LCD Power Management ICs

Charge pump, switchmode and LDO techniques are used by various ICs to power color thin film transistor (TFT) liquid crystal displays (LCDs). These ICs usually employ a combination dc-dc converter technologies to provide the multiple voltages required by an LCD.

Following are examples of LCD power management ICs

1. This IC uses a charge pump to generate +5V, +15V, ?15V from a single 3 V supply. These voltages are then used to provide 5V for the LCD controller and the gate drives ±15V for the transistors in the panel. A low dropout (LDO) voltage regulator provides a low ripple 5V output. This LDO can be shut down and an external LDO can be used to regulate the 5V doubler output and drive the input to the charge pump section that generates the ±15V outputs.

2. Another LCD IC employs a fixed frequency, current mode PWM step-up dc-dc switchmode regulator with five buffers capable of 12 V boosted output voltage. Each buffer can deliver 35 mA output current and has rail-to-rail input and output capability. It provides high efficiency, low noise operation, and excellent dynamic response. The high switching frequency allows small, cost-saving, external inductive and capacitive components.

3. Using a different approach, the LCD IC uses a four-channel dc-dc converter to boost the 2V to 14V input to a 5V to 17V output for powering the column drivers with up to 370mA @ 15V. A pair of charge pump control circuits provide outputs to allow the external generation of VON and VOFF supplies at 5V to 40V and 0V to -40V, respectively, each at up to 60mA for VBOOST = 15V. The VCOM buffer provides up to 50mA continuous output current from 2V to 13V.

4. An LCD IC operates from a single Lithium-Ion cell, 2- to 3-cell alkaline input or any voltage source between 1.5V and 4.6V. This synchronous boost converter generates a low noise, high efficiency 5.1V, 10mA supply. Internal charge pumps generate 10V, 15V, and ?5V, ?10V or ?15V. The IC controls output sequencing internally to insure proper initialization of the LCD panel. A master shutdown input reduces quiescent current to <2mA and quickly discharges each output for rapid turn off of the LCD panel.

5. A completely linear-regulator approach uses three devices: a high-performance linear regulator, positive charge-pump regulator, and negative charge-pump regulator. It has built-in power-up sequence control. and provides logic-controlled high-voltage switches to control the positive charge-pump output. This linear regulator directly steps down the input voltage to generate the supply voltage for the source-driver ICs. The two built-in charge-pump regulators generate the TFT gate-on and gate-off supplies. It includes an operational amplifier to drive the LCD backplane (VCOM) and features high output current (150mA), fast slew rate (12V/µs), and wide bandwidth (12MHz).

6. A mixed technology LCD power management IC provides the three LCD voltages. An auxiliary linear regulator controller can generate a 3.3V logic power rail for systems powered by a 5V supply rail only. This IC's main output is a 1.6-MHz fixed frequency PWM boost converter providing the source drive voltage for the LCD display. One version has a typical switch current limit of 2.3 A and the other has a typical switch current limit of 1.37 A. A fully integrated adjustable charge pump doubler/tripler provides the positive LCD gate drive voltage. An externally adjustable negative charge pump provides the negative gate drive voltage. In addition, these ICs have a system power good output to indicate when all supply rails are acceptable.