Abstract:
An adaptive LCD power supply circuit for adjusting at least one supply parameter in response to at least one load parameter of an associated load includes a feedback path configured to sense at least one load parameter and provide a control signal representative of at least one load parameter, and a regulating circuit configured to accept the control signal and regulate at least one supply parameter based on the control signal. The feedback path may include a first path to provide a first signal representative of a load condition of a first module, and a second path to provide a second signal representative of a load condition of a second load module, and a minimum decision circuit to output a third signal equal to the lesser of the first and second signals. An electronic device including an adaptive LCD power supply circuit is also provided.

Description:
FIELD OF THE INVENTION 
   This invention relates to power supplies, and more particularly to an adaptive power supply circuit for adjusting at least one supply parameter for an associated load such as a Liquid Crystal Display (LCD) panel of an electronic device. 
   BACKGROUND OF THE INVENTION 
   Liquid Crystal Display (LCD) screens are used in a variety of devices given their size, relatively low cost, and high resolution. Such devices typically include portable electronic devices and non-portable electronic devices. Portable electronic devices may include laptop computers, cell phones, pagers, personal digital assistants, and the like, while non-portable electronic devices make include televisions, desktop PCs, industrial controls, and the like. While such non-portable electronic devices consume modest amounts of power individually, in combination they consume a significant amount of power. Any improvements therefore in the energy efficiency of such non-portable devices are therefore desirable. 
   Manufacturers are also constantly striving to reduce power consumption of portable electronic devices without degradation of performance. Such portable devices are typically powered by some type of battery, but may be powered with other voltage sources such as a solar source. In the case of a rechargeable battery powered portable electronic devices, reduced power consumption enables the device to operate for longer periods of time between charging of the battery and/or to have a smaller size battery. 
   Typically, the LCD panel that drives an LCD of an electronic device and its backlight receives voltage directly from the power source of the electronic device. For example, the power source may be a battery for a portable electronic device. This decreases energy efficiency for such devices when select load modules such as the backlight and/or the LCD panel do not need the full available voltage from the power source. 
   Accordingly, there is a need in the art for an adaptive power supply circuit that overcomes the above deficiencies in the prior art to allow for appropriate power supply levels to be applied to select load modules. 
   BRIEF SUMMARY OF THE INVENTION 
   A power supply circuit consistent with the invention for adjusting at least one supply parameter in response to at least one load parameter of at least one associated load includes: a feedback path configured to sense at least one load parameter and provide a control signal based on at least one load parameter; and a regulating circuit configured to accept the control signal and regulate the at least one supply parameter based on the control signal. The feedback path may include a first path providing a first signal representative of a first load condition of a first load module and a second path providing a second signal representative of a second load condition of a second load module, wherein the control signal is based on a comparison of the first signal and the second signal. 
   An electronic device consistent with the invention includes: a liquid crystal display (LCD) panel circuit; and an adaptive LCD power supply circuit for adjusting at least one supply parameter in response to at least one load parameter of at least one associated load, wherein one of at least one associated loads is an LCD panel circuit, the adaptive LCD power supply circuit includes: a feedback path configured to sense at least one load parameter and provide a control signal based on at least one load parameter; and a regulating circuit configured to accept the control signal and regulate the at least one supply parameter based on the control signal. 
   An adaptive LCD power supply circuit for adjusting at least one supply parameter in response to a load condition from one of a plurality of load modules, the adaptive LCD power supply circuit consistent with the invention includes: a first path providing a first signal representative of a load condition of a first load module; a second path providing a second signal representative of a load condition of a second load module; a minimum decision circuit configured to accept the first signal and the second signal and output a third signal equal to the lesser of the first signal and the second signal; and a regulating circuit configured to accept a control signal based on the third signal and regulate the at least one supply parameter based on the control signal. 
   A method of adjusting at least one supply parameter in response to at least one load parameter of an LCD panel circuit consistent with the invention includes the steps of: sensing at least one load parameter; providing a signal representative of at least one load parameter; and adjusting at least one supply parameter based on the signal. 
   Another method of adjusting at least one supply parameter consistent with the invention includes the steps of: sensing at least one load parameter from a first load module and providing a first signal representative of at least one load parameter from the first load module; sensing at least one load parameter from a second load module and providing a second signal representative of at least one load parameter from the second load module; comparing the first signal and the second signal and providing a third signal equal to the lesser of the first signal and the second signal; and adjusting at least one supply parameter based on the third signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, together with other objects, features and advantages, reference should be made to the following detailed description which should be read in conjunction with the following figures wherein like numerals represent like parts: 
       FIG. 1  is a block diagram of an electronic device having an adaptive LCD power supply circuit consistent with the invention; 
       FIG. 2  is a simplified block diagram of the adaptive LCD power supply circuit of  FIG. 1 ; and 
       FIG. 3  is a more detailed block diagram of one exemplary adaptive LCD power supply circuit consistent with the invention. 
   

   DETAILED DESCRIPTION 
   Turning to  FIG. 1 , a simplified block diagram of an electronic device  100  having an adaptive LCD power supply circuit  104  consistent with the invention is illustrated. The electronic device  100  may be a portable or non-portable electronic device utilizing an LCD  108 . Portable electronic devices may include laptop computers, cell phones, pagers, personal digital assistants, and the like, while non-portable electronic devices may include televisions, desktop PCs, automotive controls, industrial controls, and the like. 
   The electronic device  100  includes a power source  102  for supplying power to all the components of the device  100  including the LCD  108  driven by the LCD panel circuit  106 . The LCD  108  may be any variety of display, e.g., an active matrix or passive matrix display, and the LCD panel circuit  106  may be any type of driving circuit known to those skilled in the art. The power source  102  may be any variety of power sources for providing power to the electronic device. For portable electronic devices, the power source  102  may be a stand alone power source such a rechargeable battery, e.g., lithium, nickel-cadmium, or nickel-metal hydride batteries, or a solar power source. The power source may also be various external adapters such as an AC/DC “block” adapter or DC “cigarette” type adapter to provide power to the portable electronic device. Such adapters may also provide power to recharge batteries for those devices having rechargeable batteries. For non-portable electronic devices the power supply may be an AC/DC converter for converting conventional 120 volt AC power from an outlet to a DC voltage level. 
   Advantageously, an electronic device  100  consistent with the invention includes an adaptive LCD power supply circuit  104  coupled to the power source  102  and the LCD panel circuit  106 . In general, the adaptive LCD power supply circuit  104  monitors at least one load parameter from the LCD panel circuit  106  and regulates at least one supply parameter supplied from the power source  102  such that the supply parameter is adjusted to match the load parameter. 
   In one exemplary embodiment, the load parameter may be a load voltage and the supply parameter may be supply voltage. In this way, the adaptive LCD power supply circuit  106  provides adaptive or load following voltage to the LCD panel circuit  106  that follows the instantaneous voltage requirements of the LCD panel circuit  106  as display conditions on the LCD  108  vary. As such, power dissipation is minimized. Therefore, the power efficiency of the device  100  is substantially improved. For a portable electronic device having a rechargeable battery for a power source  102 , this enables the device to have prolonged times between recharging of its battery, and/or to have a smaller size battery. 
   Turning to  FIG. 2 , a simplified block diagram of an adaptive LCD power supply circuit  204  consistent with the invention is illustrated. The adaptive LCD power supply circuit  204  includes a feedback path. The feedback path may include the sensor  205  and a conducting path  209  coupling the sensor  205  to the regulating circuit  207 . The feedback path may be configured to sense at least one load parameter of the LCD panel circuit  206  and provide a feedback control signal along the conducting path  209  to the regulating circuit  207 . The regulating circuit  207  may be responsive to the control signal to adjust at least one supply parameter from the power source  202  such that the supply parameter is driven to match the load parameter. 
   The regulating circuit  207  may be a variety of circuits known to those skilled in the art for regulating a predetermined power parameter. For instance, the regulating circuit may be a DC/DC converter where the supply parameter to be adjusted is DC voltage. Such a regulating circuit  207  may also include a pulse width modulated (PWM) switching transistor circuit functioning as a DC-DC converter. The PWM signal may be generated by a comparator comparing the output signal from a respective error amplifier, e.g., error amplifier  334  of  FIG. 3 , with an input ramp signal. The resulting PWM signal may then control a switching circuit to boost, e.g., with the switching transistor in parallel with the load, or buck, e.g., with a switching transistor in series with the load, the input DC voltage to the desired output DC voltage level. 
   Turning to  FIG. 3 , a block diagram of one exemplary embodiment of an adaptive LCD power supply circuit  304  is illustrated. In this exemplary embodiment, the sensed load parameter is DC load voltage and the regulated supply parameter is DC supply voltage. A feedback path to the DC/DC converter  303  may include two paths. One path may include a light source such as the Light Emitting Diode (LED)  318 , a current source  319 , and a switch  313 . This path provides a signal to the minimum decision circuit  320  at the input terminal In 1 . The current source  319  sets the current for the LED  318 . The LED  318  may function as a backlight for an LCD panel. Another path may include a sensor, e.g. transistor Q 1 , to provide another signal to the input terminal In 2  of the minimum decision circuit  320 . The first path provides a signal to the minimum decision circuit  320  which is representative of the voltage across the current source  319  when the switch  313  is closed and representative of the voltage output from the DC/DC converter when the switch  313  is open. The other path provides another control signal to the minimum decision circuit  320  representative of the voltage across transistor Q 1 . 
   The transistor Q 1  may be a variety of transistor types including a p-channel MOSFET. An LDO control  332  may be coupled to the gate of the transistor Q 1  to control the state of the transistor Q 1  and whether or not power is supplied to a load at the output terminal  330 . When power is provided by the power source  302  to a load, e.g., an LCD panel circuit, at output terminal  330 , the voltage Vpass across the source and drain of the transistor Q 1  varies in proportion to load changes and voltage requirements. Hence, as the instantaneous load requirements of an LCD load panel circuit (not illustrated) which may be coupled to the output terminal  330  changes, the voltage level Vpass across the source and drain changes proportionately. The voltage level Vpass may then be input to a sense amplifier  310 , which amplifies the voltage level relative to ground and may then be input to a second input terminal of the minimum decision circuit  320 . 
   The minimum decision circuit  320  compares the voltage signal from the first path input to input terminal In 1  with the voltage signal from the other path input to its other input terminal In 2 , and provides an output signal equal to the lesser of the two voltage levels to an error amplifier  334 . The error amplifier  334  compares a reference voltage level provided by a reference voltage source  336  to the voltage level output from the minimum decision circuit  320  to provide a control signal to the DC/DC converter  303 . When the output of the minimum decision circuit  320  is equal to the reference voltage signal provided by the reference source  336  then the control signal from the error amplifier  334  instructs no change to be made to the output voltage of the DC/DC converter  303 . If the output of the minimum decision circuit  320  is less than the reference voltage, then the control signal instructs the DC/DC converter  303  to increase its output voltage. Conversely, if the output of the minimum decision circuit  320  is greater than the reference voltage, then the control signal instructs the DC/DC converter  303  to decrease its output voltage. 
   In the exemplary embodiment of  FIG. 3 , the LED  318  provides backlight to an LCD. If the LED is on and the switch  313  is accordingly closed, an LCD panel circuit coupled to the output terminal  330  may require a lower voltage than the voltage required by the LED. As such, the voltage drop across Q 1  is relatively large and the output of the DC/DC converter  303  is controlled by the first path to maintain the voltage across the current source  319  equal to the reference voltage provided by the reference voltage source  336 . Alternatively, if the LED is off and the switch  313  is accordingly open, the output of the DC/DC converter  303  may be controlled by the second path to maintain the voltage drop across the transistor Q 1  equal to the reference voltage provided by the reference voltage source  336 . In order to minimize power losses, a minimum voltage drop across transistor Q 1  and the current source  319  set by the reference source  336  should be maintained. An adaptive liquid crystal display power supply circuit consistent with the invention keeps one of these two voltage levels equal to the reference voltage level depending on the relative signal levels input at In 1  and In 2  to the minimum decision circuit  320 . 
   For example, assume the reference voltage level provided by the reference source  336  is 0.2 volts, the voltage required at output terminal  330  is 3.3 volts at a particular instant, and the voltage drop on the LED  318  functioning as a backlight to an LCD is 8.0 volts. 
   If the LED  318  is enabled in this particular example, switch  313  is closed. As such, the voltage at the output of the DC/DC converter  303  needs to be 8.2 volts to properly supply the voltage for the backlight and to account for the voltage drop of about 0.2 volts across the current source  319 . In this instance, the voltage drop across Q 1  is higher at 4.9 volts such that the desired voltage of 3.3 volts is provided at the output terminal  330 . Since the voltage drop across the current source  319  in this instance (0.2 volts), is less than the voltage drop across transistor Q 1  (4.9 volts), the minimum decision circuit will select 0.2 volts and the control of the DC/DC converter will be controlled by the first path input to the input terminal In 1  of the minimum decision circuit  320 . 
   If the LED  318  is disabled in this particular example, switch  313  is open. As such, the voltage at the output of the DC/DC converter  303  may be advantageously decreased to 3.5 volts compared to a voltage level of 8.2 volts that may otherwise be provided at all times. Since the voltage drop across the transistor Q 1  of about 0.2 volts is less than the voltage output from the DC/DC converter  303  of 3.5 volts, the minimum decision circuit will select 0.2 volts and the control of the DC/DC converter will be controlled by the other path input to the input terminal In 2  of the minimum decision circuit  320 . 
   As such, the DC/DC converter  303  is advantageously responsive to the control signal from the error amplifier  334  to adjust its output DC voltage level to match that of the DC load voltage level. As such, an adaptive LCD power supply circuit  304  consistent with the invention includes a regulating circuit, e.g., DC/DC converter  303 , that regulates or adjusts its secondary DC voltage to match that of the instantaneous DC load voltage requirements of a load module such as the LED  318  serving as a backlight for an LCD or a separate load module such as an LCD panel circuit coupled to the output terminal  330  depending on a comparison made by the minimum decision circuit  320 . 
   The embodiments that have been described herein, however, are but some of the several which utilize this invention and are set forth here by way of illustration but not of limitation. It is obvious that many other embodiments, which will be readily apparent to those skilled in the art, may be made without departing materially from the spirit and scope of the invention.