain backlight uniformity, all LEDs must be the same brightness. As the portable devices are battery powered, the power supply available is usually between 3V and 4.2V. LED backlights also need to deliver high brightness for daylight vision and low brightness for night vision. Another challenge for LED backlights is keep the LED junction temperature low to ensure reliability.
Nearly every portable device designer strives to drive the backlight with the most efficient, smallest size, and simplest solution available. Two often-used driving solutions are inductive boost conversion driver and charge pump driver. Here we shows the charge pump conversion solution. It provides an output voltage derived from a capacitor, which may be a fixed 4.5V voltage or a variable voltage that is determined based on the forward voltage of the LEDs. The fundamental concept behind this scheme is the use of a switched-capacitor system that charges and then discharges the pump capacitors to develop a voltage higher than the input voltage. In this case the LEDs are connected in parallel and the current control unit of the driver carries out current matching for each parallel path. Charge pumps have traditionally offered lower EMI, less complexity, and a smaller footprint, including a much lower profile, than an inductor. To take advantage of those benefits, designers have had to accept a charge pump’s lower efficiency.
The inductive boost LED drivers set up as a constant current source can drive several LEDs in series, keeping brightness constant even over a wide variation in supply voltage. An inductive switching regulator has a relatively high efficiency of typically 80% to 90% over the Li-Ion input voltage range. We select a series of inductive switching drivers from leading suppliers. They are able to drive up to 10 LEDs at up to 30mA each. The brightness of the LEDs can be controlled using either PWM signal, analog signal, or serial pulse.
For dual display backlighting applications, the LM3503 provides a complete solution. The LM3503 contains two internal white LED current bypass FET (Field Effect Transistor) switches. The white LED current can be adjusted with a DC voltage from a digital to analog converter or RC filtered PWM (pulse-width-modulated) signal at the Cntrl pin. With no external compensation, cycle-by-cycle current limit, output over-voltage protection, input under-voltage protection, and dynamic white LED current control capability, the LM3503 offers superior performance over other step-up white LED drivers.
A charge pump driver is used to drive parallel LEDs. In this type of driver, capacitors are used to store electrical energy for release at some predetermined rate and time. In our solution, two capacitors (C1, C2) are used as a switch network. An advantage of the charge pump is elimination of the magnetic fields and electromagnetic interference (EMI) that comes with an inductor or transformer. We selected a range of charge pump converter to drive up to 9 LEDs at 30mA per LED.
The LTC3200 are low noise, constant frequency switched capacitor voltage doublers. They produce a regulated output voltage from a 2.7V to 4.5V input with up to 100mA of output current. Low external parts count (one flying capacitor and two small bypass capacitors at VIN and VOUT) make the device ideally suited for small, battery-powered applications. The charge-pump architecture maintains constant switching frequency to zero load and reduces both output and input ripple. High switching frequency enables the use of small ceramic capacitors. The LTC3200 have thermal shutdown capability and can survive a continuous short circuit from VOUT to GND. Built-in soft-start circuitry prevents excessive inrush current during start-up.
usually white LEDs are used as the backlight of display and keypad. Normally the white LEDs in backlighting application have 3.5-4V forward voltage and 15-20mA typical forward current. For 3-8inch display’s backlighting, 6-28 LEDs are needed; for 1.8-3inch display and keypad’s backlighting, 2-4 LEDs are enough. Of course, more and more RGB LEDs are used to improve the regular LED backlight in color. Here we select some white LEDs and RGB LEDs suitable for backlighting applications.
In order to complete the solution, we also need other devices, like MCUs, batteries, diodes, capacitors, and inductors. The MCU are used here to provide intelligence to the solution. The MCU can generates PWM signals for LED dimming and timing signal for LED on and off, and also color management. Capacitors and inductors are used as energy store parts for the application.
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1. Portable LED Backlight
2. Introduction <ul><li>Purpose </li></ul><ul><ul><li>To discuss the solution of LED backlight for portable devices </li></ul></ul><ul><li>Outline </li></ul><ul><ul><li>LED backlight basics </li></ul></ul><ul><ul><li>Challenges for LED backlight design </li></ul></ul><ul><ul><li>Block diagram for the reference design </li></ul></ul><ul><ul><li>Recommended core chips using the solution </li></ul></ul><ul><li>Content </li></ul><ul><ul><li>14 pages </li></ul></ul>
3. LED Backlighting <ul><li>One of the biggest problems any mobile display device is “daylight readable". </li></ul><ul><ul><li>LED backlighting is able to create a bright LCD displays. </li></ul></ul><ul><li>LED backlight is the most popular backlighting for small and medium LCDs. </li></ul><ul><ul><li>Typically use white, standard LEDs </li></ul></ul><ul><ul><li>Edge-lit is used in almost all these displays. </li></ul></ul><ul><li>The advantages of LED backlighting are </li></ul><ul><ul><li>Wide color gamut for lifelike images </li></ul></ul><ul><ul><li>Long lifespan </li></ul></ul><ul><ul><li>Low power consumption </li></ul></ul><ul><ul><li>Low heat generation </li></ul></ul><ul><ul><li>Environmental benefits </li></ul></ul><ul><ul><li>Reduction of product size </li></ul></ul>Edge-lit Matrix-lit LED Backlight Backlight Simulation LCD
4. LED Backlight Applications <ul><li>Nowadays products with LED backlight are entering different market segments: </li></ul><ul><ul><li>Consumer electronics </li></ul></ul><ul><ul><li>Medical devices </li></ul></ul><ul><ul><li>Industrial </li></ul></ul><ul><ul><li>Automotive </li></ul></ul><ul><ul><li>Computers & Peripherals </li></ul></ul>
5. Considerations for LED Backlight <ul><li>LEDs may be arranged along the edges of the LCD or as a matrix over the back of the LCD assembly. </li></ul><ul><li>The LED devices may be arranged in series or parallel. </li></ul><ul><li>For stable light output, a constant current driver is required to compensate for LED voltage drops and changes with temperature. </li></ul><ul><li>To maintain backlight uniformity, all LEDs must be the same brightness. </li></ul><ul><li>LED backlights must be able to deliver high brightness for daylight vision and low brightness for night vision. </li></ul><ul><li>As the portable devices are battery powered, the power supply available is usually between 3V and 4.2V. </li></ul><ul><li>Another challenge for LED backlights is keep the LED junction temperature to ensure reliability. </li></ul>
6. Charge Pump Solution
7. Inductive-Boost Driver Selection Guide PWM 1200 85 23 27 2x Series 1+4 3-6V TPS61140 TI PWM / Analog 1000 85 30 28 Series 6 1.8-6V TPS61042 TI PWM 1000 84 30 24 Series 5 2.7-5.5V NCP5010 On Semi PWM / Analog 1000 80 20 44 Series 10 2.7-5.5V LM3503 National PWM / Serial Pulse 100 91 25 24 Series 6 2.6-5.5V MAX8901A Maxim Serial-Pulse 1000 80 20 45 Series 10 2.7-5.5V LT3593 Linear Dimming Control Switching Frequency (Max) (KHz) Peak Efficiency (%) LED Current (mA) Output Voltage (Max) (VDC) LED Configuration LED Nos. Input Voltage (DC) Part Number Manufacturer
8. National’s LM3503 <ul><li>Drives up to 4, 6, 8 or 10 White LEDs for Dual Display Backlighting </li></ul><ul><li>>80% Peak Efficiency </li></ul><ul><li>Output Voltage Protection Options: 16V, 25V, 35V & 44V </li></ul><ul><li>Input Under-Voltage Protection </li></ul><ul><li>Internal Soft Start Eliminates Inrush Current </li></ul><ul><li>1 MHz Constant Switching Frequency </li></ul><ul><li>Analog Brightness Control </li></ul><ul><li>Wide Input Voltage Range: 2.5V to 5.5V </li></ul>
9. Charge Pump Driver Selection Guide I2C 750 93 230 6.5 Parallel 4+2+1 3-6V 1x/1.5x TPS60250 TI PWM 1000 90 60 (30/LED) 4.8-5.7 Parallel 2 2.7-5.5V 1x/1.5x NCP5612 On Semi PWM 750 85 120 (30/LED) Parallel 4 3-5.5V 1x/1.5x LM27951 National Digta l/ PWM / Serial Pulse 1000 94 580 (30/LED) 5 Parallel 8 2.7-5.5V 1x/1.5x/2x MAX8631 Maxim I2C 1000 93 135 (24/LED) 5 Parallel 6 2.7-5.5V 1x/1.5x MAX8647 Maxim I2C 850 90 250 5.04 Parallel 9 2.9-5.5V 1x/1.5x/2x LTC3219 Linear PWM 2000 80 150 5 Parallel 6 2.7-4.5V 1x LTC3200 Linear Dimming Control Switching Frequency (Max) (KHz) Peak Efficiency (%) LED Current (mA) Output Voltage (Max) (VDC) LED Configuration LED Nos. Input Voltage (DC) Charge Mode Part Number Manufacturer
10. Linear Technology’s LTC3200 <ul><li>Low Noise Constant Frequency Operation </li></ul><ul><li>Output Current: 100mA </li></ul><ul><li>2MHz Switching Frequency </li></ul><ul><li>Fixed 5V ± 4% Output (LTC3200-5) or ADJ </li></ul><ul><li>VIN Range: 2.7V to 4.5V </li></ul><ul><li>Automatic Soft-Start Reduces Inrush Current </li></ul><ul><li>No Inductors </li></ul><ul><li>I CC <1mA in Shutdown </li></ul>
11. LED Selection Guide 130 70 80 2.9 20 White LWL283-Q1R2-3K8L-1-Z OSRAM 120 35 40 2.9 10 White LNJ036X8ARA Panasonic 120 40 40 2.9 10 White LNJ026X8ARA1 Panasonic 120 260 -- 3.6 25 White NSCW100 Marl 140 12 -- 2 20 Yellow CMD17-21VYC/TR8 CML 140 15(R)/12(G) -- 1.7(R)/2.1(G) 20 R/G CMD15-22SRUGC/TR8 CML 170 90 60 1.9 25 Red HSMC-C191 Avago 170 15 52 2.2 20 Green HSMG-C170 Avago 130 6.5 92 3.8 20 Blue HSMB-C110 Avago 170 90 60 1.9 25 White HSMW-C191 Avago 150 71.5 -- 3.6 20 White HSMW-C265 Avago Viewing Angle Luminous Intensity (mcd) Power Dissipation (mW) Forward Voltage (V) DC Forward Current (mA) Color Part Number Manufacturer
13. Additional Resource <ul><li>For ordering solution related products, please click the part list or </li></ul><ul><li>Call our sales hotline </li></ul><ul><li>For more design resources go to </li></ul><ul><ul><li>http://www.element-14.com/community/docs/DOC-18131/ </li></ul></ul><ul><li>Visit Element 14 to post your question </li></ul><ul><ul><li>www.element-14.com </li></ul></ul><ul><li>For additional inquires contact our technical service hotline or even use our “Live Technical Chat” online facility </li></ul>