Combined LDO regulator and battery charger

[0001] The present invention relates to battery charging, and, in particular, to a circuit and method for low-drop-out (LDO) voltage regulator and battery charger circuit.

[0002] Some commonly available battery chargers for portable devices such as cellular phones, personal digital assistants (PDAs), and the like, include a dual mode operation, where the charger may provide a voltage source for a load (cellular phone, PDA, and the like) or charge the battery. Conventional linear battery chargers typically provide continuous stable current to both battery and load. Such battery chargers typically include a switch controlled by an external signal for switching between supplying the load and charging the battery.

[0003] A common problem with these battery chargers is that the load may be subjected to excess current and damaged, if the battery is inadvertently disconnected while being charged. This problem may be an issue not only during operation, but also during production testing, when batteries may be inserted and removed for the first time.

[0004] Thus, it is with respect to these considerations and others that the present invention has been made.

[0010] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific exemplary embodiments by which the invention may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Among other things, the present invention may be embodied as methods or devices. Accordingly, the present invention may take the form of an entirely hardware embodiment, or an embodiment combining software and hardware aspects. The following detailed description is, therefore, not to be taken in a limiting sense.

[0011] Briefly stated, the present invention is related to a low-drop-out (LDO) voltage regulator and battery charger circuit that does not need an external control to switch between two modes, and may provide load supply and battery charge current simultaneously, if power is available. When a battery is being charged, the circuit is in constant current mode (CC), and provides a constant charging current to the battery. When the battery is close to a full charge status, the charging current is gradually reduced, while a charge voltage supplied to the battery remains constant. This mode of operation is called constant voltage (CV) mode. When the battery is fully charged, the circuit may continue to remain in the CV mode providing constant charge voltage to the battery. In either mode, a current provided to a load is maintained substantially constant. Accordingly, the circuit is configured to provide a stable voltage to the load regardless of a battery charging status, and a stable charging current to the battery regardless of a load drain.

[0012] FIG. 1 illustrates block diagram 100 of an embodiment of a combined LDO regulator and battery charger circuit with external devices employing the circuit. External devices include power source 102, load 114, and battery 116. Circuit 120 includes two main subs-circuits: LDO regulator 122 and constant-current constant-voltage (CC-CV) current regulator 124. Voltage regulator 122 further includes power pass device device 104 and LDO control loop 112. Current regulator 124 further includes current sense device 106, current sense integrator 108, voltage sense circuit 111. FIG. 1 shows a particular arrangement of inputs and outputs of the various components. In one embodiment, all of the components of circuit 120 are included in the same chip. Alternatively, one or more of the components of circuit 120 may be off-chip.

[0013] LDO regulator is arranged to receive an unregulated input from external power source 102, and to provide a regulated output voltage to load 114 in response to control voltage V_cnt and the regulated output voltage. CC-CV current regulator 124 is configured to sense a load current and to provide a battery charge voltage V_bat to battery 116. CC-CV current regulator 124 is further configured to provide control voltage V_cnt to LDO regulator 122 such that the battery charging current is maintained constant in a constant current mode and reduced in response to a battery charge status during a constant voltage mode. In one embodiment, CC-CV current regulator 124 may also provide a portion of feedback voltage V_fb.

[0014] Power pass device 104 may be configured to regulate the unregulated input received from power source 102. LDO loop control 112 is arranged to receive a control voltage V_cnt from CC-CV regulator 124 and provide a control signal to power pass device 104. Current sense device 106 may include a current sense resistor R_sense that has low resistance. The resistance of R_sense may range from about 10 milliohms to about 100 milliohms. This may enable circuit 120 to provide the output voltage to load 114 simultaneously with charge current to battery 116. Current sense integrator 108 may provide an output current in response to an output of voltage sense circuit 111. Voltage sense circuit 111 is arranged to receive battery charge voltage V_bat. In a further embodiment, voltage sense circuit 111 may be configured to limit battery charge voltage V_bat.

[0015] In one embodiment, control voltage V_cnt may be an output of voltage sense circuit 111 that comprises a portion of battery charge voltage V_bat derived from a voltage divider. In another embodiment, V_cnt may comprise a combination of an output of current sense integrator 108 and an output of voltage sense circuit 111 as described above. In either case, control voltage V_cnt for LDO control loop 112 in an active linear regulation mode may be expressed as:
V_cnt=V_bat+(I_chg*R_sense−(V_bat−V_term)*G),
where I_chg is the battery charging current, V_term is a termination voltage for the battery indicating an upper limit for battery charge voltage, and G is a gain of voltage sense integrator included in voltage sense circuit 111.

[0016] The above described elements provide circuit 120 with a CC-CV charging profile. The use of V_cnt, generated by the charge current regulation sub-circuit, as control voltage for voltage regulator 122 instead of a constant reference voltage, enables circuit 120 to simultaneously provide load supply and battery charging without risking damage to load 114 when battery 116 is disconnected.

[0017] FIG. 2 schematically illustrates one embodiment of combined LDO regulator and battery charger circuit 220, load 114, and battery 116. Circuit 220 may be an exemplary embodiment of circuit 120 of FIG. 1. Circuit 220 includes LDO regulator 222 and CC-CV current regulator 224.

[0018] LDO regulator 222 is arranged to provide a regulated output voltage based on an unregulated input. In one embodiment, LDO regulator 222 includes power transistor M204 and LDO control loop 212. LDO control loop 212 includes amplifier A244, and resistors R₂₁ and R₂₂. Power transistor M204 is arranged to receive an analog signal VrN at the source of power transistor M204. Power transistor M204 is further arranged to provide a regulated output voltage V_OUT to load 114 at node N252. Amplifier A244 is arranged to provide a control signal to a gate of power transistor M204. A feedback loop provides feed back voltage V_fb to an input of amplifier A244 from a drain of power transistor M204 through a first voltage divider. The first voltage divider includes resistors R₂₁ and R₂₂.

[0019] CC-CV current regulator 224 is configured to provide a charging current to battery 116 and to provide control voltage V_cnt to LDO regulator 222 based on a sensed and amplified load current and battery charge voltage. In one embodiment, CC-CV current regulator 224 includes voltage sense circuit 211, current sense integrator 208, and current sense device 206. Current sense device 206 may include sense resistor R₂₃ and current sense integrator 208 may include voltage-controlled current source I242. Voltage sense circuit 211 may include voltage sense integrator 210 and a second voltage divider comprising resistors R₂₄ and R₂₅.

[0020] Control voltage V_cnt may be derived from a current provided by voltage-controlled current source I242. Voltage sense integrator 210 is configured to receive a battery charge voltage V_bat and provide a control signal to voltage-controlled current source I242 based on battery charge voltage V_bat. Current sense resistor R₂₃ is arranged to sense a load current. A voltage derived from the sensed load current and battery charge voltage V_bat may be provided through a second voltage divider to an input of amplifier A244 along with an output of the voltage-controlled current source I242 at node N251. The second voltage divider includes resistors R₂₄ and R₂₅.

[0021] In constant current mode, I242 is configured to supply a constant current. In constant voltage mode, I242's output may be reduced by voltage sense integrator 210, if battery charge voltage V_bat approaches battery limit voltage V_term. Battery charge current, I_chg, flowing through node N253 between R₂₄ and battery terminal BAT+, to battery 116 may be expressed as:

[0022] $I_{\mathrm{chg}}=\frac{V_{\mathrm{OUT}}-V_{\mathrm{bat}}}{R_{23}},$
where V_OUT is an output of LDO regulator 222, and may be expressed as:

[0023] $V_{\mathrm{OUT}}=V_{\mathrm{cnt}}*\frac{R_{21}+R_{22}}{R_{22}},$
and
LDO regulator 222 reference voltage V_cnt may be expressed as:

[0024] $V_{\mathrm{cnt}}=\frac{\left(V_{\mathrm{bat}}+I_{242}*R_{24}\right)*R_{25}}{\left(R_{24}+R_{25}\right)},$
where I₂₄₂ is the current supplied by the current source I242.

[0025] In one embodiment, resistor values may be selected such that

[0026] $\frac{R_{21}}{R_{22}}=\frac{R_{24}}{R_{25}}.$
In that case, the charge current, I_chg, may be expressed as:

[0027] $I_{\mathrm{chg}}=I_{242}*\frac{R_{24}}{R_{23}}.$

[0028] A ratio of the resistors R₂₁, R₂₂, R₂₃, R₂₄, and R₂₅ creates biasing for LDO regulator 222 to regulate V_OUT such that V_OUT is slightly higher than battery charge voltage V_bat, enabling a supply of stable voltage to load 114 and current to battery 116. The difference between V_OUT and V_bat is determined by current source I242 and the ratio of the resistors. In other words, resistors R₂₄ and R₂₅ provide another feedback path for stability to LDO regulator 222.

[0029] In constant current mode, voltage sense integrator 210 and current source I242 may operate in such a way that, an average charge current, I242, may be constant and expressed as:

[0030] $I_{242}=I_{\mathrm{trg}}*\frac{R_{23}}{R_{24}},$
where I_trg is a target full charge current for battery 116.

[0031] As mentioned above, when battery charge voltage, V_bat, reaches about termination voltage V_term, the circuit may begin to operate in constant voltage mode and the current supplied by voltage-controlled current source I242 may decrease. If battery 116 is disconnected while being charged, current flow through R₂₃ may terminate, causing battery charge voltage V_bat to approximate about V_OUT. Since current source I242 is configured to continue supplying the current, V_OUT may increase up to V_IN, but not beyond, protecting load 114. If a partially charged battery is connected to the circuit while only load 114 is being supplied, a current through R₂₃ would be added, causing V_OUT to decrease. However, the feedback path provided by R₂₄ and R₂₅ may control LDO regulator 222 to regulate V_OUT back to a value slightly higher than V_bat.

[0032] In another embodiment, current source I242 may be configured such that a limit voltage for V_OUT has a smaller value than V_IN. A power dissipation characteristic and temperature dependency of resistors R₂₃, R₂₄, and R₂₅ may affect accuracy and stability of current and voltage regulation of the circuit. In a further embodiment, current source I242 may be configured to have a temperature dependent characteristic such that an inaccuracy caused by resistors R₂₃, R₂₄, and R₂₅ may be compensated.

[0033] FIG. 3 schematically illustrates one embodiment of combined LDO regulator and battery charger circuit 320, load 114, and battery 116. Circuit 320 may be another exemplary embodiment of circuit 120 of FIG. 1. Circuit 320 includes LDO regulator 322 and CC-CV current regulator 324.

[0034] LDO regulator 322 operates substantially the same as described in conjunction with FIG. 2 and may include power transistor M304 and LDO control loop 312. LDO control loop 312 may include amplifier A344 and a first voltage divider comprising resistors R₃₁ and R₃₂. One difference between LDO regulator 222 and LDO regulator 322 is node N382 being further coupled to current mirror 375 of CC-CV current regulator 324. This connection provides for further adjustment of feedback voltage V_fb based on a current flowing through current mirror 375. Other operational aspects of LDO regulator 322 are substantially the same as LDO regulator 222 of FIG. 2.

[0035] CC-CV current regulator 324 is similarly configured to provide a charging current to battery 116 and to provide control voltage V_cnt and a portion of feedback voltage V_fb to LDO regulator 322. CC-CV current regulator 324 may include voltage sense circuit 311, current sense integrator 308, and current sense device 306. Voltage sense circuit 311 may include amplifier A372 and a second voltage divider comprising resistors R₃₄, R₃₅, and R₃₆. Current sense integrator 308 may include voltage-controlled current source I373, current mirror 375, and transistor M374. Current sense device 306 may comprise current sense resistor R₃₃.

[0036] In one embodiment, current sense resistor R₃₃ may be arranged to sense a load current. Battery charge voltage V_bat is sensed by a voltage divider including resistors R₃₄, R₃₅, and R₃₆. Amplifier A372 is arranged to receive a portion of battery charge voltage V_bat and amplify a difference between V_bat and a predetermined reference voltage V_ref. Amplifier A372 is further configured to control a gate of transistor M374 of current sense integrator 308. Another portion of the battery charge voltage V_bat may be provided to an input of amplifier A344 of LDO regulator 322 as control voltage V_cnt.

[0037] Voltage-controlled current source I373 is configured to provide a current to one source of current mirror 375 and to transistor M374. Transistor M374 is configured to control the current flowing through one source of current mirror 375. Accordingly another current flowing through another source of current mirror 375 may be controlled by transistor M374. By controlling the current through the other source of current mirror 375, a portion of feedback voltage V_fb at another input of amplifier A344 may also be controlled.

[0038] Current source I373 may be arranged to determine a full rate current setting. If resistors are selected such that:

[0039] $\frac{R_{31}}{R_{32}}=\frac{R_{34}}{\left(R_{35}+R_{36}\right)},$
charge current in constant current mode may be expressed as:

[0040] $I_{\mathrm{chg}}=I_{373}*\frac{R_{31}}{R_{33}},$
where I373 is the current supplied by current source I373.

[0041] This is similar to the operation of the circuit described in FIG. 2. As can be seen in FIG. 3, however, an additional current may be created in the feedback path for LDO regulator 322 by current mirror 375.

[0042] Constant voltage mode may begin when battery 116 is close to being charged such that V_bat approximates V_term, where V_term is determined by:

[0043] $V_{\mathrm{term}}=V_{\mathrm{ref}}*\frac{R_{34}+R_{35}+R_{36}}{R_{36}}.$
V_ref is a predetermined reference voltage. As a result, amplifier A372 may increase a gate voltage of transistor M374 causing transistor M374 to drain some of the current provided by current source I373. This may result in a decrease of a current flowing through the source terminal of current mirror 375 at node N388. As battery 116 further charges, the current flowing through current mirror 375 may continue to decrease. Another result of this decrease is a decrease in the current drawn by the other source of current mirror 375 at node N382 leading to a change in feedback voltage V_fb. LDO regulator 322 is thereby controlled such that when the current reaches about zero, charging will terminate. Accordingly, as battery 116 gets close to being charged, charge current will decrease, while charge voltage remains constant.

[0044] Amplifier A372 may be a slow response, transconductance amplifier with very low output current. This characteristic of amplifier A372 along with capacitor C₃, coupled between the output of amplifier A372 and ground, may provide for improved frequency response of CC-CV current regulator 324 and a smooth battery charging current. In typical applications such as cellular phone chargers, load current may change drastically based, in part, on an operation mode. For example, if the cellular phone begins transmitting while being powered by the circuit, load current may increase dramatically. At the same time, a smooth and stable current is preferred for charging the battery. Slow response, transconductance amplifier A372 coupled with capacitor C₃₁ may provide such smooth and stable battery charge current despite rapidly changing load current.

[0045] In one embodiment, amplifier A372 may be externally programmable to adjust for different power sources. A LDO regulator and battery charger circuit, such as described above, may be employed with a variety of power sources including, but not limited to, AC/DC adapters, Power-over-Ethernet sources, DC/DC adapters, USB ports, and the like. Since each power source may have a different characteristic and tolerance, smoothness and stability of the charging current provided to battery 116 may vary based on the power source. An external programming source may provide amplifier A372 with a control signal enabling amplifier A372 to modify its response characteristics, thereby adjusting battery charge current to varying power sources.

[0046] The above specification, examples and data provide a description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention also resides in the claims hereinafter appended.