Obviousness

Obviousness Analysis of US Patent 7,995,047 under 35 U.S.C. § 103

This analysis identifies combinations of prior art references that would render the independent claims of US Patent 7,995,047 obvious to a person having ordinary skill in the art (PHOSITA) as of the priority date, December 13, 2006.

1. The Claimed Invention (from Independent Claims 1 and 10)

US Patent 7,995,047 addresses the problem of slow calibration in current driving devices, particularly when a reference current is very small or when the reference current changes, leading to non-uniformity in display output. The invention proposes a two-stage calibration process: a "voltage supply mode" where a voltage holding part is rapidly pre-charged by a voltage source, followed by a "current supply mode" where fine-tuning and accurate storage of a reference current value occur.

• Independent Claim 1 defines a current driving device with a first voltage supply, a first current supply, multiple output terminals, and multiple current output circuits. Each current output circuit includes a current-voltage converting circuit, a voltage-current converting circuit, and a voltage holding circuit (VHC). Crucially, the VHC has one terminal connected to a reference voltage different from the first voltage. The operation involves three modes:

  • Voltage Supply Mode: The VHC receives the first voltage from the first voltage supply to one of its terminals.
  • Current Supply Mode: The VHC receives the first electric current from the first current supply, which generates a second voltage via the current-voltage converting circuit, and this first current is also supplied to the same VHC terminal.
  • Current Output Mode: An output current is generated based on the voltage held in the VHC.

• Independent Claim 10 describes a specific circuit implementation for this current driving device, detailing elements such as a current input switch (Sw1), a voltage holding circuit (C1) with a first terminal to a fixed voltage and a second terminal, a calibration switch (Sw2), voltage-current converting elements (QN1-QNm), signal response switches (G1-Gm), a connection node (N1), current output switches (So1, So2), and a high-speed switch (Sw3). The calibration mode is explicitly divided into two stages:

  • Voltage Supply Mode: The high-speed switch (Sw3) is conductive, connecting a voltage supply source to the second terminal of the VHC (C1) to rapidly boost its potential. During this, output switches (So1, So2) are non-conductive, and the calibration switch (Sw2) and signal response switches (G1-Gm) are conductive.
  • Current Supply Mode: The high-speed switch (Sw3) is non-conductive, while the current input switch (Sw1) and calibration switch (Sw2) are conductive, allowing the current from the current supply source to flow and set the accurate reference voltage in the VHC.

2. Primary Prior Art Reference: US 2005/0231241 A1 (Matsushita et al.)

US 2005/0231241 A1, titled "Current driver," is explicitly identified in the background of US7995047 as a "conventional technique". This reference (hereinafter "Matsushita") discloses a current driving device for displays with a current output part, featuring both calibration and current output functions.

Matsushita teaches:

• A current output part A comprising a voltage holding capacitance element C1 (voltage holding part), Nch transistors QN1-QNm (which serve as both current-voltage converting elements during calibration and voltage-current converting elements during output), signal response switches G1-Gm, current input switch Sw1, calibration switch Sw2, reference current source I1 (first current supply part), and current output switches So1, So2, leading to output terminals T1, T2.
• A calibration mode where current output switches So1, So2 are non-conductive, and Sw1, Sw2, and G1-Gm are conductive. The drains and gates of QN1-QNm are effectively short-circuited, acting as a diode, allowing the reference current I1 to flow and charge C1 to a gate voltage (V(N2)) corresponding to this current. This directly corresponds to the "current supply mode" and the mechanism for generating a "second voltage" in US7995047.
• A current output mode where Sw1 and Sw2 are non-conductive, C1 holds the calibrated voltage, and QN1-QNm output current based on this voltage and the states of G1-Gm.
• The overall structure of a current driving device with multiple current output parts (A0-An) and multiple output terminals (OUT1-OUTn).

However, Matsushita suffers from the exact drawback that US7995047 aims to solve: "the capacity for charging/discharging the voltage holding capacitance element C1 becomes insufficient when the current value of the reference current source I1 is very small, so that it is difficult to charge the current of the Nch transistors QN1-QNm to accurately meet the value of the current from the reference current source I1 within the calibration period." This leads to slow calibration, as illustrated in FIG. 9 of US7995047.

3. Secondary Prior Art Reference: US 6,594,606 B2 (Clare Micronix)

US 6,594,606 B2, titled "Matrix element voltage sensing for precharge" (hereinafter "Clare Micronix"), is related to display technology, specifically active matrix liquid crystal displays (AMLCDs).

Clare Micronix teaches:

• The concept of pre-charging capacitive elements (e.g., data lines in a display) with a voltage to "substantially reduc[e] the data line charge time." This is done by applying a precharge to the data lines prior to sampling the data voltage.

4. Obviousness Combination and Motivation to Combine

Combination: US 2005/0231241 A1 in view of US 6,594,606 B2.

Motivation to Combine:
A person having ordinary skill in the art (PHOSITA) in the field of current driving devices for displays, faced with the problem of slow and sometimes inaccurate calibration in the Matsushita device (due to the limitations of charging a capacitive voltage holding element solely by a small reference current), would have been motivated to combine the teachings of Matsushita with Clare Micronix.

The motivation arises from the common objective in display technology to improve efficiency and speed, particularly in charging capacitive elements. Matsushita clearly demonstrates a current driving device with a voltage holding capacitor (C1) that is charged during calibration. US7995047 itself highlights that the limitation of Matsushita is the insufficient charging capacity when the reference current is small, leading to slow calibration.

Clare Micronix, operating in the closely related field of display driving, explicitly teaches a solution to the general problem of slow charging of capacitive elements: pre-charging them with a voltage to reduce charge time. A PHOSITA would readily recognize that the voltage holding capacitance element (C1) in Matsushita is a capacitive element that requires charging. Therefore, applying the known technique from Clare Micronix of rapidly pre-charging a capacitor with a voltage before a more precise, slower process would be an obvious solution to accelerate the calibration of C1 in Matsushita.

5. Application of the Combination to Independent Claims

A. Independent Claim 1:

1. First voltage supply source for supplying a first voltage: Clare Micronix teaches applying a "precharge" voltage to data lines; a PHOSITA would implement this with a voltage supply source.
2. First current supply source for supplying a first electric current: Taught by Matsushita's reference current source I1.
3. Plurality of output terminals & current output circuits (with current-voltage, voltage-current converting, voltage holding circuits): Taught by Matsushita. The Nch transistors QN1-QNm in Matsushita function as both current-voltage converters during calibration (generating V(N2) from I1) and voltage-current converters during output (converting V(N2) to output current). C1 is the voltage holding circuit, with one terminal connected to ground (a reference voltage).
4. Operation modes:
   • Voltage Supply Mode: The PHOSITA, applying Clare Micronix's pre-charging concept to Matsushita's C1, would introduce an initial "voltage supply mode" where C1 (the VHC) receives the "first voltage" from the newly added voltage supply, supplied to its ungrounded terminal (Node N2 in Matsushita's Fig. 6A).
   • Current Supply Mode: This mode is directly taught by Matsushita's calibration mode, where C1 receives the "first current" from I1, and the Nch transistors (current-voltage converting circuit) generate the "second voltage" held in C1.
   • Current Output Mode: This mode is directly taught by Matsushita's current output mode.

B. Independent Claim 10:
Most elements of Claim 10 (current input switch Sw1, voltage holding circuit C1, calibration switch Sw2, voltage-current converting elements QN1-QNm, signal response switches G1-Gm, connection node N1, current output switches So1/So2) are directly taught by Matsushita's circuit shown in FIG. 6A.

The remaining elements, crucial to the inventive concept, are:

• High-speed switch (Sw3) for controlling connection/disconnection states of said voltage supply source with respect to the second terminal of said voltage holding circuit: A PHOSITA, motivated to implement the pre-charging taught by Clare Micronix, would introduce a standard switch (Sw3) to connect the added voltage supply source to Node N2 (the second terminal of C1) in Matsushita's circuit. This is a conventional way to switch a voltage supply.
• Calibration Mode (two stages):
  • Voltage Supply Mode: The PHOSITA would logically define this new mode where Sw3 is conductive (connecting the voltage supply to N2 to boost potential). The conditions that "all current output switches are non-conductive, and the calibration switch and all signal response switches are conductive" are either taught by Matsushita's calibration (non-conductive output switches) or represent straightforward design choices to facilitate charging and prepare for the subsequent current supply mode (conductive Sw2 and G1-Gm).
  • Current Supply Mode: The PHOSITA would then transition to the current supply mode, as taught by Matsushita's calibration (Sw3 non-conductive to isolate the pre-charge, Sw1 and Sw2 conductive for precise current calibration).

6. Conclusion

The combination of US 2005/0231241 A1 and US 6,594,606 B2, driven by the motivation to overcome the known problem of slow calibration in display drivers using a conventional and well-understood technique of voltage pre-charging for capacitive elements, would have rendered the subject matter of independent Claims 1 and 10 of US Patent 7,995,047 obvious to a PHOSITA at the time of the invention.