Power Electronics

Multiple-Output Power-Supply ICs Support LCD Panels

Recently, Maxim and Linear Technology both introduced ICs intended for powering LCDs (table). The MAX17014A multiple-output power-supply controller IC generates all the supply rails for thin-film transistor (TFT) LCD panels in TVs and monitors operating from a regulated 12-V input. It includes a stepdown and a stepup regulator, a positive and a negative charge pump, two operational amplifiers and a proprietary dual-mode logic-controlled high-voltage switch control block. The MAX17014A can operate from 8-V to 16.5-V input voltages and is optimized for LCD TV panel and LCD monitor applications running directly from 12-V supplies.

The stepup and stepdown regulators feature internal power MOSFETs and high-frequency operation allowing the use of small inductors and capacitors, resulting in a compact solution. Both switching regulators use fixed-frequency current-mode control architectures, providing fast load-transient response and easy compensation.

A current-limit function for internal switches and output-fault shutdown protect the stepup and stepdown power supplies against fault conditions. The MAX17014A provides soft-start functions to limit inrush current during startup. The MAX17014A provides adjustable power-up timing.

The positive and negative charge-pump regulators provide TFT gate-driver supply voltages. Both output voltages can be adjusted with external resistive voltage dividers. The switch control block allows the manipulation of the positive TFT gate-driver voltage.

The MAX17014A includes two high-current operational amplifiers designed to drive the LCD backplane (VCOM). The amplifier features high output current (±150 mA), fast slew rate (100 V/µs), wide bandwidth (20 MHz), and rail-to-rail inputs and outputs. A series p-channel MOSFET is integrated to sequence power to AVDD after the MAX17014A has proceeded through normal startup.

The MAX17014A is available in a 7-mm 3 7-mm, 0.8-mm profile, 48-pin thin QFN package and operates over a -40°C to +85°C temperature range.

The Linear Technology LT3513 is a highly integrated, five-output regulator designed to provide all the supply voltages typically required by large TFT LCDs. The part features a stepdown switching regulator to produce a 3.3-V or 5-V logic voltage from a wide input-voltage range, such as automotive battery. A lower-voltage logic supply can be generated from the first supply by adding an external npn driven by an internal linear regulator. The other three on-chip regulators provide the three bias voltages required by LCDs: a high-power boost regulator to generate AVDD, a low-power boost regulator to generate VON and an inverting regulator to provide VOFF.

Switching regulators are chosen over linear regulators to accommodate a wide input-voltage range (providing both stepup and stepdown functions) and to minimize power dissipation. The LT3513's 4.5-V to 30-V input range allows it to accept a variety of power sources, including automotive batteries, distributed supplies and wall transformers.

All of the regulators are synchronized to a 2-MHz internal clock, allowing the use of small, low-cost inductors and ceramic capacitors. Since different types of panels may require different bias voltages, all output voltages are adjustable for maximum flexibility. Programmable soft-start capability is included to limit inrush current.

The five-output TFT LCD power supply can accommodate an 8-V to 30-V input voltage. The first switching converter produces a 5-V logic supply using a buck regulator. The internal linear regulator with an external npn produces a 3.3-V logic supply using the 5-V supply as input. The second switcher is used to boost the 5-V supply to an 8-V, 80-mA AVDD bias supply. Another boost converter and an inverter generate VON and VOFF, which also use the 5-V supply as input.

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