LMC6009 datasheet(6/7 Pages) NSC

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Applications (Continued)

Example: Below is a calculation of pixel charge time (for a

black to black transition) in a VGA display operating at a ver-

tical refresh rate of 60 Hz, with a panel capacitance of 50 pF

per sub-pixel:

A full black to black transition represents the maximum

charging time for the panel, since it requires that the panel

capacitance be driven by a 4V swing from node V

REF1

(

Figure 7).

Total capacitive load presented to the LMC6009 is

L = 50 pFx3x640 = 96 nF

Output current of the LMC6009 is:

SC = 80 mA

Hence, slew time t

SLEW = (96 nF x 4V)/80 mA = 3.07 µs

The total line time for a VGA system is approximately 34 µs.

Therefore, the LMC6009 easily meets the drive require-

ments for the application. The input resistance seen between

the V

REFn and VREF(n+1) inputs, (where n = 0 thru 8) of the

Column Driver (

Figure 7) also draw current from the

LMC6009. Thus, the actual current available for charging the

panel capacitance is:

Ipx = 80 mA - (V

VREF1–VVREF2)/RCD

where

V REFn = Voltage at node VREFn,

VREF(n+1) = Voltage at node VREF(n+1), and

CD = Column driver input resistance between

VREFn and VREF(n+1)

Since the LMC6009 is capable of sourcing 80 mA, the pixel

charging time is primarily limited only by the length of the

CD.CL time constant. To implement a high quality display,

column drivers that allow the shortest possible time constant

(lower values of R

CD) are desirable. However, lower values

of R

CD result in increased system quiescent power dissipa-

tion. It is therefore important to optimize system performance

by carefully considering speed vs power tradeoffs.

DS012533-7

FIGURE 7.

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