LMC6009 Datasheet, PDF (6/7 Pages) National Semiconductor (TI) – 9 Channel Buffer Amplifier for TFT-LCD

Applications (Continued)
FIGURE 7.
DS012533-7
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
C_L = 50 pF x 3 x 640 = 96 nF
Output current of the LMC6009 is:
I_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 V_REF(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–V_VREF2)/R_CD
where
V_{V REFn} = Voltage at node V_REFn,
V_VREF(n+1) = Voltage at node V_REF(n+1), and
R_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
R_CD. C_L 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.
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