(12) United States Patent
`(10) Patent N0.:
`US 6,184,660 B1
`
`Hatular
`(45) Date of Patent:
`Feb. 6, 2001
`
`U8006184660B1
`
`(54) HIGH-SIDE CURRENT-SENSING SMART
`BATTERY CHARGER
`
`Smart Battery Charger Specification, Revision 1.0, Jun. 27,
`1996, Copyright 1996.
`
`(75)
`
`Inventor: Alexandru Hatular, Campbell, CA
`(US)
`
`Smart Battery Data Specification, Revision 1.0, Feb. 15,
`1995, Copyright 1996.
`
`(73) Assignee: Micro International, Ltd. (KY)
`
`( * ) Noticc:
`
`Under 35 U.S.C. 154(b), the term of this
`patent shall be extended for 0 days.
`
`Smart Battery Selector Specification, Revision 1.0, Sep. 5,
`1996, Copyright 1996.
`
`*
`
`c1 e
`'t d b
`
`y examiner
`.
`
`(21) Appl. N0.: 09/276,616
`
`(22)
`
`Filed:
`
`Mar. 25, 1999
`
`(60)
`
`Related U-S- Application Data
`Provisional application No. 60/079,509, filed on Mar. 26,
`1998
`
`Int. Cl.7 ........................................................ H02J 7/04
`(51)
`(52) US. Cl.
`............................................. 320/141; 320/139
`(58) Field of Search ..................................... 320/141, 139,
`320/145, 140
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`5,477,132 * 12/1995 Canter et al.
`........................ 320/101
`
`5 698 964
`12/1997 Kates a al
`320/22
`5:723:970
`3/1998 Bell
`.. 320/30
`............
`
`1/1999 Downs et a1.
`.
`320/134
`5,864,221 *
`579207475 *
`7/1999 Boylan et al' """""""""""" 363/127
`OTHER PUBLICATIONS
`
`System Management Bus Specification, Revision 1.0, Feb.
`15’ 1995’ Copyright 1996.
`System Management Bus BIOS Interface Specification,
`Revision 1.0, Feb. 15, 1995, Copyright 1996.
`
`Primary Examiner—Peter S. Wong
`Assistant Examiner—Lawrence Luk
`
`(74) Attorney, Agent, or Firm—D. E. Schrcibcr
`
`(57)
`
`ABSTRACT
`
`.
`.
`A battery Charger 1C. for coerIhRg 01"?”th Of a Wk
`converter Clrcult that includes a series SWItCh and a res1stor
`for sensing battery charging current. The battery charger IC
`includes a pulse-Width-modulation switch drive circuit that,
`during charging of the battery, supplies to the buck converter
`circuit with an electrical signal Which repeatedly turns-on
`and then turns-off the series switch. The battery charger IC
`.
`.
`.
`.
`also includes a charging-current sense amplifier which
`receives from the current-sensing resistor and amplifies an
`electrical signal which represents the battery charging elec-
`trical current. The charging-current sense amplifier includes
`a bridge circuit to which is coupled the electrical signal
`
`received by the charging-current sense amplifier from the
`current-sensing resistor and an auto-zero circuit.
`
`42 Claims, 6 Drawing Sheets
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`US. Patent
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`US 6,184,660 B1
`
`1
`HIGH-SIDE CURRENT-SENSING SMART
`BATTERY CHARGER
`
`CLAIM OF PROVISIONAL APPLICATION
`RIGHTS
`
`This application claims the benefit of US. Provisional
`Patent Application No. 60/079,509 filed on Mar. 26, 1998.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates generally to battery powered
`electrical devices, and, more particularly,
`to an improved
`battery charger integrated circuit (“IC”) adapted for inclu-
`sion in portable electrical devices.
`2. Decription of the Prior Art
`A system has been specified for use in battery powered
`portable devices that is identified as the System Manage-
`ment Bus (“SMBus”). The SMBus prescribes data protocols,
`device addresses, and additional electrical
`requirements
`necessary to transport commands and information among
`various subsystems of a battery powered device. The SMBus
`specification envisions the SMBus interconnecting at least a
`system host computer, a smart battery charger, and a smart
`battery that are all included in the portable device. Under the
`SMBus protocol, the smart battery provides data, via the
`SMBus, to the portable device’s host computer. A power
`management routine executed by the host computer pro-
`cesses such smart battery data to manage operation of at
`least the smart battery and the smart battery charger.
`In accordance with the SMBus specification and protocol,
`a smart battery accurately reports its characteristics to the
`host computer Via the SMBus. If a portable device includes
`more than one battery, each battery reports such character-
`istics independently via the SMBus. Providing the power
`management routine executed by the host computer with
`information about the charge state of each battery permits
`displaying the batteries’ condition, and accurately estimat-
`ing the portable device’s remaining operating time.
`However, in addition to providing information about the
`batteries” charge state,
`the information obtained via the
`SMBus is sufficient to permit electrical power management
`for the portable device, and to also permit controlling battery
`charging regardless of a battery’s particular chemistry.
`To achieve the preceding objectives, the SMBus specifies
`that,
`independent of host computer power management
`routine operation, a smart battery charger must periodically
`poll a smart battery that is being charged for the battery’s
`charging characteristics. Upon receiving a response from the
`smart battery,
`the smart battery charger then adjusts its
`output to match the smart battery’s requirements. To avoid
`battery damage, the smart battery also reports certain con-
`ditions such as over charge, over voltage, over temperature,
`and too rapid temperature increase to the smart battery
`charger. In this way the smart battery effectively controls its
`re-charging cycle. Moreover, to prolong smart battery life,
`the smart battery charger may prevent a fully charged smart
`battery from powering the portable device if a source of
`external electrical power is available.
`Analogously, the power management routine executed by
`the host computer may poll the smart battery, that powers the
`host computer’s operation, for smart battery information.
`The power management routine can request factual infor-
`mation about the battery such as the battery’s chemistry, or
`the battery’s operating temperature, voltage, or charge or
`discharge current. The power management routine can then
`
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`either display such information directly and/or to display an
`estimate of the battery’s operating capabilities, or it may
`process such information for use in the computer system’s
`power management scheme. Similar to the smart charger,
`the power management routine receives information about
`critical events if the smart battery detects a problem.
`Moreover,
`the power management routinc also receives
`smart battery estimates about end of discharge, electrical
`capacity remaining below a preset threshold value, and time
`remaining until discharge below a preset threshold value.
`As part of the host computer’s power management
`scheme, the power management routine may provide other
`routines with information about battery condition.
`Accordingly, the power management routine may query a
`device driver routine to determine if an anticipated action
`will endanger the host computer’s electrical power integrity.
`For example, before attempting to start a hard disk drive the
`power management
`routine may first determine if that
`particular opcration might cause the smart battery’s output
`voltage to drop below a threshold for host computer failure.
`Under such circumstances, the hard disk device driver’s
`response might be to increase power available for starting
`the hard disk drive by causing the power management
`routine to turn-off a non-critical power consumption such as
`liquid crystal display (“LCD”) backlighting.
`In addition to a smart battery and a smart charger, a
`portable device that implements the SMBus will, in general,
`also include a smart battery selector. The SMBus specifica-
`tion and protocol includes a smart battery selector because
`a portable device may include two or more smart batteries,
`only one of which may be in use for powering the portable
`device’s operation at any instant in time. In such multi-
`battery devices,
`the smart battery selector must arbitrate
`between or among batteries. Furthermore, the smart battery
`selector must be capable of swiftly re-configuring the por-
`table devices power if a battery were to be suddenly
`removed, such as might occur if a battery were removed
`from a laptop or notebook computer to install a floppy
`diskette drive.
`
`Additional, more detailed information about the SMBus
`specifications and protocol, and about smart batteries is
`provided by:
`System Management Bus Speczfication, Revision 1.0,
`Intel, Corporation, Feb. 15, 1995;
`System [Management Bus BIOS Interface Specification,
`Revision 1.0, Intel, Corporation, Feb. 15, 1995;
`Smart Battery Charger Speczfication Revision 1.0, Dura-
`cell Inc. and Intel Corporation, Jun. 27, 1996,
`Smart Battery Data Specification Revision 1.0, Duracell
`Inc. and Intel Corporation, Feb. 15, 1995; and
`Smart Battery Selector Specification Revision 1.0, Dura-
`cell Inc. and Intel Corporation, Sep. 5, 1996.
`The publications listed above are hereby incorporated
`herein by reference as though fully set forth here.
`US. patent application Ser. No. 08/850,335 filed May 2,
`1997, entitled “Smart Battery Selector” describes a control—
`ler IC adapted for inclusion in a portable device. The
`portable device also includes at least two batteries that are
`capable of providing battery-state data via a bus to a host
`computer also included in the portable device. A control
`electronic-circuit included in the controller directs operation
`of switch-drivers for selecting among the batteries one of
`which powers operation of the portable device. A bus-
`snooper circuit allows the controller to monitor the bus for
`battery-condition alarm-messages independently of the host
`computer. The controller may respond to messages on the
`
`8
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`

`

`US 6,184,660 B1
`
`3
`bus by independently selecting a different battery even if the
`host computer’s operation has been suspended, perhaps to
`reduce power consumption. The controller disclosed in this
`patent may also independently select a single battery for
`charging, and may terminate charging upon receiving a
`battery overcharge message. The disclosure of the Smart
`Battery Selector patent application is hereby incorporated by
`reference.
`While the SMBus specification and protocol addresses
`many significant problems associated with battery powered
`operation of portable devices, it omits details which are
`essential
`to address significant operational constraints
`involved in battery charging. For example, properly charg-
`ing a battery requires continuously monitoring the charging
`current and, if necessary, adjusting operation of the battery
`charger so a prescribed charging current is supplied to the
`battery over time. U.S. Pat. No. 5,698,964 entitled “Adap-
`tive Power Battery Charging Apparatus” (“the ’964 patent”)
`discloses a battery charger that includes a “buck converter
`circuit” battery charger having a feedback circuit which
`regulates battery charging. To permit regulating battery
`charging current, the circuit disclosed in this patent includes
`a current sensing resistor, depicted in FIG. 3, connected
`between a ground terminal of the battery being charged and
`circuit ground of the battery charger. During charging of the
`battery, current flowing through the current sensing resistor
`produces an electrical signal that is proportional to charging
`current. However, a difficulty encountered with such “low
`side” current sensing as that illustrated in the ’964 patent is
`that it requires two electrically separate ground circuits, one
`for normal operation of the battery powered device, and
`another for battery charging.
`U.S. Pat. No. 5,723,970 entitled “Battery Charging Cir-
`cuitry Having Supply Current Regulation” (“the ’970
`patent”) also discloses a battery charger that
`includes a
`feedback circuit which regulates battery charging. However,
`the feedback circuit disclosed in the ”970 patent differs from
`that disclosed in the ’964 patent by having the current
`sensing resistor located not between a battery ground and a
`charger ground, but rather between a source of battery
`charger’s electrical energy and the battery being charged.
`Consequently,
`the circuit disclosed in the ’970 patent is
`simpler than that disclosed in the ”964 patent in the sense
`that it employs only a single ground that is common both to
`the battery powered device and to the charger. However,
`during battery charging the circuit disclosed in the ’970
`patent experiences a voltage at
`the “high-side” current
`sensing resistor which may exceed 16.8 volts (“V”), or may,
`if the battery is excessively discharged, be as low as 2.5 V.
`While circuits can be built using comparatively high voltage
`semiconductor devices or processes which are capable of
`accommodating this rather wide common-mode voltage
`range, it is difficult to envision a battery charger IC built
`using a conventional 5.0 V Complementary Metal-Oxide-
`Silicon (“CMOS”) process that operates consistently
`throughout this large common-mode voltage range.
`BRIEF SUMMARY OF THE INVENTION
`
`invention is to provide an
`An object of the present
`improved, high efficiency battery charger [C that senses
`charging current.
`invention is to provide
`Another object of the present
`battery charger IC made with a low voltage IC process that
`senses charging current throughout a wide common-mode
`voltage range.
`Another object of the present invention is to provide
`battery charger IC having an amplifier for sensing charging
`current which automatically compensates for drift within the
`amplifier.
`
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`Another object of the present invention is to provide
`battery charger IC having an amplifier for sensing charging
`current which automatically compensates for mismatch
`within the amplifier.
`Briefly, the present invention in one aspect is a battery
`charger IC adapted for controlling operation of a buck
`converter circuit which may be included in a battery pow-
`ered device. Such a buck converter circuit receives electrical
`energy from an external power source and supplies electrical
`energy for charging the battery. The buck converter circuit
`includes a series switch that receives an electrical current
`from the external power source and that supplies an elec-
`trical battery charging current to the battery. In accordance
`with the present invention the buck converter circuit also
`includes a current-sensing resistor connected in series
`between the external power source and the battery. Electrical
`current supplied for charging the battery flows through the
`current-sensing resistor.
`The battery charger IC includes a pulse-width-modulation
`switch drive circuit
`that, during charging of the battery,
`supplies to the buck converter circuit an electrical signal
`which repeatedly turns-on and then turns-off the series
`switch. The battery charger IC also includes a charging-
`current sense amplifier which receives from the current-
`sensing resistor and amplifies an electrical signal which
`represents the battery charging electrical current. The
`charging—current sense amplifier in accordance with the
`present
`invention includes a bridge circuit
`to which is
`coupled the electrical signal received by the charging-
`current sense amplifier from the current-sensing resistor.
`The charging-current sense amplifier also includes an auto-
`zero circuit which automatically compensates for long-term
`drift or mismatch occurring within the charging-current
`sense amplifier.
`is a battery
`invention in another aspect
`The present
`powered device that includes a battery for energizing its
`operation, and a buck converter circuit as described above
`including the current-sensing resistor. The battery powered
`device also includes a battery charger IC in accordance with
`the present invention for controlling operation of the buck
`converter circuit. The battery charger
`IC includes the
`charging—current sense amplifier that receives from the
`current-sensing resistor and amplifies the electrical signal
`which represents the battery charging current supplied to the
`battery. The charging—current sense amplifier includes the
`bridge circuit and the auto-zero circuit which permit to in
`accordance with the present invention the battery charger IC,
`though fabricated with a conventional 5 .0 V CMOS process,
`to accommodate the large common-mode voltage range
`which may occur at the “high-side” current-sensing resistor.
`These and other features, objects and advantages will be
`understood or apparent to those of ordinary skill in the art
`from the following detailed description of the preferred
`embodiment as illustrated in the various drawing figures.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGS. 1A and 1B are block diagrams depicting a battery
`powered device, that respectively omit and include a con-
`troller for selecting between a pair of batteries, both of
`which FIGS. illustrate a battery charger IC in accordance
`with the present invention;
`FIG. 2, is a block diagram depicting in greater detail the
`battery charger IC depicted in FIGS. 1A and 1B that includes
`a SMB interface and charging control, an error summing
`circuit, a charging-current sense amplifier, and a sample-
`and-hold circuit;
`
`9
`
`

`

`US 6,184,660 B1
`
`5
`FIG. 3 is a register diagram depicting a register block
`included in the SMB interface and charging control depicted
`in FIG. 2;
`FIG. 4 is a block diagram depicting the error summing
`circuit depicted in FIG. 2;
`FIG. 5, is a circuit diagram depicting the charging-current
`sense amplifier depicted in FIG. 2 including an auto-zero
`portion that operates entirely within the battery charger IC;
`FIG. 6 is a circuit diagram depicting the sample-and-hold
`circuit depicted in FIG. 2; and
`FIG. 7 is a circuit diagram depicting part of the charging-
`current sense amplifier depicted in FIG. 5 that illustrates an
`alternative embodiment thereof in which part of the auto-
`zero portion operates externally to the battery charger IC.
`DETAILED DESCRIPTION
`
`The block diagram of FIG. 1A illustrates a device,
`referred to by the general reference character 20, whose
`operation may be energized by a rechargeable smart battery
`22. Though not specifically illustrated in FIG. 1A for peda-
`gogical reasons, when operation of the device 20 is ener-
`gized by the smart battery 22, the smart battery 22 usually
`supplies electrical power to a DC—DC converter 24. The
`DC—DC converter 24 converts such electrical power from
`the voltage supplied by the smart battery 22 to other voltages
`required for proper operation by various electronic circuits
`included in the device 20 such as an IC microprocessor host
`computer 26. Accordingly FIG. 1A depicts the DC—DC
`converter 24 as being connected to the host computer 26 by
`a power supply bus 28.
`As is well known to those skilled in the art, generally the
`host computer 26 may exchange electrical signals with other
`devices included in the device 20. Depending upon the
`precise details of the device 20, as is well known to those
`skilled in the art such devices, none of which are depicted in
`FIG. 1A, may include a random access memory (“RAM”),
`a floppy diskette drive, a hard disk drive, a compact disk
`read only memory (“CD-ROM”) drive, a display controller,
`a PC Card controller, etc. However, the illustration of FIG.
`1A specifically depicts the host computer 26 as being
`capable of exchanging electrical signals with the smart
`battery 22 via a System Management Bus SM 32. As
`described above in greater detail, the SMBus 32 permits the
`smart battery 22, and a computer program executed by the
`host computer 26, to exchange information regarding the
`status of the smart battery 22, particularly its charge state.
`The block diagram of FIG. 1A depicts the device 20
`specifically configured for charging the smart battery 22.
`Accordingly,
`the illustration of FIG. 1 includes an AC
`adaptor 42 that receives alternating current electrical power
`for energizing the operation of the device 20,
`including
`energizing charging of the smart battery 22, from an external
`power source not depicted in any of the FIGs. In the
`configuration of the device 20 depicted in FIG. 1A, the AC
`adaptor 42 supplies electrical energy to the DC—DC con-
`verter 24 via an external power line 44. The DCiDC
`converter 24 in turn supplies that electrical energy to the host
`computer 26, and to a battery charger IC 50 in accordance
`with the present invention. The AC adaptor 42 also supplies
`electrical energy via the external power line 44 to a 5 volt
`regulator 52. The 5 volt regulator 52 supplies electrical
`energy at a potential of 5.0 V via a 5 volt supply line 54
`directly to the battery charger IC 50 to energize its operation.
`Because externally supplied electricity energizes the opera-
`tion of the battery charger IC 50, the battery charger IC 50
`ceases operation immediately when the AC adaptor 42
`
`6
`becomes disconnected from the external source of electrical
`power. For purposes to be described in greater detail below,
`the 5 volt regulator 52 also supplies electrical energy at a
`potential of 5.0 V to a maximum—charging—voltage voltage
`divider 56 assembled from series connected resistors 56a
`and 56b, and to a maximum-charging-current voltage
`divider 58 assembled from series connected resistors 58a
`and 58b.
`
`While charging the smart battery 22, the AC adaptor 42
`also supplies electrical energy via the external power line 44
`to a high charging—current, pulse—width—modulated
`(“PWM”) buck converter circuit identified in FIG. 1Aby the
`general reference character 60. Accordingly, a source termi-
`nal 62s of a series switch 62 included in the PWM buck
`
`converter circuit 60 receives electrical power directly from
`the AC adaptor 42. As illustrated in FIG. 1A, the series
`switch 62 is preferably a P-type MOSFET. The external
`power line 44 also connects to circuit ground via a voltage
`divider 64 assembled from series connected resistors 64a
`and 64b. The junction of the resistors 64a and 64b supplies
`the battery charger IC 50 with a reference voltage that is
`proportional to the voltage supplied by the AC adaptor 42 as
`present on the external power line 44.
`During charging of the smart battery 22, an electrical
`signal, supplied to a gate terminal 62g of the series switch
`62 from the battery charger IC 50 via an inverting amplifier
`66, repeatedly turns the series switch 62 first on and then off.
`A drain terminal 62d of the series switch 62 connects to an
`
`inductor 68. During charging of the smart battery 22, while
`the series switch 62 is turned-on,
`the drain terminal 62d
`supplies an electrical current to the inductor 68. During each
`successive interval in which the series switch 62 is turned-
`
`flowing through the inductor 68
`on, electrical current
`increases until the series switch 62 is turned-off. During each
`successive interval in which the series switch 62 is turned-
`
`flowing through the inductor 68
`off, electrical current
`decreases either until electrical current stops flowing
`through the inductor 68, or until the series switch 62 is again
`turned on.
`While the series switch 62 is turned-on, some of the
`electrical current flowing through the inductor 68 enters a
`filter capacitor 72. While the series switch 62 is turned-off,
`electrical current flows out of the filter capacitor 72. During
`each successive interval in which the series switch 62 is
`turned-off while electrical current through the inductor 68
`decreases, electrical current flows into the inductor 68 from
`a free-wheeling zener diode 74. Accordingly, a cathode
`terminal 740 of the free—wheeling zener diode 74 connects to
`the junction between the drain terminal 62d of the series
`switch 62 and the inductor 68, while an anode terminal 74a
`of the free-wheeling zener diode 74 connects to circuit
`ground.
`To reduce power loss in the PWM buck converter circuit
`60 due to electrical current flowing through the free—
`wheeling zener diode 74, the PWM buck converter circuit 60
`also includes a synchronous-rectifier switch 76, preferably a
`N—type MOSFET, connected in parallel with the free—
`wheeling zener diode 74. A source terminal 76d of the
`synchronous-rectifier switch 76 connects to circuit ground,
`and a drain terminal 76d of the synchronous-rectifier switch
`76 connects to the junction among the drain terminal 62d of
`the series switch 62,
`the inductor 68 and the cathode
`terminal 746 of the free-wheeling zener diode 74. During
`charging of the smart battery 22, an electrical signal supplied
`to a gate terminal 76g of the synchronous-rectifier switch 76
`from the battery charger
`IC 50 repetitively turns the
`synchronous-rectifier switch 76 on after the series switch 62
`
`10
`
`15
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`10
`
`10
`
`

`

`US 6,184,660 B1
`
`8
`a battery-charger segment 32bc and a host-computer seg-
`ment 32hc. SMBus segments 32a or 32b respectively inter-
`connect the smart battery selector 102 with the smart bat-
`teries 22. Accordingly,
`the smart battery selector 102
`conveys SMBus communications between the smart battery
`22 being charged and the battery charger IC 50 via the
`battery-charger segment 32bc and one or the other of SMBus
`segments 32a or 32b. Correspondingly, the smart battery
`selector 102 conveys SMBus communications between the
`smart battery 22 that is not being charged and the host
`computer 26 via the host-computer segment 32hc and one or
`the other of SMBus segments 32a or 32b. Analogously, the
`smart battery selector 102 supplies battery charging current
`to one or the other of the smart batteries 22 via battery-
`charging-current
`line-segments 88a or 88b. Finally,
`the
`smart battery selector 102 also selects via either thermistor
`signal-line-segment 96a or 96b a temperature signal from a
`thermistor included in the smart battery 22 being charged,
`and supplies that
`temperature signal via the thermistor
`signal-line 96 to the battery charger IC 50.
`As depicted in the block diagram of FIG. 2, the battery
`charger IC 50 includes a SMB interface and charging control
`122. In accordance with the SMB protocol, the SMB inter-
`face and charging control 122 may transmit an ALRT signal,
`i.c. an interrupt, to other devices connected to the SMBus 32
`via a SMBALRT line 124 included in the SMBus 32. The
`
`SMB interface and charging control 122 receives, via a SMB
`clock (“SMBC”) line 126 included in the SMBus 32, a clock
`signal transmitted from a master device connected to the
`SMBus 32, e.g. the host computer 26 or the smart battery 22.
`Responsive to the clock signal present on the SMBC line
`126, the SMB interface and charging control 122 exchanges
`data with other devices connected to the SMBus 32, e. g. the
`host computer 26 or the smart battery 22, via a SMBD line
`128. Such inter-device communications may selectively
`cause data to be written into and/or read from a set nine (9)
`registers included in an interface register block 132 of the
`SMB interface and charging control 122. FIG. 3 depicts
`registers 132a—132i included in the interface register block
`132 which comprise:
`1. a read only chargerspecinfo register 132a;
`. a write only charger—mode register 132b;
`. a read only charger-status register 132C;
`. a read/write charging-current register 132d;
`. a read/write charging-voltage register 1326;
`. a write only alarm-warning register 132f;
`. a read only battery-temperature register 132g;
`8 . a read only battery-voltage register 132k; and
`9. a read only chipinfo register 132i.
`ChargerSpecInfo Register 132a
`The read only chargerspecinfo register 132a stores
`extended status bits which specify performance capabilities
`of the battery charger IC 50. Bit 13261113 located in the low
`nibble 132aa of the chargerspecinfo register 132a stores
`data indicating the applicable version of the specification for
`the battery charger IC 50. Bit 132aa4 stores data indicating
`whether the battery charger IC 50 supports commands for
`the optional smart battery selector 102.
`Charger Mode Register 132b
`The write only charger-mode register 132b stores data
`which specifies various operating modes for the battery
`charger IC 50. Values assigned to bits in the charger-mode
`register 132b upon b 10resetting the battery charger IC 50
`permit the battery charger IC 50 to operate in concert with
`the smart battery 22 without intervention of the host com-
`
`2 3 4 5
`
`6 7
`
`7
`turns-off, and then turns the synchronous-rectifier switch 76
`off before turning the series switch 62 on. To protect the
`synchronous-rectifier switch 76 from an inadvertent appli-
`cation of an excessively high voltage between the junction
`among the drain terminal 76d of the synchronous-rectifier
`switch 76, the drain terminal 62d of the series switch 62 and
`the terminal of the inductor 68 and circuit ground while
`permitting proper operation of the PWM buck converter
`circuit 60, usually the free-wheeling zener diode 74 has a
`zener breakdown voltage of approximately 40 V to 50 V.
`A current-sensing resistor 82 connects in series with the
`inductor 68 at the junction between the inductor 68 and the
`filter capacitor 72. The voltages present respectively at
`opposite terminals of the current-sensing resistor 82 during
`charging of the smart battery 22 are supplied through
`isolation resistors 84p and 84m to the battery charger IC 50.
`The isolation resistors 84p and 84m preferably have a
`resistance of 1 megohm (“M9”), and are matched to within
`0.1%.
`
`A terminal of the current-sensin g resistor 82 furthest from
`the inductor 68 connects to a drain terminal 86d of a
`
`reverse-current-protection switch 86, preferably a P-type
`MOSFET. A source terminal 865 of the reverse-current-
`protection switch 86 supplies the smart battery 22 with the
`charging current via battery-charging-current line 88. Dur-
`ing charging of the smart-battery 22, if the battery charging
`current exceeds a pre—established threshold an electrical
`signal supplied to a gate terminal 86g of the reverse-current-
`protection switch 86 by the battery charger IC 50 turns the
`reverse-current-protection switch 86 on to provide a low
`resistance path between the PWM buck converter circuit 60
`and the smart battery 22. When the battery charging current
`decreases below the pre-established threshold, the electrical
`signal supplied by the battery charger IC 50 turns the
`reverse-current-protection switch 86 oil, and the charging
`current flows from the PWM buck converter circuit 60 to the
`
`smart battery 22 via a drain body diode 86db of the reverse-
`current—protection switch 86. Consequently, for charging
`currents less than the threshold below which the reverse-
`
`current—protection switch 86 is turned-off, the drain terminal
`86d blocks any possible reverse current flowing from the
`smart battery 22 into the PWM buck converter circuit 60.
`A feedback voltage divider 92, assembled from series
`connected resistors 92a and 92b, connects between the
`battery—charging—current
`line 88 and circuit ground. The
`junction of the resistors 92a and 92b supplies the battery
`charger
`IC 50 with a feed-back voltage signal
`that
`is
`proportional to the voltage feed-back (“VFB”) signal present
`on the battery-charging-current line 88 and applied across
`the smart battery 22 during charging.
`In addition to the connection between the smart battery 22
`and the battery charger IC 50 via the SMBus 32, a thermistor
`signal-line 96 also supplies the battery charger IC 50 with an
`electrical signal produced by a thermistor included in the
`smart battery 22 in accordance with the SMBus specifica-
`tion. A bias-current-programming resistor 98 also couples a
`terminal of the battery charger IC 50 to circuit ground.
`The block diagram of FIG. 1B depicts a device 20 which
`differs from the device 20 depicted in FIG. 1A by being
`specifically configured to include a smart ba