Electricity: power supply or regulation systems – In shunt with source or load – Using choke and switch across source
Reexamination Certificate
2000-09-15
2001-10-02
Sterrett, Jeffrey (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using choke and switch across source
C323S282000
Reexamination Certificate
active
06297622
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a power supply circuit for feeding power with a small loss, and an electro-optical device to which power is fed using the power supply circuit and which is characterized by low power consumption.
2. Description of Related Art
To begin with, a description will be made of a conventional power supply circuit, for example, a power supply circuit for producing a high voltage Vout from a low voltage Vin and supplying the high voltage Vout.
FIG. 9
is a block diagram showing the circuitry of a conventional power supply circuit
400
. In the drawing, a switch
412
is realized with a transistor in practice, and turned on when a signal CTR that is a gate signal is driven high. One terminal of the switch
412
is connected on a line, over which the voltage Vin is applied, via an inductor (coil)
414
. The other terminal thereof is connected on a ground line over which a reference potential GND is applied. One terminal of the switch
412
is connected to one terminal of a capacitor
418
through the conduction in a forward direction of a diode
416
. Consequently, a held voltage is output as the voltage Vout The other terminal of the capacitor
418
is connected on the ground line.
The output voltage Vout developed at one terminal of the capacitor
418
has a fraction thereof developed due to resistors
420
and
422
. The fractional voltage is applied to a negative input terminal of a comparator
424
. For a clear description, a voltage to be applied to the negative input terminal of the comparator
424
shall be a voltage Vout′. A reference voltage Vref is applied to a positive input terminal of the comparator
424
. When the voltage Vout′ falls below the reference voltage Vref, an output signal Cow of the comparator
424
is, as shown in
FIG. 10
, driven high. In contrast, when the voltage Vout′ exceeds the reference voltage vref, the output signal CMP is driven low. When the output signal CMP of the comparator
424
is driven high, a control circuit
426
outputs, as shown in
FIG. 10
, a pulsating signal CTR having a certain pulse width W.
Next, the action of the power supply circuit
400
having the foregoing components will be described. First, the switch
412
is turned on. This causes a current Ion to flow from the line, over which the voltage Vin is applied, through the inductor
414
towards the ground line. Energy is accumulated in the inductor. When the switch
412
is turned off, an off current loff flows through the inductor
414
. The accumulated energy is added to the voltage Vin through the conduction in the forward direction of the diode
416
according to the series feed. The energy thus moves to the capacitor
418
. When the energy accumulated in the inductor
414
has entirely moved to the capacitor
418
, the diode
416
is inversely biased. Therefore, the energy accumulated in the capacitor
418
will not flow backward through the diode
416
.
On the other hand, the output voltage Vout drops gradually according to the magnitude of a load. As shown in
FIG. 10
, when the voltage Vout′ falls below the Ad reference voltage Vref, the output signal CMP of the comparator
424
makes a low-to-high transition. This causes the control circuit
426
to output the pulsating signal CTR. The switch
412
is then turned on. After energy is accumulated in the inductor
414
, the switch
412
is turned off, and the energy moves to the capacitor
418
. Consequently, the output voltage Vout rises. In other words, when the voltage Vout′ falls below the reference voltage Vref, control is given to raise the output voltage Vout.
When the output voltage Vout rises, the voltage Vout′ may exceed the reference voltage Vref. In this case, the output signal CMP of the comparator
424
remains low. The switch
412
is not therefore turned on or off, and the capacitor
418
discharges according to a load. Consequently, the output voltage Vout drops gradually. In other words, when the voltage Vout′ exceeds the reference voltage Vref, the discharge of the capacitor
418
causes control to be given for lowering the output voltage Vout.
As a whole, the output voltage Vout is stabilized when control given for raising the output voltage Vout and a control given for lowering it are balanced, that is, when the voltage Vout′ becomes equal to the reference voltage Vref. Herein, the voltage Vout′ is a fraction of the output voltage Vout developed due to the resistors
420
and
422
. Assuming that the resistances of the resistors
420
and
422
are R
1
and R
2
, Vout′=Vout×R
2
/(R
1
+R
2
) is established. The voltage Vout′ is stabilized when becoming equal to the reference voltage Vref In other words, the power supply circuit
400
boosts the voltage Vin and outputs the voltage Vout stabilized when becoming equal to Vref(R
1
+R
2
)/R
2
.
SUMMARY OF THE INVENTION
In the foregoing circuitry, a current always flows from the other terminal of the capacitor
418
, which serves as the output terminal of the power supply circuit, through the resistors
420
and
422
towards the line over which the ground potential GND is applied. This may pose a problem in that a large magnitude of power consumption occurs in the power supply circuit If the resistances of the resistors
420
and
422
were increased, the problem would be solved. However, it is hard to form a resistor exhibiting a high resistance on an IC chip. Otherwise, the resistor exhibiting a high resistance cannot help being large in size. This may be disadvantageous in realizing a compact and simple power supply circuit in the form of an integrated circuit. Moreover, when the resistances of the resistors
420
and
422
are increased, another problem may occur in that the power supply circuit becomes susceptible to noises.
Moreover, from the viewpoint of simple circuitry, a voltage “Vin-GND” that has not been boosted should be used as a supply voltage to be applied to the control circuit
426
and comparator
424
. When the voltage “Vin-GND” is used as the supply voltage to be applied to the comparator
424
, the voltage Vout′ and reference voltage Vref with which the voltage is compared must fall below “Vin-GND.” Noted is that when the voltage “Vin-GND” is used as the supply voltage to be applied to the comparator
424
in an effort to realize simple circuitry, the voltage must not be compared directly with the boosted voltage Vout. Namely, a lower voltage (fractional voltage) produced by stepping down the voltage Vout must be used as a voltage with which the voltage “Vin-GND” is compared.
The present invention attempts at least to break through the foregoing situation. An object of the present invention is to at least provide a power supply circuit capable of minimizing power consumption and contributing to a compact and simple design, and to also provide an electro-optical device using the power supply circuit.
According to an exemplary aspect of the present invention, there is provided a power supply circuit for converting a first voltage that is an input voltage into a second voltage that is an output voltage and outputting the second voltage. The power supply circuit preferably has two or more resistive elements, a pair of first switching elements, and a holding element. The two or more resistive elements cause a fraction of the second voltage to develop. The pair of first switching elements is turned on or off in order to enable or disable flow of a current through the two or more resistive elements. When the pair of first switching elements is turned on, the holding element holds the fractional voltage developed due to the resistive elements. In the power supply circuit, control is given so that the voltage held by the holding element will be equal to a reference voltage. Owing to these constituent features, only when the pair of first switching elements is turned on, a current flows through the two or more resistive elements, causing a fraction of the second voltage
Seiko Epson Corporation
Sterrett Jeffrey
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