Applications Information Low-Cost Single-Inductor Triple-Output TFT LCD Supply By the Power Management Business Unit Staff This application note describes an easy method of conversion from a single power supply to three distinct voltage levels. This method eliminates all but one inductor, thereby reducing matching effort during design, as well as limiting final application size and cost. The Allegro™ A8483 device is examined as an integral component of this solution. The vast majority of portable devices today feature thin-film transistor (TFT) LCD displays. Such displays need to operate from a low-voltage input supply or a battery source, in the range of +3 to +5 volts. A typical TFT LCD bias supply requires a main supply, also referred to as AVDD (typically in the range of +8 V to +12 V), a positive gate-drive bias (typically in the range of +12 V to +18 V), and a negative gate-drive bias (typically in the range of –5 V to –10 V). Because portable electronics applications demand small size and low profile, the supply must operate at sufficiently high frequency to reduce the size of external filter components. Additionally, a single dc-to-dc controller generating three outputs is highly desirable, because it limits the cost of the solution and minimizes the PCB area required. The circuit shown in the figure below uses a combination boost regulator and charge pump circuitry to accomplish this task. Boost Regulator The circuit employs a single dc-to-dc controller (the A8483, a current-mode–controlled boost regulator) to generate a +8.75 V main supply, capable of delivering up to 50 mA. (Selecting an appropriate feedback voltage divider can change the output voltage.) The 1.2 MHz operating D5 BAT54S C8 1 μF 25 V C5 D3 1 μF 16 V BAT54S C3 D2 1 μF 16 V BAT54S L1 10 μH 4 C1 1 μF 6.3 V D1 RB421D 1 SW VIN EN A8483 GND 2 Enable C9 2.2 μF 25 V R4 470 Ω D6 15 V C6 2.2 μF 16 V D4 7.5 V C10 2.2 μF 25 V R7 15 kΩ VOUT2 –7.5 V C7 2.2 μF 16 V R5 7.5 kΩ C4 1 μF 16 V VSW 3.3 V 5 VOUT1 +15 V R6 1 kΩ R1 39 kΩ FB VOUT3 8.75 V 50 mA R2 512 kΩ 3 R3 39 kΩ C2 2.2 μF 16 V This circuit delivers three output voltages from a single supply, by combining regulator topologies. AN295043 frequency reduces the inductor size and allows operation with tiny ceramic capacitors. Internal compensation simplifies circuit design and reduces external component count. The low-profile (0.75 mm nominal height) MLP/TDFN package allows the boost regulator to be mounted on either side of the PCB to optimize footprint and minimize height. Positive and Negative Charge Pumps When the internal switch of the A8483 is on, the SW node is near 0 V, with low impedance to circuit ground. When the internal switch of the A8483 switches off in order to provide boost action, the SW node goes up to approximately 9.05 V. This operation allows for an easy implementation of the charge pump with diodes and capacitors, to generate gate-drive supplies at +15 V and -7.5 V with 1 mA output-current capability. The circuit diagram shows optional Zener diodes across gatedriver outputs. These can help to provide good regulation. Additional sequencing features can be added to this solution as needed, by providing pass elements in series with the outputs. Allegro MicroSystems, LLC reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the information being relied upon is current. Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the failure of that life support device or system, or to affect the safety or effectiveness of that device or system. The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC assumes no responsibility for its use; nor for any infringement of patents or other rights of third parties which may result from its use. Copyright ©2007-2013, Allegro MicroSystems, LLC For the latest version of this document, visit our website: www.allegromicro.com AN295043 Allegro MicroSystems, LLC www.allegromicro.com (508) 853-5000 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 2