TFT- LCD DC-DC Converters with Operational Amplifiers

EC9219

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2009/09/29

Typical Operating Circuit

The EC9219 Typical Operating Circuit (Figure 2) is a complete power-supply system for TFT LCDs. The

circuit generates a +9.6V source-driver supply and +18V and -6V gate-driver supplies. The input voltage

range for the IC is from +2.6V to +5.5V. The listed load currents in Figure 1 are available from a +4.5V to

+5.5V supply. Typical Operating Circuit recommended components,.

Applications Information

The EC9219 is a high frequency, high efficiency boost regulator operated at constant frequency PWM

mode. The boost converter stores energy from an input voltage source and deliver it to a higher output

voltage. The input voltage range is 2.6V to 5.5V and output voltage range is 5V to 18V The switching

frequency is selectable between 640KHz and 1.2MHz allowing smaller inductors and faster transient

response. An external compensation pin gives the user greater flexibility in setting output transient

response and tighter load regulation. The converter soft-start characteristic can also be controlled by

external C08 capacitor. The SHDN pin allows the user to completely shut-down the device.

Main Step-Up Regulator

The main step-up regulator employs a current-mode, fixed-frequency PWM architecture to maximize loop

bandwidth and provide fast transient response to pulsed loads typical of TFT-LCD panel source drivers.

The 1.2MHz switching frequency allows the use of low profile inductors and ceramic capacitors to

minimize the thickness of LCD panel designs. The integrated high-efficiency MOSFET and soft-start

function controls inrush currents. The output voltage can be set from VIN to 13V with an external resistive

voltage-divider. The regulator controls the output voltage and the power delivered to the output by

modulating the duty cycle (D) of the internal power MOSFET in each switching cycle. The duty cycle of the

MOSFET is approximated by:

MAIN

IN

MAIN

V

V

V

D

−

=

Figure 1 shows the Functional Diagram of the step-up regulator. An error amplifier compares the signal at

FB to 1.228V and changes the COMP output. The voltage at COMP sets the peak inductor current. As the

load varies, the error amplifier sources or sinks current to the COMP output accordingly to produce the

inductor peak current necessary to service the load. To maintain stability at high duty cycles, a

slope-compensation signal is summed with the current-sense signal. On the rising edge of the internal

clock, the controller sets a flip-flop, turning on the n-channel MOSFET and applying the input voltage

across the inductor. The current through the inductor ramps up linearly, storing energy in its magnetic field.

Once the sum of the current-feedback signal and the lope compensation exceeds the COMP voltage, the

controller resets the flip-flop and turns off the MOSFET. Since the inductor current is continuous, a

transverse potential develops across the inductor that turns on the diode (D1). The voltage across the

inductor then becomes the difference between the output voltage and the input voltage. This discharge

condition forces the current through the inductor to ramp back down, transferring the energy stored in the

magnetic field to the output capacitor and the load. The MOSFET remains off for the rest of the clock cycle.