Digital controlled battery charging system

This application claims the benefit of CN application No. 201510051503.6, filed on Feb. 2, 2015, and incorporated herein by reference.

The present invention relates to electrical circuit, more particularly but not exclusively relates to battery charging system.

With development of portable electronic device, battery charging system is widely used in application of portable electronic device. FIG. 1 shows a conventional battery charging system 100 employing step-down converter topology. Battery charging system 100 comprises a plurality of control loops, and each control loop comprises an error amplifier and a compensation network. Take a battery voltage control loop as an example, during each switching period, a RS flip-flop FF 0 is set by a clock signal CLK to turn ON a high-side switch PM 1 and turn OFF a low-side switch PM 2 by a switching control signal PWM. A current following through high-side switch PM 1 and an inductor L 1 charges an output capacitor Cout. A battery voltage Vout increases and a feedback signal Vx 1 representing battery voltage Vout increases accordingly. An error amplifier A receives feedback signal Vx 1 and a battery voltage reference signal REF 1 , and provides an error amplifier signal Vcom 1 . When error amplifier signal Vcom 1 decreases less than a peak signal PK, a comparison circuit CP is configured to provide a signal to reset RS flip-flop FF 0 , and switching control signal PWM is configured to turn OFF high-side switch PM 1 and turn ON low-side switch PM 2 . Output capacitor Cout is configured to power a load RL and battery voltage Vout decreases. When clock signal CLK set RS flip-flop FF 0 again, a new switching period starts.

As shown in FIG. 1 , each control loop needs at least one error amplifier, and each error amplifier needs different compensation network for loop stability, as a result, battery charging system 100 becomes complicated, and has poor transplantation for different power stages. Besides, transition between each loop is hard to design per error amplifier signals Vcom 1 ˜Vcomn may be very closed to each other.

It is one of the objects of the present invention to provide a battery charging system and a control circuit for the battery charging system to resolve one or more technical problems.

One embodiment of the present invention discloses a battery charging system, comprising: an input terminal, configured to receive an input voltage and an input current; an output terminal, coupled to a battery and configured to provide a battery voltage and a battery charging current to charge the battery; a first switch, coupled between the input terminal and the output terminal; an input voltage control loop, configured to receive an input voltage feedback signal representative of the input voltage, a slope compensation signal and an input voltage reference signal, and configured to provide a first loop control signal via comparing the input voltage reference signal with a first compounded signal comprising the input voltage feedback signal and the slope compensation signal; an input current control loop, configured to receive an input current feedback signal representative of the input current of the battery charging system, the slope compensation signal and an input current reference signal, and configured to provide a second loop control signal via comparing the input current reference signal with a second compounded signal comprising the input current feedback signal and the slope compensation signal; a battery voltage control loop, configured to receive a battery voltage feedback signal representative of the battery voltage, the slope compensation signal and a battery voltage reference signal, and configured to provide a third loop control signal via comparing the battery voltage reference signal with a third compounded signal comprising the battery voltage feedback signal and the slope compensation signal; a battery charging current control loop, configured to receive a battery charging current feedback signal representative of the battery charging current, the compensation signal, and a battery charging current reference signal, and configured to provide a fourth loop control signal via comparing the battery charging current reference signal with a fourth compounded signal comprising the battery charging current feedback signal and the slope compensation signal; and a digital control unit, configured to receive the first loop control signal, the second loop control signal, the third loop control signal and the fourth loop control signal, and configured to provide a switching control signal to control the first switch automatically in response to one of the input voltage control loop, the input current control loop, the battery voltage control loop and the battery charging current control loop.

Another embodiment of the present invention discloses a battery charging system having an input terminal and an output terminal coupled to a battery, comprising: a first switch, coupled between the input terminal of the battery charging system and the output terminal of the battery charging system; a plurality of analog control loops, configured to receive a plurality of reference signals, a plurality of feedback signals and a slope compensation signal, and configured to provide a plurality of loop control signals based on the plurality of reference signals, the plurality of feedback signals and the slope compensation signal; and a digital control unit, configured to receive the plurality of loop control signals, and configured to provide a switching control signal to control the first switch automatically in response to one of the plurality of analog control loops, wherein when one of the plurality of loop control signals transits to a first state, the switching control signal is configured to turn OFF the first switch.

Yet another embodiment of the present invention discloses a control circuit for a battery charging system, the battery charging system having a switching circuit, and the control circuit comprising: a plurality of analog control loops, configured to receive a plurality of reference signals, a plurality of feedback signals and a slope compensation signal, and configured to provide a plurality of loop control signals based on the plurality of reference signals, the plurality of feedback signals and the slope compensation signal; a digital control unit, configured to receive the plurality of loop control signals and provide a switching control signal to control the switching circuit based on the plurality of loop control signals, the digital control unit is further configured to provide a digital slope control signal to adjust an amplitude of the slope compensation signal, wherein the digital control unit automatically chooses one of the plurality of analog control loops to control the switching circuit; and a slope generating unit, configured to provide the slope compensation signal based on the digital slope control signal and the switching control signal.

In one embodiment, each analog control loop employs a comparison circuit to replace complicated error amplifier, and transition among analog control loops are distinguishable and cleared. The control circuit is compatible and transplantable for power stages having different specs.

Non-limiting and non-exhaustive embodiments are described with reference to the following drawings.

FIG. 1 illustrates a conventional battery charging system 100 .

FIG. 2 schematically illustrates a circuit block diagram of a battery charging system 200 according to an embodiment of the present invention.

FIG. 3 schematically illustrates a battery charging system 200 a according to an embodiment of the present invention.

FIG. 4 schematically illustrates a slope generating unit 26 according to an embodiment of the present invention.

FIG. 5 schematically illustrates a battery charging system 200 b according to an embodiment of the present invention.

FIG. 6 shows waveforms of battery charging system 200 b shown in FIG. 5 according to an embodiment of the present invention.

FIG. 7 schematically illustrates a battery charging system 200 c according to an embodiment of the present invention.

The use of the same reference label in different drawings indicates the same or like components.

In the present application, numerous specific details are provided, such as examples of circuits, components, and methods, to provide a thorough understanding of embodiments of the invention. These embodiments are exemplary, not to confine the scope of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention. Some phrases are used in some exemplary embodiments. However, the usage of these phrases is not confined to these embodiments.

Several embodiments of the present invention are described below with reference to battery charging system and associated control circuit. As used hereinafter, the term “couple” generally refers to multiple ways including a direct connection with an electrical conductor and an indirect connection through intermediaries, such as diodes, resistors, and/or capacitors.

FIG. 2 schematically illustrates a circuit block diagram of a battery charging system 200 according to an embodiment of the present invention. Battery charging system 200 comprises a switching circuit 21 and a control circuit 22 . Switching circuit 21 comprises an input terminal In and an output terminal Out. Input terminal In may be coupled to a power supply, and output terminal Out may be coupled to a battery. Switching circuit 21 comprises a switch. Control circuit 22 is configured to provide switching control signal PWM to turn ON and turn OFF the switch in switching circuit 21 .

Control circuit 22 comprises a plurality of analog control loops 24 _ 1 - 24 _N, a digital control unit 25 , a plurality of digital-to-analog conversion (DAC) units 23 _ 1 - 23 _N and a slope generating unit 26 , where N is an integer larger than 1. Analog control loops 24 _ 1 - 24 _N are configured to receive a slope compensation signal Ramp, reference signals Ref 1 -RefN, and feedback signals FB 1 -FBN respectively, and provide a plurality of loop control signals Ct 1 -CtN. Each analog control loop 24 _ x is configured to provide a loop control signal Ctx based on corresponding reference signal Refx, corresponding feedback signal FBx and the slope compensation signal Ramp, where x is an integer larger than or equal 1 , and less than or equal N. Digital control unit 25 is coupled to analog control loops 24 _ 1 - 24 _N to receive loop control signals Ct 1 -CtN, and provides switching control signal PWM based on loop control signals Ct 1 -CtN and a time period control signal. Digital control unit 25 is further configured to provide a plurality of digital reference signals Dref 1 -DrefN and a digital slope control signal CRA. Digital control unit 25 is configured to turn ON or turn OFF the switch in switching circuit 21 automatically based on one of the analog control loops 24 _ 1 - 24 _N. Digital-to-analog conversion units 23 _ 1 - 23 _N are coupled to digital control unit 25 to receive corresponding digital reference signals DRef 1 -DRefN, and provide reference signals Ref 1 -RefN. Take digital-to-analog conversion unit 23 _ x as one example, digital-to-analog conversion unit 23 _ x is configured to receive digital reference signal DRefx, and provides reference signal Refx based on digital reference signal DRefx. Slope generating unit 26 is coupled to digital control unit 25 to receive digital slope control signal CRA and switching control signal PWM, and provides slope compensation signal Ramp based on digital slope control signal CRA and switching control signal PWM. As shown in FIG. 2 , control circuit 22 is compatible and transplantable for different power stages per employing digital control unit 25 .

FIG. 3 schematically illustrates a battery charging system 200 a according to an embodiment of the present invention. Battery charging system 200 a comprises switching circuit 21 and control circuit 22 . In the embodiment shown in FIG. 3 , switching circuit 21 employs step-down topology as one example. However, one of ordinary skill in the art should appreciate that switching circuit 21 may employ other suitable topology. Switching circuit 21 comprises a switch S 1 , a switch S 2 , an inductor L 1 , and a capacitor C 1 . Switch S 1 and switch S 2 may be any suitable controllable semiconductor device, e.g., Metal Oxide Semiconductor Field Effect Transistor (MOSFET), Junction Field Effect Transistor (JFET), Insulated Gate Bipolar Transistor (IGBT) and so on. One terminal of switch S 1 is coupled to input terminal In of switching circuit 21 , the other terminal of switch S 1 is coupled to one terminal of switch S 2 , the other terminal of switch S 2 is coupled to a system ground. One terminal of inductor L 1 is coupled to a common terminal of switch S 1 and switch S 2 , the other terminal of inductor L 1 is coupled to one terminal of capacitor C 1 , and the other terminal of capacitor C 1 is coupled to the system ground. A common terminal of inductor L 1 and capacitor C 1 is coupled to output terminal Out of switching circuit 21 .

Control circuit 22 comprises digital-to-analog conversion units 23 _ 1 - 23 _ 4 , analog control loops 24 _ 1 - 24 _ 4 , digital control unit 25 and slope generating unit 26 . Battery charging system 200 a takes four digital-to-analog conversion units 23 _ 1 - 23 _ 4 and four analog control loops 24 — 1 - 24 _N as one example, however, one of ordinary skill in the art should also appreciate that control circuit 22 may comprises more digital-to-analog conversion units and analog control loops.

Digital-to-analog conversion unit 23 _ 1 is configured to receive digital reference signal DRef 1 and provide reference signal Ref 1 via digital-to-analog converting. Digital-to-analog conversion unit 23 _ 2 is configured to receive digital reference signal DRef 2 and provide reference signal Ref 2 via digital-to-analog converting. Digital-to-analog conversion unit 23 _ 3 is configured to receive digital reference signal DRef 3 and provide reference signal Ref 3 via digital-to-analog converting. Digital-to-analog conversion unit 23 _ 4 is configured to receive digital reference signal DRef 4 and provide reference signal Ref 4 via digital-to-analog converting.

In one embodiment, analog control loop 24 _ 1 is an input voltage control loop. The input voltage control loop comprises an operation circuit 241 and a comparison circuit 242 . Operation circuit 241 comprises a first input terminal configured to receive feedback signal FB 1 representing an input voltage, a second input terminal configured to receive slope compensation signal Ramp, and an output terminal configured to provide a difference between feedback signal FB 1 and slope compensation signal Ramp (FB 1 -Ramp). Comparison circuit 242 comprises an inverting terminal coupled to the output terminal of operation circuit 241 and a non-inverting terminal coupled to the output terminal of digital-to-analog conversion unit 23 _ 1 to receive reference signal Ref 1 , and an output terminal configured to provide a loop control signal Ct 1 to control the input voltage. In one embodiment, analog control loop 24 _ 2 is an input current control loop. The input current control loop comprises an operation circuit 243 and a comparison circuit 244 . Operation circuit 243 comprises a first input terminal configured to receive feedback signal FB 2 representing an input current, a second input terminal configured to receive slope compensation signal Ramp, and an output terminal configured to provide a summation of feedback signal FB 2 and slope compensation signal Ramp (FB 2 +Ramp). Comparison circuit 244 comprises a non-inverting terminal coupled to the output terminal of operation circuit 243 and an inverting terminal coupled to the output terminal of digital-to-analog conversion unit 23 _ 2 to receive reference signal Ref 2 , and an output terminal configured to provide a loop control signal Ct 2 to control the input current. In one embodiment, analog control loop 24 _ 3 is a battery voltage control loop. The battery voltage control loop comprises an operation circuit 245 and a comparison circuit 246 . Operation circuit 245 comprises a first input terminal configured to receive feedback signal FB 3 representing a battery voltage, a second input terminal configured to receive slope compensation signal Ramp, and an output terminal configured to provide a summation of feedback signal FB 3 and slope compensation signal Ramp (FB 3 +Ramp). Comparison circuit 246 comprises a non-inverting terminal coupled to the output terminal of operation circuit 245 and an inverting terminal coupled to the output terminal of digital-to-analog conversion unit 23 _ 3 to receive reference signal Ref 3 , and an output terminal configured to provide a loop control signal Ct 3 to control the battery voltage. In one embodiment, analog control loop 24 _ 4 is a battery charging current control loop. The battery charging current control loop comprises an operation circuit 247 and a comparison circuit 248 . Operation circuit 247 comprises a first input terminal configured to receive feedback signal FB 4 representing a battery charging current, a second input terminal configured to receive slope compensation signal Ramp, and an output terminal configured to provide a summation of feedback signal FB 4 and slope compensation signal Ramp (FB 4 +Ramp). Comparison circuit 248 comprises a non-inverting terminal coupled to the output terminal of operation circuit 247 and an inverting terminal coupled to the output terminal of digital-to-analog conversion unit 23 _ 4 to receive reference signal Ref 4 , and an output terminal configured to provide a loop control signal Ct 4 to control the battery charging current. In the embodiment shown in FIG. 3 , analog control loops 24 _ 1 - 24 _ 4 receives slope compensation signal Ramp, however, in other embodiments, analog control loops 24 _ 1 - 24 _ 4 may employ slope compensation signals having different amplitudes.

Digital control unit 25 comprises an OR gate 251 and a RS flip-flop 252 . OR gate 251 comprises a first input terminal, a second input terminal, a third input terminal, a fourth input terminal and an output terminal, wherein the first input terminal is coupled to the output terminal of comparison circuit 242 to receive loop control signal Ct 1 , the second input terminal is coupled to the output terminal of comparison circuit 244 to receive loop control signal Ct 2 , the third input terminal is coupled to the output terminal of comparison circuit 246 to receive loop control signal Ct 3 , the fourth input terminal is coupled to the output terminal of comparison circuit 248 to receive loop control signal Ct 4 , and the output terminal is configured to provide a reset signal Reset. RS flip-flop 252 comprises a set terminal S, a reset terminal R, a non-inverting terminal Q and an inverting terminal /Q, wherein set terminal S is configured to receive a time period control signal TCON, reset terminal R is coupled to the output terminal of OR gate 251 , the non-inverting output terminal Q is configured to provide switching control signal PWM 1 to turn ON and turn OFF switch S 1 , and the inverting terminal /Q is configured to provide switching control signal PWM 2 to turn ON and turn OFF switch S 2 . When one of loop control signals Ct 1 -Ct 4 becomes high voltage level, corresponding analog control loop works to turn OFF switch S 1 . In one embodiment, switch S 1 is turned ON based on time period control signal TCON, and an OFF time period of switch S 1 is controlled by time period control signal TCON. Digital control unit 25 is further configured to provide digital slope control signal CRA and digital reference signals DRef 1 -DRef 4 . In one embodiment, digital slope control signal CRA and digital reference signals DRef 1 -DRef 4 may be stored in an on-chip memory, or may be set via a communication bus, such as Inter-Integrated Circuit (I2C), Power Management Bus (PMBus), System Management Bus (SMBus) and so on.

In the embodiment shown in FIG. 3 , analog control loops 24 _ 1 - 24 _ 4 employs comparison circuits to provide loop control signals Ct 1 -Ct 4 which are logic signals having high voltage level or low voltage level, and when any one of loop control signals Ct 1 -Ct 4 becomes high voltage level, digital control unit 25 is configured to turn OFF switch S 1 automatically based on corresponding analog control loop via OR gate 251 , as a result, reliable transition between different analog control loops is automatically achieved with simple circuit structure, while complex error amplifier circuits and associated compensation network are eliminated.

FIG. 4 schematically illustrates a slope generating unit 26 according to an embodiment of the present invention. Slope generating unit 26 comprises a switch S 3 , a current source CS and a capacitor C 2 . Capacitor C 2 has a first terminal configured to receive a charging current provided by current source CS and a second terminal coupled to the system ground, and a voltage across capacitor C 2 is slope compensation signal Ramp. Switch S 3 is coupled to capacitor C 2 in parallel. Switch S 3 has a first terminal coupled to the first terminal of capacitor C 2 , a second terminal coupled to the second terminal of capacitor C 2 , and a control terminal configure to receive switching control signal PWM 1 . Switch S 3 is turned ON and turned OFF based on switching control signal PWM 1 . In one embodiment, when switch S 1 is turned ON by switching control signal PWM 1 , switch S 3 is turned OFF, capacitor C 2 is charged by current source CS, slope compensation signal Ramp increases; and when switch S 1 is turned OFF by switching control signal PWM 1 , switch S 3 is turned ON, capacitor C 2 is discharged through switch S 3 , slope compensation signal Ramp decreases to zero volt. In one embodiment, the charging current provided by current source CS is adjusted to adjust an increasing slope rate of slope compensation signal Ramp based on digital slope control signal CRA, as a result, amplitude of slope compensation signal Ramp is adjusted. In one embodiment, capacitance of capacitor C 2 is adjusted to adjust an increasing slope rate of slope compensation signal Ramp based on digital slope control signal CRA, as a result, amplitude of slope compensation signal Ramp is adjusted.

FIG. 5 schematically illustrates a battery charging system 200 b according to an embodiment of the present invention. As shown in FIG. 5 , battery charging system 200 b further comprises an analog-to-digital conversion unit 27 . Analog-to-digital conversion unit 27 comprises a multiplexer 271 and an analog-to-digital conversion (ADC) device 272 . Multiplexer 271 is configured to receive feedback signals FB 1 -FB 4 , provides one of feedback signals FB 1 -FB 4 at an output terminal based on a control signal CTRL. Analog-to-digital conversion device 272 has an input terminal coupled to the output terminal of multiplexer 271 , and an output terminal configured to provide a digital signal DFB via analog-to-digital converting of one of feedback signals FB 1 -FB 4 . Control signal CTRL is configured to control a timing sequence of analog-to-digital converting, e.g., at a first time, converting feedback signal FB 1 to digital signal DFB, at a second time, converting feedback signal FB 2 to digital signal DFB, at a third time, converting feedback signal FB 3 to digital signal DFB, and at a fourth time, converting feedback signal FB 4 to digital signal DFB.

Continuing with FIG. 5 , digital control unit 25 further comprises a multiplexer 253 , an operation module 254 , a digital compensation loop 255 , an output module 256 , operation modules 257 - 259 and 250 . Multiplexer 253 is configured to receive digital reference signals DRef 1 -DRef 4 , and is configured to choose one of digital reference signals DRef 1 -DRef 4 as a digital reference signal DRef at its output terminal based on control signal CTRL. Operation module 254 has a first input terminal coupled to the output terminal of analog-to-digital conversion device 272 to receive digital signal DFB, a second input terminal coupled to the output terminal of multiplexer 253 to receive digital reference signal DRef, and an output terminal configured to provide a digital error signal Der based on a difference between digital reference signal DRef and digital signal DFB (DRef-DFB). Digital compensation loop 255 is coupled to the output terminal of operation module 254 to receive digital error signal Der, and is configured to provide an error compensation signal Vc via a digital compensating. The digital compensating may comprise Proportional Integral (PI) compensating, Proportional Integral Differential (PID) compensating, and non-linear compensating. Output module 256 is coupled to the output terminal of digital compensation loop 255 , and is configured to provide error compensation signals Vc 1 -Vc 4 based on control signal CTRL and error compensation signal Vc. Error compensation signals Vc 1 -Vc 4 may be either positive or negative. When analog-to-digital conversion device 272 provides digital signal DFB based on feedback signal FB 1 , multiplexer 253 provides digital reference signal DRef based on digital reference signal DRef 1 , digital compensation loop 255 is configured to provide error compensation signal Vc based on digital reference signal DRef 1 and feedback signal FB 1 , and output module 256 is configured to provide error compensation signal Vc to operation module 257 as an input voltage error compensation signal Vc 1 . When analog-to-digital conversion device 272 provides digital signal DFB based on feedback signal FB 2 , multiplexer 253 provides digital reference signal DRef based on digital reference signal DRef 2 , digital compensation loop 255 is configured to provide error compensation signal Vc based on digital reference signal DRef 2 and feedback signal FB 2 , and output module 256 is configured to provide error compensation signal Vc to operation module 258 as an input current error compensation signal Vc 2 . When analog-to-digital conversion device 272 provides digital signal DFB based on feedback signal FB 3 , multiplexer 253 provides digital reference signal DRef based on digital reference signal DRef 3 , digital compensation loop 255 is configured to provide error compensation signal Vc based on digital reference signal DRef 3 and feedback signal FB 3 , and output module 256 is configured to provide error compensation signal Vc to operation module 259 as a battery voltage error compensation signal Vc 3 . When analog-to-digital conversion device 272 provides digital signal DFB based on feedback signal FB 4 , multiplexer 253 provides digital reference signal DRef based on digital reference signal DRef 4 , digital compensation loop 255 is configured to provide error compensation signal Vc based on digital reference signal DRef 4 and feedback signal FB 4 , and output module 256 is configured to provide error compensation signal Vc to operation module 250 as a battery charging current error compensation signal Vc 4 . Operation module 257 comprises a first input terminal coupled to output module 256 to receive input voltage error compensation signal Vc 1 , a second input terminal configured to receive digital reference signal DRef 1 , and an output terminal coupled to an input terminal of digital-to-analog conversion unit 23 _ 1 to provide a summation of digital reference signal DRef 1 and input voltage error compensation signal Vc 1 . Digital-to-analog conversion unit 23 _ 1 has an output terminal configured to provide reference signal Ref 1 based on the summation of digital reference signal DRef 1 and input voltage error compensation signal Vc 1 . Operation module 258 comprises a first input terminal coupled to output module 256 to receive input current error compensation signal Vc 2 , a second input terminal configured to receive digital reference signal DRef 2 , and an output terminal coupled to an input terminal of digital-to-analog conversion unit 23 _ 2 to provide a summation of digital reference signal DRef 2 and input current error compensation signal Vc 2 . Digital-to-analog conversion unit 23 _ 2 has an output terminal configured to provide reference signal Ref 2 based on the summation of digital reference signal DRef 2 and input current error compensation signal Vc 2 . Operation module 259 comprises a first input terminal coupled to output module 256 to receive battery voltage error compensation signal Vc 3 , a second input terminal configured to receive digital reference signal DRef 3 , and an output terminal coupled to an input terminal of digital-to-analog conversion unit 23 _ 3 to provide a summation of digital reference signal DRef 3 and battery voltage error compensation signal Vc 3 . Digital-to-analog conversion unit 23 _ 3 has an output terminal configured to provide reference signal Ref 3 based on the summation of digital reference signal DRef 3 and battery voltage error compensation signal Vc 3 . Operation module 250 comprises a first input terminal coupled to output module 256 to receive battery charging current error compensation signal Vc 4 , a second input terminal configured to receive digital reference signal DRef 4 , and an output terminal coupled to an input terminal of digital-to-analog conversion unit 23 _ 4 to provide a summation of digital reference signal DRef 4 and battery charging current error compensation signal Vc 4 . Digital-to-analog conversion unit 23 _ 4 has an output terminal configured to provide reference signal Ref 4 based on the summation of digital reference signal DRef 4 and battery charging current error compensation signal Vc 4 .

Reference signals Ref 1 -Ref 4 are calibrated by error compensation signals Vc 1 -Vc 4 to reduce a static error caused by slope compensation signal Ramp. In the embodiment shown in FIG. 5 , digital control unit 25 employs only one digital compensation loop 255 to calibrate reference signals Ref 1 -Ref 4 .

FIG. 6 shows waveforms of battery charging system 200 b shown in FIG. 5 according to an embodiment of the present invention. The waveforms shown in FIG. 6 is switching control signal PWM 1 , input voltage feedback signal FB 1 , a summation of reference signal Ref 1 and slope compensation signal Ramp, reference signal Ref 2 , the summation of input current feedback signal FB 2 and slope compensation signal Ramp, reference signal Ref 3 , the summation of battery voltage feedback signal FB 3 and slope compensation signal Ramp, reference signal Ref 4 , and the summation of battery charging current feedback signal FB 4 and slope compensation signal Ramp. In the embodiment shown in FIG. 6 , switch S 1 is controlled by analog control loop 24 _ 4 as one example. One of ordinary skill in the art should understand that switch S 1 may also be controlled by one of analog control loops 24 _ 1 - 24 _ 3 . At time T 1 , switching control signal PWM 1 becomes high voltage level to turn ON switch S 1 , slope compensation signal Ramp increases. At time T 2 , the summation of battery charging current feedback signal FB 4 and compensation signal Ramp increases larger than reference signal Ref 4 , and loop control signal Ct 4 becomes high voltage level. Meanwhile, the summation of reference signal Ref 1 and slope compensation signal Ramp is less than input voltage feedback signal FB 1 , and loop control signal Ct 1 is low voltage level; the summation of input current feedback signal FB 2 and slope compensation signal Ramp is less than reference signal Ref 2 , and loop control signal Ct 2 is low voltage level; the summation of battery voltage feedback signal FB 3 and slope compensation signal Ramp is less than reference signal Ref 3 , and loop control signal Ct 3 is low voltage level. Reset signal Reset becomes high voltage level to reset RS flip-flop 252 since loop control signal Ct 4 becomes high voltage level, switching control signal PWM 1 becomes low voltage level to turn OFF switch S 1 accordingly, and slope compensation signal Ramp decreases to zero quickly. At time T 3 , OFF time period T_off of switch S 1 equals to a predetermine time period controlled by time period control signal TCON, and switching control signal PWM 1 becomes high voltage level to turn ON switch S 1 again, a new switching period starts.

In summary, battery charging system 200 b is able to automatically choose one of analog control loops 24 _ 1 - 24 _ 4 to turn OFF switch S 1 with simple structure and without complex error amplifier.

FIG. 7 schematically illustrates a battery charging system 200 c according to an embodiment of the present invention. Battery charging system 200 c is configured to automatically choose one of analog control loops 24 _ 1 - 24 _ 4 to turn ON switch S 1 .

Analog control loop 24 _ 1 comprises an operation circuit 641 and a comparison circuit 642 . Operation circuit 641 is configured to receive input voltage feedback signal FB 1 and slope compensation signal Ramp, and provide a summation of input voltage feedback signal FB 1 and slope compensation signal Ramp at an output terminal. Comparison circuit 642 has a non-inverting terminal coupled to the output terminal of operation circuit 641 , an inverting terminal coupled to the output terminal of digital-to-analog conversion unit 23 _ 1 to receive reference signal Ref 1 , and an output terminal configured to provide input voltage loop control signal Ct 1 . Analog control loop 24 _ 2 comprises an operation circuit 643 and a comparison circuit 644 . Operation circuit 643 has a first input terminal configured to receive input current feedback signal FB 2 and a second input terminal configured to receive slope compensation signal Ramp, and an output terminal configured to provide a difference between input current feedback signal FB 2 and slope compensation signal Ramp. Comparison circuit 644 has a non-inverting terminal coupled to the output terminal of digital-to-analog conversion unit 23 _ 2 to receive reference signal Ref 2 , an inverting terminal coupled to the output terminal of operation circuit 643 , and an output terminal configured to provide input current loop control signal Ct 2 . Analog control loop 24 _ 3 comprises an operation circuit 645 and a comparison circuit 646 . Operation circuit 645 has a first input terminal configured to receive battery voltage feedback signal FB 3 and a second input terminal configured to receive slope compensation signal Ramp, and an output terminal configured to provide a difference between battery voltage feedback signal FB 3 and slope compensation signal Ramp. Comparison circuit 646 has a non-inverting terminal coupled to the output terminal of digital-to-analog conversion unit 23 _ 3 to receive reference signal Ref 3 , an inverting terminal coupled to the output terminal of operation circuit 645 , and an output terminal configured to provide battery voltage loop control signal Ct 3 . Analog control loop 24 _ 4 comprises an operation circuit 647 and a comparison circuit 648 . Operation circuit 647 has a first input terminal configured to receive battery charging current feedback signal FB 4 and a second input terminal configured to receive slope compensation signal Ramp, and an output terminal configured to provide a difference between battery charging current feedback signal FB 4 and slope compensation signal Ramp. Comparison circuit 647 has a non-inverting terminal coupled to the output terminal of digital-to-analog conversion unit 23 _ 4 to receive reference signal Ref 4 , an inverting terminal coupled to the output terminal of operation circuit 647 , and an output terminal configured to provide battery charging current loop control signal Ct 4 .

Digital control unit 25 comprises an AND gate 651 and a RS flip-flop 652 . AND gate 651 comprises a first input terminal coupled to the output terminal of comparison circuit 642 to receive input voltage loop control signal Ct 1 , a second input terminal coupled to the output terminal of comparison circuit 644 to receive input current loop control signal Ct 2 , a third input terminal coupled to the output terminal of comparison circuit 646 to receive battery voltage loop control signal Ct 3 , a fourth input terminal coupled to the output terminal of comparison circuit 648 to receive battery charging current loop control signal Ct 4 , and an output terminal configured to provide a set signal Set. RS flip-flop 652 has a set terminal S coupled to the output terminal of AND gate 651 to receive set signal Set, a reset terminal R configured to receive time period control signal TCON, a non-inverting output terminal Q configured to provide switching control signal PWM 1 to control switch S 1 , and an inverting output terminal /Q configured to provide switching control signal PWM 2 to control switch S 2 . When all of loop control signals Ct 1 -Ct 4 is high voltage level, switch S 1 is turned ON. In one embodiment, switch S 1 is turned OFF when an ON time period of switch S 1 equals a predetermined time period controlled by time period control signal TCON. Digital control unit 25 further provides digital slope control signal CRA and digital reference signal DRef 1 -DRef 4 .

In one embodiment, battery charging system 200 c is configured to generate reference signal Ref 1 based on a difference between digital reference signal DRef 1 and input voltage error compensation signal Vc 1 , generate reference signal Ref 2 based on a difference between digital reference signal DRef 2 and input current error compensation signal Vc 2 , generate reference signal Ref 3 based on a difference between digital reference signal DRef 3 and battery voltage error compensation signal Vc 3 , and generate reference signal Ref 4 based on a difference between digital reference signal DRef 4 and battery charging current error compensation signal Vc 4 . One of ordinary skill in the art should understand that error compensation signals Vc 1 -Vc 4 may be positive or negative. Digital control unit 25 further comprises operation modules 657 - 659 and 650 . Operation module 657 has a first input terminal coupled to the output terminal of output module 256 to receive input voltage error compensation signal Vc 1 , a second input terminal configured to receive digital reference signal DRef 1 , and an output terminal coupled to the input terminal of digital-to-analog conversion unit 23 _ 1 to provide a difference between digital reference signal DRef 1 and input voltage error compensation signal Vc 1 . The output terminal of digital-to-analog conversion unit 23 _ 1 is configured to provide reference signal Ref 1 based on the difference between digital reference signal DRef 1 and input voltage error compensation signal Vc 1 . Operation module 658 has a first input terminal coupled to the output terminal of output module 256 to receive input current error compensation signal Vc 2 , a second input terminal configured to receive digital reference signal DRef 2 , and an output terminal coupled to the input terminal of digital-to-analog conversion unit 23 _ 2 to provide a difference between digital reference signal DRef 2 and input current error compensation signal Vc 2 . The output terminal of digital-to-analog conversion unit 23 _ 2 is configured to provide reference signal Ref 2 based on the difference between digital reference signal DRef 2 and input current error compensation signal Vc 2 . Operation module 659 has a first input terminal coupled to the output terminal of output module 256 to receive battery voltage error compensation signal Vc 3 , a second input terminal configured to receive digital reference signal DRef 3 , and an output terminal coupled to the input terminal of digital-to-analog conversion unit 23 _ 3 to provide a difference between digital reference signal DRef 3 and battery voltage error compensation signal Vc 3 . The output terminal of digital-to-analog conversion unit 23 _ 3 is configured to provide reference signal Ref 3 based on the difference between digital reference signal DRef 3 and battery voltage error compensation signal Vc 3 . Operation module 650 has a first input terminal coupled to the output terminal of output module 256 to receive battery charging current error compensation signal Vc 4 , a second input terminal configured to receive digital reference signal DRef 4 , and an output terminal coupled to the input terminal of digital-to-analog conversion unit 23 _ 4 to provide a difference between digital reference signal DRef 4 and battery voltage error compensation signal Vc 4 . The output terminal of digital-to-analog conversion unit 23 _ 4 is configured to provide reference signal Ref 4 based on the difference between digital reference signal DRef 4 and battery voltage error compensation signal Vc 4 .

While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.