Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-01-20
2002-10-15
Chauhan, Ulka J. (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S039000, C345S040000, C345S061000, C345S069000, C345S083000, C345S084000, C345S102000, C345S212000
Reexamination Certificate
active
06466188
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to the field of DC to DC converters used with non-linear devices such as light emitting diodes (LED) in back lighting for liquid crystal displays (LCD).
BACKGROUND INFORMATION
Many crystal displays (LCDs) take advantage of light emitting diode (LED) back lighting technology. The LEDs are used to provide a back lighting source for the LCDs so that the displays may be more efficiently viewed. However, illuminosity of LEDs is sensitive to current fluctuations and is directly dependant upon the current flowing through the LEDs. Therefore, back lighting LED circuitry for LCDs must regulate the current flow through the LED to ensure a constant current during all operating conditions. thus providing a constant lighting source for the LCDs.
LEDs utilized for back lighting purposes are typically aligned in an array.
FIG. 1
illustrates a block diagram of a typical circuit utilizing a LED array in the prior art. LED
101
is placed in series with a predetermined number of equivalent LED
101
s. This multiple LED arrangement provides a stack
105
of four LEDs aligned in series. Those skilled in the art will understand the stack is not limited to four LEDs and that more or less than four LEDs may be used to achieve similar results.
Subsequent stacks
106
and
107
are placed in parallel with stack
105
. Stacks
106
and
107
each have the same LED arrangement as stack
105
, each stack consisting of the same number and type of LEDs. The parallel arrangement of stacks
105
,
106
and
107
provides an LED array
110
for the back lighting of the LCD (not shown).
Power requirements of LEDs encourage the LEDs to be stacked in series, with each stack then placed in parallel with other stacks as shown in
FIG. 1
in circuit
110
. As diodes, LEDs require some voltage to forward bias the LED and permit proper operation of the light emitting aspect of the diode. Typical LEDs might require as much as 1.2 volts or more to forward bias the diode depending on the diode used. Thus, when four LEDs are connected in series as a LED stack 4.8 volts may be required to forward bias the stack. Additional circuit losses might force the voltage requirements of the stack to be as high as 8 volts to forward bias the LED stack. This level typically exceeds the standard voltage used in the display circuitry requiring a step-up or boosting circuitry to provide the additional voltage. Many in the art have solved this problem by using a DC-DC converter to step-up or boost the voltage available to the diode stacks. Thus, voltage source
125
is input to a DC-DC boosting converter
120
which typically provides a boosted voltage across the LED array
110
. Current regulating device
130
senses the current across resistor
135
and regulates the current through LED array
110
in order to provide a constant current through display
110
. Those skilled in the art will recognize that the arrangement of stacks
105
,
106
and
107
along with the current limiting device
130
provide a constant voltage and current source across LED array
110
, thus providing a constant illuminosity output of display
110
. In this way, those skilled in the art will recognize that each LED will receive the same current flow as every other LED in the array
110
, ensuring a constant luminosity across the LED array
110
for any given current flow.
FIG. 2
illustrates a traditional DC-DC converter
120
utilized in a boosting circuit to provide the necessary voltage needed for back lighting LEDs. Shown in
FIG. 2
is a DC/DC converter control chip
203
. DC/DC converter control chip
203
is a generic, standard DC/DC converter control chip such as Advance Micro Device's ADP 1110. Various inputs are needed for DC/DC converter control chip
203
to operate in its normal mode. Pin
1
235
is supplied with V
IN
250
which may correspond to the bus voltage within the display. Additionally, DC/DC converter control chip
203
receives a feedback signal (FB)
245
at pin
3
. The feedback voltage samples the voltage across a typical voltage divider resistor circuitry. The voltage divider circuitry consists of resistors
210
and
220
. The DC-DC converter
120
compares the feedback voltage with a reference voltage determined by DC/DC converter control chip
203
, internal to the DC-DC converter
120
. The reference voltage source commonly used in DC-DC voltage converters varies from 0.22 volts to 1.245 volts or more.
Switch (SW)
230
at pin
2
regulates the current through inductor
202
. By regulating the current through inductor
202
, those skilled in the art will recognize that, in combination with rectifier
205
and capacitor
225
, inductor
202
will operate to provide a boosted, V
BUS
260
which is greater than V
IN
. Using a resistor divider feedback circuitry, a constant V
BUS
260
is maintained. Thus, a separate and distinct current regulating circuit as shown in
FIG. 1
, at
130
, is needed to provide a constant current through the LEDs.
FIG. 3
provides an example of typical current regulating circuitry
130
utilized in a back light LED circuit in prior art circuits. In
FIG. 3
, a certain V
BUS
260
is provided from DC-DC converter
120
as a constant voltage source to the current regulating circuit
130
. Four LEDs
101
are placed in series to create LED stack
105
as described above in FIG.
1
. Transistor
320
and low ohmic resistor R
1
135
are placed in series with LED stack
105
. Operational amplifier
330
senses the voltage across resistor
135
and compares the voltage across resistor
135
with a reference voltage
340
maintained by xenor diode
335
and resistors
336
and
338
which are aligned in a typical resistor divider network. When current flow through LED stack
105
varies from a predetermined range, those skilled in the art will recognize that Operational amplifier
330
will sense the current divergence by comparing the two voltages. If the current flow through the LED stack
105
is less than the predetermined value, the circuitry will bias transistor
320
on or off accordingly to maintain the proper current flow through LED stack
105
. It is important to note that in the prior art example shown in
FIG. 3
, a constant voltage source
260
is provided as a V
BUS
as described above. Thus, the current regulating circuitry
130
directly affects the current flow through LED stack
105
without affecting the voltage
260
.
Because LEDs tend to be non-linear resistive devices, a stack of LEDs is unable to be used in a voltage divider network in order to regulate the current through the stack. Stated another way, LED stack
105
cannot replace resistor
135
, simply because of LED's non-linear characteristics do not allow for a predictive voltage to occur across a LED or LED stack. Thus, the voltage across a LED or LED stack cannot be used as the feedback voltage for a constant current output. Hence, in prior art solutions, a DC-DC converter
120
alone is unable to regulate the current through the LED back light display
110
to maintain a constant current through the LEDs. Thus, prior art solutions incorporated a separate current sensing circuit
130
electrically coupled to the DC-DC converter to regulate the current through the LED stack to within the proper limits to maintained the desired luminosity as described above. Using two separate circuits to provide the necessary voltage and current flow through the LED array
110
is inefficient because of the additional circuitry needed.
Thus, a need in the art exists for a simplified DC-DC converter/current regulating device that, all in one, provides the proper voltage range, while maintaining the required current needed to provide a constant luminescence in the back light LED art.
SUMMARY OF THE INVENTION
Accordingly, provided is a DC-DC voltage converting means that takes the input voltage and either boosts or bucks the voltage to an output voltage, the output voltage being placed across a LED array and low ohmic resistor, such that the voltage across the low ohmic resist
Chauhan Ulka J.
International Business Machines - Corporation
Lesperance Jean
Schelkopf J. Bruce
Winstead Sechrest & Minick P.C.
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