Vcom AND Gamma COMPENSATION

The architecture used for the amplifiers in the LM2702 re-

quires external compensation on the output. Depending on

the equivalent capacitive load of the TFT-LCD panel, exter-

nal components at the amplifier outputs may or may not be

necessary. If the capacitance presented by the load is equal

to or greater than 5nF no external components are needed

as the TFT-LCD panel will act as compensation itself. Dis-

tributed resistive and capacitive loads enhance stability and

increase performance of the amplifiers. If the capacitance

presented by the load is less than 5nF external components

will be required as the load itself will not ensure stability. No

external compensation in this case will lead to oscillation of

the amplifier and an increase in power consumption. A single

5nF or greater capacitor on the output will ensure a stable

amplifier with no oscillations. For applications requiring a

higher slew rate, a good choice for compensation is to add a

50

Ω (R

C

This allows for driving zero to infinite capacitance loads with

no oscillations, minimal overshoot, and a higher slew rate

than using a large capacitor. The high phase margin created

by the external compensation will guarantee stability and

good performance for all conditions.

For noise sensitive applications greater output capacitance

may be desired. When the power supply for the amplifiers

(AV

the output ripple of the regulator will produce ripple at the

output of the amplifiers.

LAYOUT CONSIDERATIONS

The LM2702 uses a single ground connection, GND. The

feedback, softstart, delay, and compensation networks

should be connected directly to a dedicated analog ground

plane and this ground plane must connect to the GND pin, as

shown in Figure 3. If no analog ground plane is available

then the ground connections of the feedback, softstart, de-

lay, and compensation networks must tie directly to the GND

pin, as show in Figure 4. Connecting these networks to the

PGND plane can inject noise into the system and effect

performance.

The input bypass capacitor C

IC. This will reduce copper trace resistance which effects

input voltage ripple of the IC. For additional input voltage

filtering, a 100nF bypass capacitor can be placed in parallel

with C

noise to ground. The output capacitor, C

placed close to the IC. Any copper trace connections for the

C

directly effects output voltage ripple and efficiency. The feed-

back network, resistors R1 and R2, should be kept close to

the FB pin, and away from the inductor, to minimize copper

trace connections that can inject noise into the system.

Trace connections made to the inductor and schottky diode

should be minimized to reduce power dissipation and in-

crease overall efficiency.

20051152

FIGURE 3. Multi-Layer Layout

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