Download 1) Compute the transmissivity of a NWTN LCD panel given:

Take Home
13-Nov-2012
4 points / each answer
1) What features of a liquid-crystal molecule are most responsible for its dual behaviors, part
liquid -- part crystal.
a)
b)
c)
d)
its molecular weight and size
its natural polarization and benzene rings
its rigid head and wiggling tail
none of the above
2) The dielectric constant of the liquid-crystal molecule in each of the two perpendicular
directions is averaged to one characterizing value because
a)
b)
c)
d)
the molecule rotates on its long (parallel) axis
the molecule is polar and flips end to end
the electric field is reversed typically 60 times a second.
none of the above
3) As the applied voltage increases across a subpixel of an LCD display that operates in
normally-white mode, the display will:
a)
b)
c)
d)
turn white
brighten
not possible to know
darken
4) The liquid crystal order used in nearly all active-matrix, LC displays used today in laptop PCs,
desktop monitors and TVs is:
a)
b)
c)
d)
smectic
nematic
cholesteric
discotic
5) A nematic liquid crystal experiences a torque in an electric field because:
a)
b)
c)
d)
of the passing of light through the molecule
it is a polar molecule
it an anisotropic dielectric
of its bend, twist and splay spring constants
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6) The reason RMS averaging of the voltage applied across an LCD is used to determine the
average optical transmission through the display is:
a)
b)
c)
d)
the power lost in the display is due to resistive heating
the energy stored in the springs depends on the square of the applied voltage
the induced torque on the molecule depends on the square of the electric field
none of the above
5v
3v
5v
Problems 7 and 8.
The following figure refers to problems 7 and 8.
The four voltages in the figure below are
continuously applied, sequentially, frame-by-frame
to a liquid crystal display sub-pixel (red, green or
blue dot).
0v
8) Regarding the waveform in the figure above,
what is the RMS average of the voltage across the LC in this case?
9) Assume that all the directors of the molecules in a liquid crystal cell are oriented at 5 degrees
to the surface of the substrate electrodes. Applying єy= 1+ [ocos2() + esin2()], what is the
capacitance of the cell given:
A = 150um x 50um (area of cell)
d = 5um (thickness of the cell)
e = 8
o = 3
e
5um
5o
10) What material is typically used to make the transparent electrodes in an LCD display?
a) polyimide
b) indium-tin oxide (ITO)
c) sputtered aluminum
d) SiO2
11) The need for Vcom in a active-matrix LCD arises from what feature of the TFT:
a)
b)
c)
d)
the capacitance from gate to column line (Cgs)
the capacitance from gate to pixel electrode (Cgd)
the capacitance from pixel electrode to column line (Cds)
the off-resistance of the channel
2
3v
5v
3v
7) Regarding the waveform in the figure above,
what is the average value (DC content of
waveform) of the voltage across the LC in this case?
12) What would V be in figure below, given the following values:
Cgd = 0.1 pF
Clc = 1 pF
Cstore = 4 pF
Cgd
Vgate = 25 volts
V
Vgate
Cstore
Clc
13) In a three level row drive system, the purpose is to make V = 0 for all values of Clc. Clc
changes of course with gray level making V dependent on gray level. What voltage does
Vgate(n-1)_ 2-3 need to be in equation below, to make Vtotal = 0 given:
Cgd = 0.1 pf
Cs = 1.5 pF
Vgate(n)_ 1-2 = 30 volts
 C gd  Vgate ( n ) _12  CS  Vgate ( n1) _ 23 
Vtotal  

C

C

C
gd
lc
s


14) In the timing figure below, what is the function of the OE signal as applied to the row driver?
OE
Row N
Row N + 1
Row N + 2
Row N + 3
Line N
a)
b)
c)
d)
Line N+1
Line N+2
Line N+3
to clock the row driver
to assure that the row signal falls before the column signal changes
to force all outputs to the de-select (low) state.
b and c above
Problems 15 and 16
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15) In the column driver gamma curve shown above, how many bits are required to specify a
gray level:
a)
b)
c)
d)
63
6
8
10
16) In the column driver gamma curve shown above, how many total, individual resistors are
required for the ladder:
a)
b)
c)
d)
16
29
63
71
Problems 17 through 18
In this circuit, there are four gamma references being applied to 8 column drivers. The
references drive each end of the RDAC resistances, R.
A
R
R
R
R
R
R
R
R
B
C
R
R
R
R
R
R
R
R
CD1
CD2
CD3
CD4
CD5
CD6
CD7
CD8
D
17) In the circuit above, what is the voltage at node C with respect to ground?
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18) In the circuit above, the voltage the resistors labeled R represent the total resistance of each
RDAC in each of eight column drivers. There is an upper and a lower RDAC in each column
driver. If each R is 16K ohms, what will the equivalent resistance between nodes A and B due to
the column drivers only?
19) The voltage shown as ground in the circuit to the right is the center of the column driving
waveform in the circuit above. This center voltage is the voltage halfway between node B and
node C in the above circuit. What is this voltage?
Cgd
Vgate
Vsource
Clc
Cstore
Vcom
20) A particular column driver output produces a constant offset error of +5mV. If the ideal,
intended output voltage is a continuous +/-3.8 volts, changing polarity each frame, what is the
equivalent Vrms from the CD?
a)
b)
c)
d)
3.800001423 volts
3.805000000 volts
3.800000000 volts
3.800003289 volts
21) What would the RMS voltage be if the error is +5 millivolts when the intended output
voltage is +3.8 volts but -5 millivolts when the intended output voltage is -3.8?
a)
b)
c)
d)
3.800001423 volts
3.805000000 volts
3.800000000 volts
3.800003289 volts
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The I-V characteristic of a TFT is given in the figure to the right and is used in problems 22 through
24. This characteristic curve is measured using a 10-volt reference (to ground) at the drain with the
source terminal grounded (i.e. zero volts). Under these channel conditions the gate-source voltage
(Vgs) is swept though a range of voltages while the drain current is plotted as a function of that
sweep.
22) For the test conditions captured in the curve above, what would be the ideal potential to choose
for the gate de-select voltage (the voltage used to turn the transistor off)?
a) -7 volts
b) 0 volts
c) -3 volts
d) +7 volts
23) There are several disadvantages to a high gate select voltage. One is that higher voltage widens
the stray field effect from the row line. The optical effect from the stray fields from the row line must
be visually masked from the viewer reducing the active area of the screen. Another issue is that kickback voltage, the ΔV drop on the subpixel voltage which occurs when the TFT is turned off, varies
linearly with the amplitude of the gate select to deselect voltage.
Suppose that in a system using the TFT characterized as above, the gate select voltage is chosen as
20 volts what is the approximate on-current to off-current ratio when Vgate-off is set to -5V.
a) 3.8 x 105
b) 4.1 x 108
c) 2.5 x 107
d) 3.3 x 106
24) The resistance between the source and drain terminals of the TFT, under the test conditions, can
be determined from the characterization curve above. For example at zero volts of Vgs, the channel
current is approximately 500 x 10-12. Since R=V/I and V across the channel is 10 volts, R can found
as approximately 2 x 1010 ohms.
Assume the test conditions of the characterized TFT above. What is the on-resistance of the TFT
with Vgs = 20-volts?
a) 20 k-ohms
b) 2 Mega ohms
c) 200 k-ohms
d) 20 Mega ohms
25) The time constant of an RC circuit characterizes the rate at which the capacitance (representing
the subpixel capacitance in this case) will be charged or discharged through the resistor (representing
the TFT channel in this case). Suppose that we consider the pixel to be fully charged to the column
line voltage after 5 time constants (5 x R x C).
Given a TFT on-resistance of 2 M-Ohm, what value of C, the total capacitance of the cell, is required
to have 5 time constants in 22 microseconds (the line time of a typical XGA panel).
a) 5.10 pF
b) 4.70 pF
c) 6.21 pF
d) 2.20 pF
The diagram below represents two frames of voltage data in a particular panel. A frame is the time it
takes to refresh all the lines on a display. Tx is the time for each line to be refreshed, call a line time.
The timing and voltage diagram is used in problems 26 and 28.
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26) The multi-level gate voltage is used to eliminate the subpixel voltage drop caused by the charge
extraction from the fall of the gate voltage. It does this through injection of charge from the rise of
the previous gate line which exactly offsets the charge lost when the current gate line falls.
Compute the RMS value of the bottom waveform over both frames in the above figure when each
line time (T) is 80 microseconds and there are 200 lines per frame (three shown plus 197 between T3
and T1).
Note: the voltage function has been altered slightly into steps to make the computations simpler.
a) 2.999 volts
b) 3.000 volts
c) 3.001 volts
d) 3.002volts
27) Suppose the number of lines per frame is extended to 800 (4 times larger) while the line time is
reduced to 20 microseconds (4 times shorter) what will be the RMS value of the bottom waveform in
this case?
a) 3.002volts
b) 2.999 volts
c) 3.001 volts
d) 3.000 volts
28) Usually there is some dead time delay at the end of the line-by-line refresh of the display before
the start of the next frame to allow time to reset pointers and perform other “bookkeeping” tasks in
logic. What is the approximate refresh rate (frames per second) in the previous problem if after the
800 lines of the display refresh, an additional 33 dummy lines of delay are added before starting the
next frame?
a) 58.2 Hz
b) 62.5 Hz
c) 61.8 Hz
d) 60.0 Hz
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