Portable LED Backlight
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The document discusses solutions for LED backlighting in portable devices. It outlines the basics of LED backlighting and challenges in the design. It then provides recommendations for core chips that can be used in an LED backlight reference design, including charge pump and inductive boost driver options from various manufacturers. Key considerations for LED backlight design such as output voltage, current, efficiency and dimming control are also covered.
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Portable LED Backlight
- 7. Inductive-Boost Driver Selection Guide Manufacturer Part Number Input Voltage (DC) LED Nos. LED Configuration Output Voltage (Max) (VDC) LED Current (mA) Peak Efficiency (%) Switching Frequency (Max) (KHz) Dimming Control Linear LT3593 2.7-5.5V 10 Series 45 20 80 1000 Serial-Pulse Maxim MAX8901A 2.6-5.5V 6 Series 24 25 91 100 PWM / Serial Pulse National LM3503 2.7-5.5V 10 Series 44 20 80 1000 PWM / Analog On Semi NCP5010 2.7-5.5V 5 Series 24 30 84 1000 PWM TI TPS61042 1.8-6V 6 Series 28 30 85 1000 PWM / Analog TI TPS61140 3-6V 1+4 2x Series 27 23 85 1200 PWM
- 9. Charge Pump Driver Selection Guide Manufacturer Part Number Charge Mode Input Voltage (DC) LED Nos. LED Configuration Output Voltage (Max) (VDC) LED Current (mA) Peak Efficiency (%) Switching Frequency (Max) (KHz) Dimming Control Linear LTC3200 1x 2.7-4.5V 6 Parallel 5 150 80 2000 PWM Linear LTC3219 1x/1.5x/2x 2.9-5.5V 9 Parallel 5.04 250 90 850 I2C Maxim MAX8647 1x/1.5x 2.7-5.5V 6 Parallel 5 135 (24/LED) 93 1000 I2C Maxim MAX8631 1x/1.5x/2x 2.7-5.5V 8 Parallel 5 580 (30/LED) 94 1000 Digta l/ PWM / Serial Pulse National LM27951 1x/1.5x 3-5.5V 4 Parallel 120 (30/LED) 85 750 PWM On Semi NCP5612 1x/1.5x 2.7-5.5V 2 Parallel 4.8-5.7 60 (30/LED) 90 1000 PWM TI TPS60250 1x/1.5x 3-6V 4+2+1 Parallel 6.5 230 93 750 I2C
- 11. LED Selection Guide Manufacturer Part Number Color DC Forward Current (mA) Forward Voltage (V) Power Dissipation (mW) Luminous Intensity (mcd) Viewing Angle Avago HSMW-C265 White 20 3.6 -- 71.5 150 Avago HSMW-C191 White 25 1.9 60 90 170 Avago HSMB-C110 Blue 20 3.8 92 6.5 130 Avago HSMG-C170 Green 20 2.2 52 15 170 Avago HSMC-C191 Red 25 1.9 60 90 170 CML CMD15-22SRUGC/TR8 R/G 20 1.7(R)/2.1(G) -- 15(R)/12(G) 140 CML CMD17-21VYC/TR8 Yellow 20 2 -- 12 140 Marl NSCW100 White 25 3.6 -- 260 120 Panasonic LNJ026X8ARA1 White 10 2.9 40 40 120 Panasonic LNJ036X8ARA White 10 2.9 40 35 120 OSRAM LWL283-Q1R2-3K8L-1-Z White 20 2.9 80 70 130
- 12. Peripheral Solution Farnell Newark Microcontroller Click Click Battery Click Click Diode Click Click Capacitor Click Click Inductors Click Click
Editor's Notes
- 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.
- Thank you for taking the time to view this presentation on Portable LED Backlight. If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simply call our sales hotline. For more design resources you may either visit the element14 site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility. You may visit Element 14 e-community to post your questions.