Integrated Multi-Channel DC/DC Converter for TFT LCD Panels

The RT9921 includes a high-performance boost regulator, a low dropout linear regulator (LDO), a gate pulse modulator (GPM), a voltage detector, a VCOM buffer (unity-gain OPA), and a VGH charge pump controller for active-matrix thin-film transistor (TFT) liquid-crystal displays (LCDs). The boost converter provides the regulated supply voltage for the panel source driver ICs. The converter is a high switching frequency (640kHz or 1.2MHz) current-mode regulator with an integrated 16V N-Channel 0.2W MOSFET that allows the use of ultra-small inductors and ceramic capacitors. It provides fast transient response to pulsed loading while achieving efficiency over 90%. The device can produce output voltage as high as 15V from an input as low as 2.5V. Soft-Start is programmed by external capacitor, which sets the input-current ramp-rate.

The low-dropout (LDO) linear regulator can supply up to 350mA current while input voltage is 3.3V. It uses an internal P-MOSFET as the pass device. It is suitable for the supply voltage of the timing controller.

The GPM is controlled by frame signals from timing controller to modulate the Gate-On voltage, VGHM, which acts a flicker compensation circuit to reduce the coupling effect between gate lines and pixels. It also can delay the Gate-On voltage while power-on for achieving a corrected power-on sequence for gate driver ICs. Both of power-on delay time and the falling time of the Gate-On voltage are programmable by external capacitor and resistor.

The voltage detector monitors the supply voltage to issue a reset signal while the detected voltage is too low. The detecting level is decided by an external resistor divider and the delay time is programmable by an external capacitor.

The VCOM buffer can drive the LCD VCOM voltage that features high short-circuit current (140mA), fast slew rate (12V/μs), wide bandwidth (12MHz) and rail-to-rail inputs and outputs.

The VGH charge pump controller provides regulated TFT Gate-On voltage. The regulation of the positive charge pump is generated by the internal comparator that senses the output voltage and compares it with an internal reference.