United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,986,437
`
`Lee
`[45] Date of Patent:
`Nov. 16, 1999
`
`
`U8005986437A
`
`[54] POWER SUPPLY SYSTEM FOR PORTABLE
`ELECTRONIC DEVICES
`
`[75]
`
`Inventor: Chang-Hum Lee, Anyang-shi, Rep. of
`Korea
`.
`.
`[73] Assignee: SamSung Electronlcs CO., Ltd.,
`SuW0n> Japan
`
`[21] App1.NO.:08/992,098
`
`[22]
`
`Filed:
`
`Dec. 17, 1997
`
`[30]
`
`Foreign Application Priority Data
`,
`Rep. ot Korea ...................... 96—66818
`[KR]
`Dec. 17, 1996
`[51]
`Int. Cl.6 ........................................................ H02J 7/00
`[52] US. Cl.
`............................................. 320/162; 320/125
`[58] Field of Search ..................................... 320/161, 162,
`320/1 63, 1 25
`
`[56]
`
`References CitEd
`U.S. PATENT DOCUMENTS
`
`4,843,299
`5,049,804
`5,150,031
`571687205
`5,175,485
`5,241,259
`5,382,893
`5,411,816
`
`6/1989 Hutchings.
`9/1991 Hutchings.
`9/1992 James et 211.
`12/1992 Kan et al'
`'
`12/1992 Joo.
`8/1993 Patino et al.
`1/1995 Dehnel.
`5/1995 Patino .
`
`~
`
`.
`
`.
`
`11/1995 Narita et a1.
`5,465,039
`12/1995 Brainard.
`5,479,083
`1/1996 Qualich et al. .
`5,481,175
`7/1996 Sengupta et a].
`5,541,490
`7/1996 Fernandez et a1.
`5,541,492
`10/1996 Sengupta et a].
`5,561,361
`5,568,038 10/1996 r t
`‘
`.
`5,592,069
`“1997 1321;511:1211”
`5,625,275
`4/1997 Tanikawa et al.
`5,637,981
`6/1997 Nagai et a1.
`.
`5,642,029
`6/1997 Seragnoli.
`5,739,667
`4/1998 Matsuda et a1.
`
`.
`
`.
`
`.
`
`.
`
`........................ 320/162
`
`Primary Examiner—Peter S. Wong
`Assistant Examiner—K. Shin
`Attorney, Agent, or Firm—Robert E. Bushnell, Esq.
`_
`ABSTRACT
`[37]
`A power supply system for a portable device such as
`portable personal computer, to be operated from a DC power
`source supplied from a rechargeable battery or an AC
`adapter includes: an input current detecting circuit for
`detecting an input current supplied from the AC adapter and
`for generating a detection s1gna1 haV1ng a magnitude Wthh
`varies depending upon the input current, and a charging
`current control circuit for controlling the amount of the
`charging current in response to the detection signal. The
`power supply system has low power losses and effects a high
`efficiency battery charging.
`
`6 Claims, 5 Drawing Sheets
`
`100v~ 240v
`AC --
`
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`Adapter
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`

`

`US. Patent
`
`Nov. 16,1999
`
`Sheet 1 0f5
`
`5,986,437
`
`
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`2
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`US. Patent
`
`Nov. 16,1999
`
`Sheet 2 0f 5
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`US. Patent
`
`Nov. 16,1999
`
`Sheet 3 0f5
`
`5,986,437
`
`Fig. 3
`
`Current(A)
`2.0A
`
`AC Adapter_
`Input Current
`
`1.5A
`
`Current(A)
`2.0A
`
`AC Adapter_
`Input Current
`
`1.5A
`
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`PoweriOn
`
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`US. Patent
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`Nov. 16,1999
`
`Sheet 5 0f 5
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`5,986,437
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`5,986,437
`
`1
`POWER SUPPLY SYSTEM FOR PORTABLE
`ELECTRONIC DEVICES
`
`CLAIM OF PRIORITY
`
`incorporates the
`This application makes reference to,
`same herein, and claims all benefits accruing under 35
`U.S.C. §119 from an application for POWER SUPPLY
`SYSTEM FOR PORTABLE ELECTRONIC DEVICES ear-
`lier filed in the Korean Industrial Property Oflice on Oct. 17,
`1996 and there duly assigned Serial No. 66818/1996.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention generally relates to a power supply
`system for portable electronic devices provided with
`rechargeable batteries, and more particularly, to a system for
`charging a rechargeable battery in a portable computer.
`2. Related Art
`
`Rechargeable batteries are commonly used to supply
`power to portable electronic devices, such as personal
`computers, wordprocessors, a variety of portable handsets
`and so on when conventional power outlets are not available.
`An AC (alternating current) adapter is also typically pro-
`vided to power a portable device when the user does have
`access to a power outlet. The AC adapter further provides
`power to a battery charging circuit, which recharges the
`batteries, of the portable device.
`When a rechargeable battery is used as a power supply
`system for portable personal computers as disclosed, for
`example, in US. Pat. No. 5,561,361 for Computer Power
`Supply And Battery Recharging System issued to Sengupta
`et al., the battery may be charged by the battery charger in
`two different charging modes, that is, a run and charging
`mode and a run or charging mode. During the run and
`charging mode, the battery is charged regardless whether the
`computer is operating or not, and as a result, the charging
`operation may be difficult to control. In contrast to the run
`and charging mode, the run or charging mode allows the
`battery to be charged selectively depending upon whether or
`not the computer is operating.
`Generally, charging operations of such a battery may be
`classified into four modes, that is, a fast or rapid charging,
`a quick charging, a standard charging and a trickle charging,
`in relation to the charging capacities or charging current of
`the battery.
`In the fast charge mode, a relatively large
`charging current (e.g., about 2 amperes or so) is supplied to
`a battery over a relatively short period of time (e.g., one or
`two hours). In the quick charge mode, a relatively large
`charging current ( e.g., about 1 ampere to high hundreds of
`milliamperes or so) is supplied to the battery over a rela-
`tively short period of time (e.g., four to six hours). In the
`standard charge mode, a relatively small charging current
`(e.g., about low hundreds of milliamperes or so) is supplied
`to the battery over a relatively long period of time (e.g. eight
`to ten hours). In the trickle charge mode, the battery charg—
`ing circuit provides a relatively even smaller constant cur-
`rent (e.g., about 100 milliamperes or so) to the battery
`without interruption. In this mode, the battery may not be
`charged but
`its power loss due to natural discharge is
`compensated for. During all the charging operations,
`the
`charge level of the battery is monitored so that, when the
`battery reaches its maximum charge level,
`the charging
`operation is terminated in order to prevent overcharging and
`damage of the battery. Exemplars of contemporary battery
`chargers are disclosed in US. Pat. Nos. 4,843,299 and
`
`5
`
`10
`
`15
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`5,049,804 for Universal Battery Charging System issued to
`Hutchings, US. Pat. No. 5,168,205 for Method And Appa-
`ratus For Charging A Battery In High Amp And Automatic
`Charging Modes issued to Kan et al., US. Pat. No. 5,175,
`485 for Apparatus For Controlling Charging Of A Storage
`Battery issued to Joo, US. Pat. No. 5,241,259 for Method
`and Apparatus For Rapidly Charging A Battery At High
`Temperature issued to Patino et al., US. Pat. No. 5,411,816
`for Method And Apparatus For Determining Battery Char-
`acteristics issued to Patino, US. Pat. No. 5,479,083 for
`Non-Dissipative Battery Charger Equalizer issued to
`Brainard, US. Pat. No. 5,541,492 for Method For Charging
`A Multiple Voltage Electrochemical Cell issued to Fernan-
`dez et al., US. Pat. No. 5,592,069 for Battery Charger issued
`to Dias et al., US. Pat. No. 5,637,981 for Method For
`Charging A Secondary Battery And Charge Used Therefor
`Using Constant Current And Constant Voltage issued to
`Nagai et al., and the co-pending Korean application Ser. No.
`96-45546 filed by the present applicant.
`The following additional patents each disclose features in
`common with the present invention: US. Pat. No. 5,465,039
`to Power Supply For Electronic Device, And Electronic
`Device System issued to Narita et al., US. Pat. No. 5,625,
`275 for Power Supply Which ProvidesA Variable Charging
`Current To A Battery In A Portable Computer System issued
`to Tanikawa et al., US. Pat. No. 5,481,175 for System And
`Method For Charging Auxiliary Batteries issued to Qualich
`et al., US. Pat. No. 5,462,029 for Constant Current Battery
`Charger With Auxiliary Output For Portable Apparatus
`issued to Seragnoli, US. Pat. No. 5,541,490 for Computer
`Power Supply System, issued to Sengupta et al., US. Pat.
`No. 5,568,038 for Portable Electric Equipment And
`Rechargeable Built-In Batteries issued to Tatsumi, US. Pat.
`No. 5,382,893 for Maximum Power Regulated Battery
`Charger issued to Dehnel, and US. Pat. No. 5,150,031 for
`Battery Charging System issued to James et al.
`SUMMARY OF THE INVENTION
`
`Accordingly, it is an object of the present invention to
`provide a novel power supply system for a portable elec-
`tronic device having a rechargeable battery, which adjusts
`the battery charging current depending upon the current
`consumed by the portable device.
`It is another object of the invention to provide a portable
`device power supply system having low power loss and high
`efficiency battery charging.
`It is still another object of the invention to provide a
`portable device power supply system capable of charging a
`rechargeable battery faster and reducing the charging time of
`the battery without enlarging the size of an AC adapter.
`These and other objects of the present invention can be
`achieved by a power supply system for a portable electronic
`device operable on an internal rechargeable battery, which
`comprises: an AC adapter for supplying a variable amount of
`input power including a variable amount of system power
`for operating the portable device and a variable amount of
`charging power for charging the battery, a battery charging
`circuit for supplying a variable amount of charging current
`flowing through the output terminal to the battery, an input
`current detecting circuit for detecting an input current sup-
`plied from the AC adapter and for generating a detection
`signal having a magnitude which varies depending upon the
`input current, and a charging current control circuit for
`controlling the amount of the charging current in response to
`the detection signal.
`In an embodiment, the charging current control circuit
`controls the charging current so as to be constant when the
`
`7
`
`

`

`5,986,437
`
`3
`amplitude of the detection signal is less than a predeter-
`mined threshold level, and so as to decrease the amount of
`the charging current depending upon the amplitude of the
`detection signal when the amplitude of the detection signal
`is not less than the predetermined threshold level.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Amore complete appreciation of the invention, and many
`of the attendant advantages thereof, will be readily apparent
`as the same becomes better understood by reference to the
`following detailed description when considered in conjunc-
`tion with the accompanying drawings in which like refer-
`ence symbols indicate the same or similar components,
`wherein:
`
`FIG. 1 is a schematic drawing showing an exemplary
`portable electronic device operable on a battery;
`FIG. 2 is a circuit diagram of an earlier portable device
`power supply system having a battery IS charging circuit for
`charging a rechargeable battery;
`FIG. 3 is a schematic representation illustrating a typical
`battery charging operation of the earlier power supply sys-
`tem'
`
`FIG. 4 is a block diagram and FIG. 5 is a detailed circuit
`diagram of a novel portable device power supply system
`according to a preferred embodiment of the present inven-
`tion; and
`FIG. 6 is a schematic representation illustrating the charg-
`ing operation of the power supply system of FIGS. 4 and 5.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`
`While the invention has been described in terms of an
`
`exemplary embodiment, it is contemplated that it may be
`practiced as outlined above with modifications within the
`spirit and scope of the appended claims.
`FIG. 1 is a schematic drawing showing an exemplary
`portable electronic device operable on a battery as noted in
`the Background of the Invention above. Rechargeable bat-
`teries 8 are commonly used to power portable electronic
`devices 6 such as personal computers, wordprocessors, a
`variety of portable handsets and so on when conventional
`power outlets 2 are not available. An AC adapter 4 is also
`typically provided to power a portable device when the user
`does have access to a power outlet 2. The AC adaptor further
`provides power to a battery charging circuit which recharges
`the batteries 8 of the portable device.
`FIG. 2 illustrates a power supply system for portable
`computers. Referring to FIG. 2,
`the portable computer
`power supply system includes an AC adapter 4 which
`converts AC voltage to a stable DC (direct current) voltage.
`The AC adapter 4 is connected to a DC/DC converter 10.
`The DC/DC converter 10 operates in a conventional manner
`to supply power for operating a system unit 20 of the
`portable computer system having various components. The
`AC adapter 4 is also connected to a battery charging circuit
`30 for recharging a battery pack 8. The battery charging
`circuit 30 includes a microcomputer 130 for controlling the
`overall charging operation of the battery pack 8. This
`microcomputer 130 is supplied with power by a regulator
`circuit 110. The battery charging circuit 30 further includes
`an adapter voltage detecting circuit 120, a battery voltage
`detecting circuit 140, and a constant current charging control
`circuit 150.
`
`The constant current charging control circuit 150 includes
`a switching circuit 151, an energy storing circuit 152 for
`
`10
`
`15
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`storing the electrical energy coming from the switching
`circuit 151, a charging current detecting circuit 153 for
`detecting the charging current that flows from the switching
`circuit 151 to the battery pack 8 using the voltage drop
`across the resistor RI 53 and outputting a current detection
`signal having an amplitude corresponding to the detected
`current, and a pulse width modulation (PWM) switching
`controller 154 which controls the ON/OFF time of the
`transistor 0152 within the switching circuit 151 by output-
`ting a pulse signal which has a duty cycle corresponding to
`the voltage level of its feedback terminal FB connected to
`the output terminal of the charging current detecting circuit
`153. The charging current flowing from the input terminal 5
`to the output terminal 7 is controlled according to the control
`voltage which is inputted to the feedback terminal FB of the
`PWM controller 154.
`
`In most portable computer systems, the batteries may be
`charged in two charging modes, that is, the fast charging
`mode when the computer systems are powered off and the
`quick charging mode when powered on. The computer
`power supply system thus supplies two constant levels of
`battery charging current. To ensure that there is always
`suflicient current available for charging the battery when the
`computer system is operating,
`the level of the charging
`current outputted by the battery charger must be selected in
`accordance with the maximum current which could be
`
`drawn by the system. Therefore, the AC adapter must be
`designed for the situation in which the computer is using
`maximum power.
`FIG. 3 is a schematic representation which illustrates a
`typical battery charging operation of such a computer power
`supply system. Referring to FIG. 3, all of the input current
`51 from the AC adapter 4 is used for charging the battery 8
`until the system unit 20 is turned on. When the system unit
`20 is turned on,
`the charging current
`is decreased to a
`constant current S3, considering a maximum system current
`S2 drawn by the system unit 20.
`However, the system unit 20 does not always consume its
`maximum current S2 when it
`is operating. That
`is,
`the
`current 52a consumed by the system unit 20 varies in
`accordance with the operating status of the system unit 20.
`In the power supply system, thus, there is a remainder S2b
`of the current that is not used by either the system unit 20 or
`the battery charging circuit 30. As a result, a power loss and
`inefficient battery charging in the power system occur. If it
`is also intended to charge the battery faster, the size of the
`AC adapter must be larger and heavier than necessary for
`typical operation. This adapter may be therefore be unsuit-
`able for portable devices.
`FIGS. 4 and 5 illustrate a power supply system according
`to a preferred embodiment of the present invention. The
`power supply system is suitable for a portable computer to
`be operated from a DC power source supplied from a
`rechargeable battery or an AC adapter. Referring to FIGS. 4
`and 5, the power supply system includes an AC adapter 4 for
`supplying an amount of DC power (e.g., 24 to 30 watts) to
`a portable computer.
`In a common portable computer
`system,
`the AC converter, for example, converts an AC
`voltage of 100 to 240 volts to a stable DC (direct current)
`voltage of about 12 to 15 volts.
`The power supply system further comprises a DC/DC
`converter 10 for supplying a variety of operating voltages
`necessary for a system unit 20 of the portable computer
`having various components and a battery charging circuit 30
`for charging a rechargeable battery pack 8.
`The battery charging circuit 30 has an input terminal 5
`connected to the AC adapter 4, a node N1 connected to the
`
`8
`
`

`

`5,986,437
`
`5
`DC/DC converter 10 and an output terminal 7 connected to
`the battery pack 8. In the battery charging circuit 30, a
`microcomputer 130 is provided to control the overall charg-
`ing operation of the battery pack 8. This microcomputer 130
`is supplied with power by a regulator circuit 110. The battery
`charging circuit 30 further includes an adapter voltage
`detecting circuit 120 composed of the resistors R121 and
`R122, and the capacitor C121, a battery voltage detecting
`circuit 140 composed of the resistors R141 and R142, and
`the capacitor C141, and a constant current charging control
`circuit 150. The adapter voltage detecting circuit 120 and the
`battery voltage detecting circuit 140 detect the output volt-
`ages of the AC adapter 4 and the battery pack 8 and generate
`an adapter voltage detection signal and a battery voltage
`detection signal, respectively, which are supplied to the
`microcomputer 130. The microcomputer 130 controls the
`operation of the constant current control circuit 150 in
`response to the detection signals. In particular, when the
`battery voltage reaches a predetermined voltage, the micro-
`computer 130 terminates the battery charging to protect the
`battery from damage due to overcharging. The microcom—
`puter 130 also controls the operation of the constant current
`control circuit 150 in accordance with the ambient tempera-
`ture of the battery pack 8 which is detected via a
`temperature-sensitive resistor TP within the battery pack 8.
`As shown in FIGS. 4 and 5, the constant current charging
`control circuit 150 includes a switching circuit 151, an
`energy storing circuit 152, a charging current detecting
`circuit 153, and a pulse width modulation (PWM) switching
`controller 154. The switching circuit 151 is composed of the
`resistors R151 and R152, the diodes D151 and D152, and the
`transistors 0151 and 0152, and is provided to supply a
`variable amount of charging current to the battery pack 8.
`The PWM switching controller 154 controls the ON/OFF
`time of the transistor 0152 within the switching circuit 151
`by outputting a pulse signal which has a duty cycle corre-
`sponding to the voltage level of its feedback terminal FB.
`The energy storing unit 152, composed of the inductor
`L151, the diode D153 and the capacitor C153 is provided to
`store the electrical energy coming from the switching circuit
`151.
`
`The charging currcnt dctccting circuit 153 is connected
`between the energy storing circuit 152 and the battery pack
`8. The charging current detecting circuit 153 has a resistor
`R153 for detecting the charging current and an amplifier
`circuit composed of resistors R152~R158, the diode D154
`and the operational amplifier U151. The resistor R153 has a
`resistance of several hundreds millions. The amplifier circuit
`amplifies the voltage drop across the resistor R153 and
`generates a charging current detection signal.
`In other
`words, the detecting circuit 153 detects the charging current
`flowing from the switching circuit 151 to the battery pack 8
`using the voltage dropped by the current detecting resistor
`R153 and outputs to the feedback terminal FB of the PWM
`controller 154 the charging current detection signal having
`an amplitude corresponding to the detected current.
`The battery charging circuit further includes an input
`current detecting circuit 170 which is placed between the AC
`adapter 4 and the constant charging current control circuit
`150. The input current detecting circuit 170 has a current
`detecting resistor R161 connected between the input termi-
`nal 5 and the node N1, and an amplifier circuit which is
`composed of the resistors R162~R172, the diode D171 and
`the operational amplifier U161. The resistor R161 has a
`resistance of several millions to tens of millions to reduce
`power loss as much as possible. The amplifier circuit ampli-
`fies the voltage drop across the resistor R161 and generates
`
`6
`an input current detection signal. Consequently, the input
`current detecting circuit 170 detects a variable amount of
`input current flowing from the input terminal 5 to the node
`N1 and generates the input current detection signal 8 having
`a magnitude that varies depending upon the input current
`from the AC adapter 4.
`FIG. 6 is a schematic representation which illustrates a
`battery charging operation of the power supply system of
`FIGS. 4 and 5.
`
`Referring to FIG. 6, all of the input current, i.e., a rated
`current 81 (e.g., 2A) of the AC adapter 4 is used for charging
`the battery 8 until the system unit 20 is turned on.
`When the system unit 20 is powered on and the input
`current does not exceed the rated current of the AC adapter
`4, namely, the sum of system current drawn by the system
`unit 20 and the charging current drawn by the battery
`charging circuit 30 is not greater than the rated current of the
`AC adapter, the charging current detecting circuit 153 sup-
`plies a fixed voltage level of the charging current detection
`signal to the feedback terminal FB of the PWM controller
`154. Therefore, the constant charging current control circuit
`150 supplies a predetermined charging current S3+S3' (e.g.,
`about 1.0~1.5 A) to the battery pack 8.
`On the other hand, when the system current drawn by the
`system unit 20 is increased, the input current does exceed the
`rated current of the AC adapter 4. Thus, the voltage level of
`the feedback terminal FB of the PWM controller 154 is
`increased so that
`the constant charging current control
`circuit 150 decreases the charging current supplied to the
`battery pack 8. At this time, the charging current is decreased
`in proportion to the increment of input current due to the
`increment of the system current consumed by the system
`unit 20. For example, if the rated current of the AC ad pter
`is 2.0 A and the system current consumed by the system unit
`20 is 1.0 A, the constant charging current control circuit 150
`will supply 1.0 A of the charging current to the battery pack
`8. Under the same condition, if the system unit 20 consumes
`USA of the system current, the constant charging current
`control circuit 15 will supply 1.5 A of the charging current
`to the battery pack 8. Like this, the input current will retain
`the magnitude of the rated current of the AC adapter until the
`batter charging is terminated. Also, if the charging current no
`longer is supplied to the battery pack because of the termi-
`nation of the battery charging, the input current will retain
`below the rated current of the AC adapter.
`According to the invention as described above, when the
`current drawn by the portable device is less than its maxi-
`mum SZ, the remainder S3' of the current can be used for
`charging the battery. This allows the AC adapter to be
`smaller and less heavy, and maximi7es the battery charging
`efficiency. Also, the portable device power supply system
`according to the present invention has low power losses and
`effects high efliciency battery charging, compared to the
`earlier systems, and further, it can charge the battery faster
`and reduce the charging time of the battery.
`Although the preferred embodiment of the present inven-
`tion has been described and illustrated above, those skilled
`in the art will appreciate that various changes and modifi-
`cations to the present invention do not depart from the spirit
`of the invention.
`What is claimed is:
`1. A system for charging a rechargeable battery used to
`supply power to a portable device, comprising:
`an input terminal for receiving an externally supplied
`variable amount of input power,
`the input power
`including a variable amount of system power for oper-
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`7
`ating said portable device and a variable amount of
`charging power for charging said battery;
`a means for supplying the system power to said portable
`device;
`an output terminal connected to said battery;
`a means for supplying a variable amount of charging
`current flowing through said output terminal to the
`battery;
`a means for detecting an input current flowing through
`said input terminal and for generating a detection signal
`having a magnitude which varies depending upon the
`input current; and
`a means for controlling the amount of the charging current
`in response to the detection signal, said means for
`controlling the charging current controlling the charg—
`ing current so as to be constant when the amplitude of
`the detection signal is less than a predetermined thresh-
`old level, and so as to decrease the amount of charging
`current depending upon the amplitude of the detection
`signal when the amplitude of the detection signal is not
`less than the predetermined threshold level.
`2. A system for charging a rechargeable battery used to
`supply power to a portable device, comprising:
`an AC adapter for supplying a variable amount of input
`current including a variable amount of system current
`for operating said portable device and a variable
`amount of charging current for charging said battery;
`a first current detecting circuit for detecting the amount of
`charging current and for generating a first detection
`signal having a magnitude which varies depending
`upon the charging current;
`a second current detecting circuit for detecting the amount
`of input current and for generating a second detection
`signal having a magnitude which varies depending
`upon the input current; and
`a charging current control circuit for controlling the
`amount of the charging current in response to the first
`and second detection signals,
`the charging current
`control circuit including an input terminal connected to
`said AC adapter, a node for supplying the system
`current to said portable device and an output terminal
`connected to said battery, the control circuit controlling
`the charging current so as to be constant when the
`amplitude of the second detection signal is less than a
`predetermined threshold level, and so as to decrease the
`amount of charging current depending upon the ampli-
`tude of the second detection signal when the amplitude
`of the second detection signal
`is not less than the
`predetermined threshold level.
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`3. The system according to claim 2, said charging current
`control means comprising a switch element connected
`between said node and said output terminal, and a switch
`control circuit for controlling ON/OFF time of said switch
`element in response to the first and second detection signals.
`4. The system according to claim 2, said first current
`detecting circuit comprising a resistor connected between
`said node and said output terminal, and an amplifier circuit
`for amplifying a voltage drop across said resistor in accor-
`dance with a predetermined gain and for generating an
`output signal of the amplified voltage drop as the first
`detection signal.
`5. The system according to claim 2, said second current
`detecting circuit comprising a resistor connected between
`said input terminal and said node, and an amplifier circuit for
`amplifying a voltage drop across said resistor in accordance
`with a predetermined gain and for generating an output
`signal of the amplified voltage drop as the second detection
`signal.
`6. Apower supply for a portable computer system oper-
`able on an internal rechargeable battery, comprising:
`an AC adapter for supplying a variable amount of DC
`power including a variable amount of system power for
`operating said portable computer system and a variable
`amount of charging power for charging said battery;
`a DC/DC converter for supplying a variety of operating
`voltages necessary for said system unit using the sys-
`tem power; and
`a battery charging circuit for charging said battery using
`the charging power;
`said battery charging circuit comprising:
`an input terminal connected to said AC adapter;
`an output terminal connected to said battery;
`a means for supplying a variable amount of charging
`current through said output terminal to said battery;
`a means for detecting input current flowing from said
`AC adapter to said input terminal and for generating
`a detection signal having a magnitude which varies
`depending upon the input current; and
`a means for controlling the amount of the charging
`current
`in response to the detection signal, said
`means for controlling the charging current control—
`ling the charging current so as to be constant when
`the amplitude of the detection signal is less than a
`predetermined threshold level, and so as to decrease
`the amount of charging current depending upon the
`amplitude of the detection signal when the amplitude
`of the detection signal is not less than the predeter-
`mined threshold level.
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