JP2013021815A - Charging circuit and electronic apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a charging circuit that has a reduced circuit area while ensuring a tolerance to overvoltage input.SOLUTION: A switching transistor M1 comprises a high voltage element. A pulse modulator 10 generates a pulse signal S1 having a duty ratio depending on a voltage Vthat is a combination of output voltages V-Vof error amplifiers EA1-EA3. A backflow prevention circuit 12 (1) turns on a first switch SW1 parallel with a body diode D1 with an anode directed to an input terminal P1 if V>V, and (2) turns on a second switch SW2 parallel with a body diode D2 with a cathode directed to the input terminal P1 if V>V.

Description

  The present invention relates to a charging circuit for charging a secondary battery.

  Battery-powered devices such as mobile phones, PDAs (Personal Digital Assistants), notebook personal computers, and portable audio players incorporate a rechargeable battery and a charging circuit for charging it. The charging circuit receives a DC voltage from an external AC adapter or USB bus, and charges the secondary battery based on the DC voltage.

FIG. 1 is a circuit diagram illustrating a configuration of an electronic device including a charging circuit studied by the present inventors. The electronic device 1 includes a secondary battery 2 and a charging circuit 100r that charges the secondary battery 2. A DC voltage _VIN from an external power source 4 such as an AC adapter or a USB bus is input to the charging circuit 100r.

  The charging circuit 100r mainly includes an OVP (Over Voltage Protection) circuit 50, a DC / DC converter 60, and a linear charger 70.

The level of the DC voltage _VIN from the external power supply 4 is determined by the specification or standard of the electronic device 1. However, the bad external power supply 4 made by a third party may generate a high DC voltage _VIN outside the range defined by the specification. The OVP circuit 50 is provided to protect the internal circuit of the charging circuit 100r from such an overvoltage.

The OVP circuit 50 includes a high voltage transistor 52, a comparator 54, and a charge pump circuit 56. The high breakdown voltage transistor 52 is an NPN type power transistor, and the breakdown voltage is determined according to the overvoltage level of the assumed input voltage _VIN . For example, it is composed of a high breakdown voltage element of about 24V. The comparator 54 compares the input voltage V _IN with the overvoltage threshold voltage V _TH, and asserts the overvoltage protection signal S _OVP when V _IN > V _TH is detected. The charge pump circuit 56 performs a step-up operation while the overvoltage protection signal S _OVP is negated, that is, when the input voltage _VIN is in a normal range, and supplies a high level gate voltage to the high breakdown voltage transistor 52. The high breakdown voltage transistor 52 is fully turned on. As a result, the input voltage _VIN is supplied to the DC / DC converter 60 in the next stage.

When the overvoltage protection signal S _OVP is asserted, that is, when an overvoltage state of the input voltage _VIN is detected, the charge pump circuit 56 stops the boosting operation and lowers the gate voltage of the high breakdown voltage transistor 52 to reduce the high breakdown voltage transistor. 52 is turned off. Thereby, overvoltage can be prevented from being supplied to the DC / DC converter 60.

The DC / DC converter 60 steps down the input voltage _VIN that has passed through the OVP circuit 50, and generates a DC voltage _VDC . The linear charger 70 receives the DC voltage V _DC and charges the secondary battery 2. The linear charger 70 includes an output transistor 72 and a control circuit 74. The output transistor 72 is provided on a charging path from the output of the DC / DC converter 60 to the secondary battery 2. Based on the current detection signal CS corresponding to the charging current I _CHG for the secondary battery 2 and the voltage detection signal VS corresponding to the battery voltage V _BAT , the control circuit 74 has a resistance component (between drain and source) of the channel of the output transistor 72. Voltage). The control circuit 74 outputs the output transistor so that (1) the charging current I _CHG is constant (CV: constant current mode) or (2) the battery voltage V _BAT is constant (CV: constant voltage mode). 72 gate voltage is controlled.

JP 2006-60977 A JP 2006-304500 A

  In the charging circuit 100 of FIG. 1, at least four power transistors, specifically, a high voltage transistor 52 of the OVP circuit 50, an output transistor 72 of the linear charger 70, a switching transistor (not shown) inside the DC / DC converter 60, and A synchronous rectification transistor is required.

  The power transistor occupies a very large area on the semiconductor substrate. Therefore, when the charging circuit 100r of FIG. 1 is integrated on a semiconductor substrate, the chip size of the charging circuit 100r increases. Further, in the charging circuit 100r of FIG. 1, power loss occurs due to the on-resistance of the high voltage transistor 52 and the output transistor 72. In order to reduce the power loss, the transistor size must be further increased. The chip size of 100r becomes larger.

  The present invention has been made in view of such a problem, and one of exemplary purposes of an embodiment thereof is to provide a charging circuit having a reduced circuit area while having resistance to an overvoltage input.

  One embodiment of the present invention relates to a charging circuit that receives a DC input voltage and charges a battery. The charging circuit includes an input terminal that receives an input voltage, a switching terminal for connecting one end of the inductor, a switching transistor that is provided between the input terminal and the switching terminal, is configured with a high-voltage element, the switching terminal and the ground A synchronous rectification transistor provided between the terminals, and a first error for generating a first error voltage corresponding to an error between a first detection voltage corresponding to an input current flowing from the input terminal to the switching transistor and a predetermined first reference voltage An amplifier, a second error amplifier that generates a second error voltage corresponding to an error between a second detection voltage corresponding to the voltage of the battery and a predetermined second reference voltage, and a third detection voltage corresponding to a charging current flowing through the battery And a third error amplifier that generates a third error voltage corresponding to an error of a predetermined third reference voltage, and a delay corresponding to a voltage obtained by synthesizing the third error voltage from the first error voltage. A pulse modulator that generates a pulse signal having a duty ratio; a driver that complementarily switches the switching transistor and the synchronous rectification transistor based on the pulse signal; and a first that is provided between the back gate and the source of the switching transistor. The switch, the second switch provided between the back gate and the drain of the switching transistor, and the input voltage and the battery voltage are compared. (1) When the input voltage is higher, the first switch and the second switch Among them, one of the anodes in parallel with the body diode provided in the direction of the input terminal is turned on and the other is turned off. (2) When the voltage of the battery is higher, the cathode of the first switch and the second switch Turns on one side in parallel with the body diode provided on the input terminal side and turns off the other Comprising a backflow prevention circuit that, the.

  According to this aspect, two power transistors, ie, a switching transistor and a synchronous rectification transistor are sufficient, so that the circuit area can be reduced. In addition, resistance to overvoltage input is realized by configuring the switching transistor with a high voltage element. Further, the ground fault of the input terminal can be detected by the backflow prevention circuit, and when the ground fault occurs, the direction of the body diode of the switching transistor can be switched to prevent a large current from flowing from the battery to the ground.

The charging circuit of an aspect may further include an overvoltage detection comparator that compares the input voltage with a predetermined threshold voltage and generates an overvoltage detection signal that is asserted when the input voltage is higher than the threshold voltage. The driver may fix the switching transistor off when the overvoltage detection signal is asserted.
In this case, it is possible to prevent an overvoltage from being applied to the synchronous rectification transistor, and the reliability can be further improved.

  Another embodiment of the present invention relates to an electronic device. The electronic device includes a battery and a charging circuit that receives an input voltage from an external power source and charges the battery.

  Note that any combination of the above-described constituent elements and the constituent elements and expressions of the present invention replaced with each other among methods, apparatuses, systems, and the like are also effective as an aspect of the present invention.

  According to the present invention, it is possible to reduce the circuit area while having resistance to overvoltage input.

It is a circuit diagram which shows the structure of an electronic device provided with the charging circuit which the present inventors examined. It is a circuit diagram which shows the structure of an electronic device provided with the charging circuit which concerns on embodiment.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

In this specification, “the state in which the member A is connected to the member B” means that the member A and the member B are physically directly connected, or the member A and the member B are electrically connected to each other. Including the case of being indirectly connected through other members that do not substantially affect the state of connection, or do not impair the functions and effects achieved by the combination thereof.
Similarly, “the state in which the member C is provided between the member A and the member B” refers to the case where the member A and the member C or the member B and the member C are directly connected, as well as their electric It includes cases where the connection is indirectly made through other members that do not substantially affect the general connection state, or that do not impair the functions and effects achieved by their combination.

  FIG. 2 is a circuit diagram illustrating a configuration of the electronic device 1 including the charging circuit 100 according to the embodiment. The electronic device 1 is a battery-driven information terminal device such as a mobile phone terminal, a PDA, or a notebook PC. The electronic device 1 includes a charging circuit 100 and a battery 2.

The battery 2 is a secondary battery such as a lithium ion battery or a nickel metal hydride battery, and outputs a battery voltage V _BAT . The electronic device 1 is provided with an adapter terminal 3 to which an external power supply 4 such as an AC adapter or USB (Universal Serial Bus) can be attached and detached, and receives a voltage (hereinafter referred to as an external voltage) _VEXT from the external power supply 4. .

An external voltage V _EXT (also referred to as an input voltage _VIN ) is input to the input terminal P1 of the charging circuit 100. The charging circuit 100 steps down the input voltage _VIN and charges the battery 2.

  The charging circuit 100 includes a switching transistor M1, a synchronous rectification transistor M2, a first error amplifier EA1 to a third error amplifier EA3, a pulse modulator 10, a driver 11, a backflow prevention circuit 12, and an overvoltage detection comparator 14.

  The charging circuit 100 is a functional IC (Integrated Circuit) integrated on a single semiconductor substrate, and constitutes a step-down DC / DC converter together with an inductor L1 and a capacitor C1 that are externally attached. The first terminal of the capacitor C1 is grounded, and the second terminal is connected to the battery 2. The inductor L1 is provided between the first terminal of the capacitor C1 and the switching terminal P2 of the charging circuit 100.

  The switching transistor M1 is provided between the input terminal P1 and the switching terminal P2, and is composed of a high voltage device. In the present embodiment, the switching transistor M1 is a P-channel MOSFET. The synchronous rectification transistor M2 is provided between the switching terminal P2 and the ground terminal P3. The synchronous rectification transistor M2 is an N-channel MOSFET.

The first error amplifier EA1 is generating a first error voltage _{V ERR1} according to the error of the first detection voltage V1 and a predetermined first reference voltage _{V REF1} in response to the input current _{I IN} flows from the input terminal P1 to the switching transistor M1 To do. The second error amplifier EA2 generates a second error voltage V _ERR2 corresponding to an error between the second detection voltage V2 corresponding to the voltage of the battery 2 (battery voltage V _BAT ) and a predetermined second reference voltage V _REF2 . The third error amplifier EA3 generates a third error voltage V _ERR3 corresponding to an error between the third detection voltage V3 corresponding to the charging current I _CHG flowing through the battery 2 and a predetermined third reference voltage V _REF3 .

Pulse modulator 10 receives the voltage _{V ERR} where the first error voltage _{V ERR1} synthesizes a third error voltage _{V ERR3,} generates a pulse signal S1 having a duty ratio corresponding to voltage _{V ERR.} For example, each of the error amplifiers EA1 to EA3 includes an output stage of an open collector type (open drain), and the drains are connected in common, so that the output voltages V _{ERR1 to} V _ERR3 of the three error amplifiers EA1 to _EA3 are Synthesized. The pulse modulator 10 includes a voltage width, peak current mode, or average current mode pulse width modulator. Since the pulse width modulator can be configured using a known technique, description thereof is omitted here. The driver 11 complementarily switches the switching transistor M1 and the synchronous rectification transistor M2 based on the pulse signal S1.

  The first switch SW1 is provided between the back gate and the source of the switching transistor M1. The second switch SW2 is provided between the back gate and the drain of the switching transistor M1.

The backflow prevention circuit 12 compares the input voltage _VIN and the battery voltage _VBAT , and controls the first switch SW1 and the second switch SW2 according to the comparison result. Specifically, (1) when the input voltage _VIN is higher, one of the first switch SW1 and the second switch SW2 that is in parallel with the body diode D1 provided in the direction of the input terminal P1 side (the first switch SW1) One switch SW1) is turned on and the other (second switch SW2) is turned off. When the battery voltage _VBAT is higher, the backflow prevention circuit 12 has one of the first switch SW1 and the second switch SW2 in parallel with the body diode D2 provided with the cathode facing the input terminal P1 side. (SW2) is turned on and the other (SW1) is turned off.

The overvoltage detection comparator 14 compares the input voltage _VIN with a predetermined threshold voltage V _OVP, and an overvoltage detection signal S2 that is asserted (for example, high level) when the input voltage _VIN is higher than the threshold voltage V _OVP. Is generated. When the overvoltage detection signal S2 is asserted, the driver 11 fixes the switching transistor M1 to be off.

  The above is the configuration of the charging circuit 100. Next, the operation will be described.

When the external power supply 4 is connected to the adapter terminal 3, the input voltage _VIN is supplied to the charging circuit 100. When the input voltage _VIN is a normal level of about 5V, V _BAT <V _IN <V _OVP is established, and the first switch SW1 is turned on.

In a state where the battery voltage V _BAT is low, feedback via the third error amplifier EA3 is dominant, and the duty ratio of the pulse signal S1 is such that the charging current I _CHG becomes a constant value corresponding to the reference voltage V _REF3. Adjusted. When the battery voltage V _BAT increases to some extent, feedback via the second error amplifier EA2 becomes dominant, and the duty ratio of the pulse signal S1 is adjusted so that the battery voltage V _BAT becomes a level corresponding to the reference voltage V _REF2. The Further, when the input current I _IN increases, the feedback via the first error amplifier EA1 becomes dominant, and the input current I _IN is adjusted to be equal to or lower than the upper limit value corresponding to the reference voltage V _REF1 .

  In this way, constant current control and constant voltage control are performed while the input current is limited, and the battery 2 is charged.

When the external power supply 4 is a poor product, the input voltage _VIN may rise to nearly 30V. Also in this case, since the switching transistor M1 is composed of a high breakdown voltage element, the reliability of the charging circuit 100 is maintained. At this time, since the overvoltage detection comparator 14 turns off the switching transistor M1, no overvoltage is applied to the synchronous rectification transistor M2, so the withstand voltage of the synchronous rectification transistor M2 may be low.

Consider a situation where the adapter terminal 3 is grounded or the input voltage _VIN is extremely low. In this case, even if the switching transistor M1 is in an OFF state, a large current may flow from the battery 2 toward the adapter terminal 3 via the diode D2, the first switch SW1, and the input terminal P1.

Therefore, the backflow prevention circuit 12 detects a state in which the input voltage _VIN is low by comparing the two voltages _VIN and _VBAT . When V _IN <V _BAT is detected, the first switch SW1 is turned off and the second switch SW2 is turned on. Thus, the current flowing from the battery 2 toward the adapter terminal 3 can be blocked by the diode D1.

The above is the operation of the charging circuit 100.
The charging circuit 100 is configured by using two power transistors (M1, M2), and the number thereof is halved compared to the four transistors in FIG. Therefore, the circuit area of the charging circuit 100 can be greatly reduced.

  Further, instead of omitting the OVP circuit 50 of FIG. 1, the switching transistor M1 can be made to function as the high breakdown voltage transistor 52 of FIG. 1 by configuring the switching transistor M1 with a high breakdown voltage element. As a result, even when a bad external power supply 4 is connected and an overvoltage is applied, it is possible to prevent the reliability of the charging circuit 100 from being lowered.

Further, in the charging circuit 100r of FIG. 1, since the DC / DC converter 60 generates the DC voltage _VDC and the battery 2 is charged by the linear charger 70, the control circuit 74 of the linear charger 70 and the DC / DC Since the control circuit (not shown) of the converter 60 exists separately, the system is complicated. However, in the charging circuit 100 of FIG. 2, the charging control is performed by the DC / DC converter, so that the system can be simplified.

  Further, in FIG. 1, useless power loss occurs due to the high breakdown voltage transistor 52 and the output transistor 72 present on the charging path. However, in the charging circuit 100 of FIG. Power can be reduced and efficiency can be increased.

  Those skilled in the art will understand that the above-described embodiment is an exemplification, and that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are also within the scope of the present invention. is there.

  In the embodiment, the case where the switching transistor M1 is fixed to be off and charging is stopped when an overvoltage is input has been described. However, the charging operation may be performed even when the overvoltage is input. In this case, the synchronous rectification transistor M2 may also be composed of a high breakdown voltage element.

  In the embodiment, the switching transistor M1 is a P-channel MOSFET. However, the switching transistor M1 may be an N-channel MOSFET. In this case, since the direction of the body diode is opposite to that of the P-channel MOSFET, the switch control by the backflow prevention circuit 12 is opposite to that of the embodiment.

  Although the present invention has been described using specific terms based on the embodiments, the embodiments only illustrate the principles and applications of the present invention, and the embodiments are defined in the claims. Many variations and modifications of the arrangement are permitted without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1 ... Electronic device, 2 ... Battery, 4 ... External power supply, 100 ... Charging circuit, M1 ... Switching transistor, M2 ... Synchronous rectification transistor, SW1 ... 1st switch, SW2 ... 2nd switch, P1 ... Input terminal, P2 ... Switching Terminal P3, ground terminal, EA1, first error amplifier, EA2, second error amplifier, EA3, third error amplifier, 10 pulse modulator, 11 driver, 12 backflow prevention circuit, 14 overvoltage detection comparator.

Claims (3)

A charging circuit that charges a battery in response to a DC input voltage,
An input terminal for receiving the input voltage;
A switching terminal for connecting one end of the inductor;
A switching transistor provided between the input terminal and the switching terminal and configured by a high withstand voltage element;
A synchronous rectification transistor provided between the switching terminal and a ground terminal;
A first error amplifier that generates a first error voltage corresponding to an error between a first detection voltage corresponding to an input current flowing from the input terminal to the switching transistor and a predetermined first reference voltage;
A second error amplifier that generates a second error voltage corresponding to an error between the second detection voltage corresponding to the voltage of the battery and a predetermined second reference voltage;
A third error amplifier that generates a third error voltage corresponding to an error between a third detection voltage corresponding to a charging current flowing through the battery and a predetermined third reference voltage;
A pulse modulator that generates a pulse signal having a duty ratio corresponding to a voltage obtained by synthesizing the third error voltage from the first error voltage;
A driver that complementarily switches the switching transistor and the synchronous rectification transistor based on the pulse signal;
A first switch provided between a back gate and a source of the switching transistor;
A second switch provided between a back gate and a drain of the switching transistor;
The input voltage and the voltage of the battery are compared. (1) When the input voltage is higher, the body diode in which the anode is provided in the direction toward the input terminal of the first switch and the second switch One in parallel with the other and off the other, (2) when the voltage of the battery is higher, of the first switch and the second switch, the cathode is provided in the direction toward the input terminal A backflow prevention circuit that turns on one side in parallel with the diode and turns off the other;
A charging circuit comprising: An overvoltage detection comparator that compares the input voltage with a predetermined threshold voltage and generates an overvoltage detection signal that is asserted when the input voltage is higher than the threshold voltage;
The charging circuit according to claim 1, wherein the driver fixes the switching transistor off when the overvoltage detection signal is asserted. Battery,
The charging circuit according to claim 1 or 2, wherein the battery is charged by receiving an input voltage from an external power source;
An electronic device comprising:

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