CN107482722B - A constant current charger circuit - Google Patents

Abstract

The invention discloses a constant current charger circuit, which mainly solves the problems of charging current precision and circuit stability caused by offset voltage in the lithium battery charging process.The invention comprises the following steps: the sampling unit (2) samples the charging unit (1) to obtain a sampling current signal I_BFETH(ii) a The switched capacitor circuit unit (3), the voltage compensation unit (4) and the signal conversion unit (5) form negative feedback and are used for ensuring sampling precision. Sampling current signal I_BFETHThe sampling voltage is converted by a signal conversion unit (5) and output to a charging current regulation unit (6). The charging current adjusting unit (6) obtains a charging current adjusting signal according to the sampling voltage and feeds the charging current adjusting signal back to the charging unit (1) for controlling and adjusting the magnitude of the constant current charging current. The digital control unit (7) is used for switching the working state of the switched capacitor circuit unit (3) so as to eliminate the influence of offset voltage of the operational amplifier. The invention can keep the charging current accurate and stable under different offset voltages, and can be used for a lithium battery charger.

Description

A constant current charger circuit

technical field

The invention belongs to the technical field of electronic circuits, in particular to a constant current charging circuit with high precision and high stability, which can be used in a single-cell lithium battery charger.

Background technique

With the popularity of mobile phones, tablets, and wearable electronic devices, power management for these mobile electronic devices becomes more and more important. Lithium batteries are widely used in mobile electronic devices due to their high operating voltage and long life. According to the chemical characteristics of lithium batteries, the most popular charging method is the three-stage charging method: trickle charging, constant current charging, and constant voltage charging. Among them, constant current charging occupies the main process of lithium battery charging.

Figure 1 is a schematic diagram of a traditional lithium battery constant current charging circuit. Among them, SYS is the stable output of the DC-DC (direct current-direct current) regulator. In the constant current charging stage, the SYS voltage is always higher than the battery voltage by △V1 (△V1>0V), and the N1 tube is the power tube of the lithium battery charger. , the S1 tube is the sampling tube. BAT is the lithium battery to be charged, R1 is an external zero-temperature high-precision resistor, REF is the reference level, and PUMPC is the output of the charge pump. The principle of constant current charging is that the gate, drain and substrate voltages of the sampling tube S1 and the power tube N1 are equal respectively, and the source voltages of S1 and N1 are also equalized through the deep negative feedback of the operational amplifier. In the case where the voltages at the four terminals are all equal, the sampling ratio is only related to the width-length ratio of the MOS tube. The sampling current I _BFETH flows through the resistor R1 to generate the sampling voltage, which is compared with the reference voltage to adjust the gate terminal voltage PUMP of the power tube, so as to control the charging current at a constant value.

Due to design and process reasons, the op amp A1 has an offset voltage (offset), which will cause errors in the sampling current, which in turn lead to deviations in the constant-current charging current. The bias current is:

It can be seen from this formula that the offset voltage (offset) error will be amplified by (V _PUMP -V _BAT -V _TH ) times, and (V _PUMP -V _BAT -V _TH ) is the overdrive voltage of the power transistor N1, which means that when The greater the preset charging current, the greater the current deviation. At this time, it is easy to cause the current to exceed the adjustment range of the constant current charging loop, and the constant current charging function will fail. Nowadays, with the continuous increase of battery capacity and the strict requirements of users for charging time, the constant current charging current will continue to increase, which means that the circuit stability problem caused by the offset voltage will be continuously amplified.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose a constant current charger circuit with high precision and high stability in view of the above-mentioned deficiencies of the prior art. By using the switched capacitor technology, the offset voltage (offset) of the operational amplifier can be eliminated and the current can be reduced. The sampling error enables the lithium battery charger to keep the charging current accurate and the charging loop stable during high current constant current charging.

In order to achieve the above purpose, a constant current charger circuit of the present invention includes: a charging unit 1, a sampling unit 2, a signal conversion unit 5, a charging current adjustment unit 6, and a digital control unit 7, and is characterized by comprising: a switched capacitor circuit unit 3 , voltage compensation unit 4;

The charging unit 1 is used for constant current charging of the lithium battery; it is provided with two inputs, the first input is connected to the output voltage SYS of the front-stage DC-DC (direct current-direct current) converter, and the second input is connected to the output of the charging current adjustment unit 6; the output terminal is connected to the anode voltage BAT of the lithium battery to be charged;

The sampling unit 2 has two inputs for proportional sampling of the charging current; the first input end is connected to the output voltage SYS of the DC-DC (direct current-direct current) converter, and the second input end is connected to the charging unit The input end of 1; the output end BFETH outputs the sampling current signal I _BFETH ;

The switched capacitor circuit unit 3 has four inputs, the first input terminal is connected to the anode voltage BAT of the lithium battery to be charged, the second input terminal is connected to the output terminal BFETH of the sampling unit 2, and the third input terminal is connected to the anode voltage BAT of the lithium battery to be charged. For the logic control signal P1 output by the digital control unit 7, the fourth input end is connected to the logic control signal P1X output by the digital control unit 7; the output end is connected to the input end of the voltage compensation unit 4;

The output terminal of the voltage compensation unit 4 is connected to the first input terminal of the signal conversion unit 5 for compensating the output voltage of the switched capacitor circuit unit 3 and reducing the sampling accuracy error;

The signal conversion unit 5 is provided with two inputs, and its second input terminal is connected to the sampling current signal I _BFETH ; the switched capacitor circuit unit 3, the voltage compensation unit 4 and the signal conversion unit 5 form a feedback loop to ensure sampling The unit 2 can accurately sample the charging current in proportion; the output terminal is connected to the first input terminal of the charging current adjusting unit 6;

The charging current adjustment unit 6 is provided with two inputs, the second input terminal is connected to the reference voltage REF, and the output terminal is connected to the input terminal of the signal sampling unit 2 and the input terminal of the charging unit 1 to adjust the charging current, Ensure that the charging current is constant;

The digital control unit 7 is used to generate two logic control signals P1 and P1X, wherein the first logic control signal P1 controls the linkage switches S1-S6 in the switched capacitor circuit unit 3, and the second logic control signal P1X controls the switched capacitor circuit. Linkage switches S7-S12 in unit 3.

The above-mentioned charging unit 1 is composed of a first transistor MN1, which is composed of N (N>1) identical NMOS transistors in parallel; the gate terminal of the first transistor MN1 is connected to the output terminal of the charging current adjustment unit 6, and the source The terminal is connected to the anode voltage BAT of the lithium battery, and the drain terminal is connected to the output voltage SYS of the front-stage DC-DC converter, and SYS=VBAT+△V1 (△V1>0V).

The above-mentioned sampling unit 2 is composed of a second transistor MN2, the size of which is exactly the same as that of the NMOS transistor that constitutes the first transistor MN1, and the number ratio is 1:N; The drain terminals are connected, the gate terminal of the second transistor MN2 is connected to the gate terminal of the first transistor MN1, and the source terminal of the second transistor MN2 is used as the output terminal BFETH of the sampling unit 2 to output the sampling current signal I _BFETH .

The above-mentioned switched capacitor circuit unit 3 includes a first operational amplifier A1, a second operational amplifier A2, a first capacitor C1, a second capacitor C2 and 12 linkage switches S1-S12:

The first linkage switch S1 is connected to the output end BFETH of the sampling unit 2 and the upper plate of the first capacitor C1, and the lower plate of the first capacitor C1 is connected to the reverse input end of the first operational amplifier A1;

The second linkage switch S2 is connected to the anode voltage BAT of the lithium battery and the forward input terminal of the first operational amplifier A1;

The third linkage switch S3 is connected to the output end of the first operational amplifier A1 and the input end A of the voltage compensation unit 4;

The fourth linkage switch S4 is connected across the reverse input end and the output end of the second operational amplifier A2;

The fifth linkage switch S5 is connected to the upper plate of the second capacitor C2 and the forward input terminal of the second operational amplifier A2;

The sixth linkage switch S6 is connected to the forward input terminal of the second operational amplifier A2 and the input terminal A of the voltage compensation unit 4;

The seventh linkage switch S7 is connected across the reverse input end and the output end of the first operational amplifier A1;

The eighth linkage switch S8 is connected to the upper plate of the first capacitor C1 and the forward input terminal of the first operational amplifier A1;

The ninth linkage switch S9 is connected to the forward input terminal of the first operational amplifier A1 and the input terminal A of the voltage compensation unit 4;

The tenth linkage switch S10 is connected to the output end BFETH of the sampling unit 2 and the upper plate of the second capacitor C2, and the lower plate of the second capacitor C2 is connected to the reverse input end of the second operational amplifier A2;

The eleventh linkage switch S11 is connected to the anode voltage BAT of the lithium battery and the forward input terminal of the second operational amplifier A2;

The twelfth linkage switch S12 is connected to the output end of the second operational amplifier A2 and the input end A of the voltage compensation unit 4 .

The above-mentioned voltage compensation unit 4 includes a third transistor MN3 and a constant current source I1;

The gate terminal of the third transistor MN3 is used as the input terminal of the voltage compensation unit 4, the drain terminal is connected to the power supply voltage VCC, and the source terminal is used as the output terminal of the voltage compensation unit 4;

The current source I1 is connected between the source terminal of the third transistor MN3 and the ground.

The above-mentioned signal conversion unit 5 includes a fourth transistor MP1 and a first resistor R1;

The gate terminal of the fourth transistor MP1 is used as the input terminal of the signal conversion unit 5, the source terminal is connected to the output terminal BFETH of the sampling unit 2, and the drain terminal is used as the output terminal of the signal conversion unit 5;

The first resistor R1 is connected between the drain terminal of the fourth transistor MP1 and the ground.

The above-mentioned charging current adjustment unit 6 includes a third operational amplifier A3, a fifth transistor MN4 and a second resistor R2;

The reverse input terminal of the third operational amplifier A3 is used as the first input terminal of the charging current adjustment unit 6, and the forward input terminal is used as the second input terminal of the charging current adjustment unit 6, and is connected to the reference voltage REF; its output terminal is connected to to the gate terminal of the fifth transistor MN4;

The source terminal of the fifth transistor MN4 is connected to the ground, and its drain terminal is connected to the external charge pump output voltage PUMPC through the second resistor R2; the common terminal PUMP of the fifth transistor MN4 and the second resistor R2 is used as the charging current adjustment unit 6 output.

Compared with the prior art, the present invention has the following advantages:

1. The present invention records and stores the offset voltage of the amplifier in the capacitor by introducing a switched capacitor circuit, and releases and offsets the original offset voltage during the sampling process, thereby ensuring the sampling accuracy of the charging current, thereby solving the problem of sampling errors due to sampling errors. The resulting problem that the constant current charging loop is unstable and easy to fail during high current charging greatly improves the current accuracy of the constant current charging and the stability of the circuit system.

2. The present invention corrects the DC operating point of the operational amplifier in the offset voltage storage stage by introducing a voltage compensation circuit to reduce the influence of the parasitic capacitance at the input end of the operational amplifier on the circuit accuracy when the switched capacitor circuit is switched.

Description of drawings

Figure 1 is a schematic diagram of the operation of a traditional lithium battery constant current charging circuit;

2 is a block diagram of a lithium battery constant current charging system of the present invention;

3 is a circuit diagram of a switched capacitor circuit unit of the present invention;

4 is a circuit diagram of a voltage compensation unit of the present invention;

5 is a circuit diagram of a signal conversion unit of the present invention;

6 is a circuit diagram of a charging current adjustment unit of the present invention;

7 is a schematic diagram of the logic control signal generated by the digital control unit of the present invention;

8 is a schematic diagram of the overall operation of the constant current charging circuit of the present invention;

9 is an equivalent circuit diagram of the constant current charging circuit of the present invention in different working states;

Detailed ways

The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

2, a constant current charger circuit provided by the present invention includes a charging unit 1, a sampling unit 2, a switched capacitor circuit unit 3, a voltage compensation unit 4, a signal conversion unit 5, a charging current adjustment unit 6, and a digital control unit. 7, of which:

The charging unit 1 is used for constant current charging of the lithium battery; it is provided with two inputs, the first input is connected to the output voltage SYS of the front-stage DC-DC (direct current-direct current) converter, and the second input is connected to the output of the charging current adjustment unit 6; the output terminal is connected to the anode voltage BAT of the lithium battery to be charged;

The sampling unit 2 has two inputs for proportional sampling of the charging current; the first input end is connected to the output voltage SYS of the DC-DC (direct current-direct current) converter, and the second input end is connected to the charging unit The input end of 1; the output end BFETH outputs the sampling current signal I _BFETH ;

The switched capacitor circuit unit 3 has four inputs, the first input terminal is connected to the anode voltage BAT of the lithium battery to be charged, the second input terminal is connected to the sampling current signal I _BFETH , and the third input terminal is connected to the digital control The logic control signal P1 of the output of the unit 7, the fourth input end is connected to the logic control signal P1X of the output of the digital control unit 7; the output end is connected to the input end of the voltage compensation unit 4;

The output terminal of the voltage compensation unit 4 is connected to the first input terminal of the signal conversion unit 5 for compensating the output voltage of the switched capacitor circuit unit 3 and reducing the sampling accuracy error;

The signal conversion unit 5 is provided with two inputs, the second input terminal of which is connected to the sampling current signal I _BFETH ; the output terminal is connected to the first input terminal of the charging current adjustment unit 6 . The switched capacitor circuit unit 3 , the voltage compensation unit 4 and the signal conversion unit 5 form a feedback loop to ensure that the sampling unit 2 can accurately sample the charging current in proportion.

The charging current adjustment unit 6 is provided with two inputs, the second input terminal is connected to the reference voltage REF, and the output terminal is connected to the input terminal of the signal sampling unit 2 and the input terminal of the charging unit 1 to adjust the charging current, Ensure that the charging current is constant;

The digital control unit 7 is used to generate two logic control signals P1 and P1X, wherein the first logic control signal P1 controls the linkage switches S1-S6 in the switched capacitor circuit unit 3, and the second logic control signal P1X controls the switched capacitor circuit. Linkage switches S7-S12 in unit 3.

Further, referring to FIG. 2 , the charging unit 1 is composed of a first transistor MN1, which is composed of N (N>1) identical NMOS transistors in parallel; the sampling unit 2 is composed of a second transistor MN2 , the size of which is exactly the same as that of the NMOS transistor constituting the first transistor MN1, and the number ratio is 1:N. The drain terminal of the second transistor MN2 is connected to the drain terminal of the first transistor MN1, the gate terminal of the second transistor MN2 is connected to the gate terminal of the first transistor MN1, and the source terminal of the second transistor MN2 outputs the sampling current signal I _BFETH ; a feedback loop is formed by the switched capacitor circuit unit 3, the voltage compensation unit 4 and the signal conversion unit 5 to ensure that the source voltage of the second transistor MN2 is equal to the source voltage of the first transistor MN1 in the charging unit 1, so that It is ensured that the current flowing through the second transistor MN2 is 1/N of the current flowing through the first transistor MN1, so as to achieve the purpose of current sampling. At the same time, the sampled current signal I _BFETH is converted into a sampled voltage by the signal conversion unit 5 and output to the charging current adjustment unit 6 . The charging current regulating unit 6 compares the sampled voltage with the reference voltage REF, and obtains a charging current regulating signal which is fed back to the gate terminal of the first transistor MN1 for controlling and regulating the magnitude of the constant current charging current.

The drain terminal of the first transistor MN1 is connected to the output voltage SYS of the previous-stage DC-DC converter, and SYS=BAT+ΔV1 (ΔV1>0V), and the gate terminal is connected to the output of the charging current adjustment unit 6 terminal, the source terminal is connected to the anode voltage BAT of the lithium battery to be charged. Since the SYS voltage is constantly higher than the BAT voltage by ΔV, which means that the source-drain voltage of the power tube is fixed, the current through the power tube is uniquely determined by the gate terminal voltage.

Referring to FIG. 3 , the switched capacitor circuit unit 3 includes a first operational amplifier A1, a second operational amplifier A2, a first capacitor C1, a second capacitor C2 and 12 linkage switches S1-S12:

The first linkage switch S1 is connected to the output end BFETH of the sampling unit (2) and the upper plate of the first capacitor C1, and the lower plate of the first capacitor C1 is connected to the reverse input end of the first operational amplifier A1;

The second linkage switch S2 is connected to the anode voltage BAT of the lithium battery and the forward input terminal of the first operational amplifier A1;

The third linkage switch S3 is connected to the output end of the first operational amplifier A1 and the input end A of the voltage compensation unit (4);

The fourth linkage switch S4 is connected across the reverse input end and the output end of the second operational amplifier A2;

The fifth linkage switch S5 is connected to the upper plate of the second capacitor C2 and the forward input terminal of the second operational amplifier A2;

The sixth linkage switch S6 is connected to the forward input terminal of the second operational amplifier A2 and the input terminal A of the voltage compensation unit (4);

The seventh linkage switch S7 is connected across the reverse input end and the output end of the first operational amplifier A1;

The eighth linkage switch S8 is connected to the upper plate of the first capacitor C1 and the forward input terminal of the first operational amplifier A1;

The ninth linkage switch S9 is connected to the forward input terminal of the first operational amplifier A1 and the input terminal A of the voltage compensation unit (4);

The tenth linkage switch S10 is connected to the output end BFETH of the sampling unit (2) and the upper plate of the second capacitor C2, and the lower plate of the second capacitor C2 is connected to the reverse input end of the second operational amplifier A2;

The eleventh linkage switch S11 is connected to the anode voltage BAT of the lithium battery and the forward input terminal of the second operational amplifier A2;

The twelfth linkage switch S12 is connected to the output end of the second operational amplifier A2 and the input end A of the voltage compensation unit (4).

4, the voltage compensation unit 4 includes a third transistor MN3 and a constant current source I1; the gate terminal of the third transistor MN3 is used as the input terminal A of the voltage compensation unit 4, the drain terminal is connected to the power supply voltage VCC, and the source terminal is used as the input terminal A of the voltage compensation unit 4. The output terminal B of the voltage compensation unit 4; the current source I1 is connected across the source terminal of the third transistor MN3 and the ground; , reducing the influence of the parasitic capacitance of the positive and negative input terminals of the first operational amplifier A1 and the second operational amplifier A2 in the switched capacitor circuit unit 3 on the accuracy.

5 , the signal conversion unit 5 includes a fourth transistor MP1 and a first resistor R1; the gate terminal of the fourth transistor MP1 is used as the input terminal of the signal conversion unit 5, the source terminal is connected to the output terminal BFETH of the sampling unit 2, and the drain terminal is connected to the output terminal BFETH of the sampling unit 2. The terminal is used as the output terminal of the signal conversion unit 5; the first resistor R1 is connected between the drain terminal of the fourth transistor MP1 and the ground.

Further, referring to FIGS. 3 , 4 and 5 ; the output voltage BFETH of the sampling unit 2 passes through the switched capacitor circuit unit 3 , and the voltage compensation unit 4 obtains the output voltage B and connects to the gate terminal of the fourth transistor MP1 , the fourth transistor MP1 The output voltage B is fed back to the input end of the switched capacitor circuit unit 3 to form a negative feedback loop. In order to ensure that the voltages of the two input terminals BFETH and BAT of the switched capacitor circuit unit 3 are equal, that is, to ensure that the voltage of the drain terminal of MN2 in the sampling unit 2 is equal to the voltage of the drain terminal of MN1 in the charging unit 1, at this time, the source voltage of MN2 and MN1 is equal. The voltages of the terminal, the gate terminal and the drain terminal are all equal, which ensures that the sampling unit 2 can accurately sample the charging current in proportion.

Referring to FIG. 6 , the charging current adjusting unit 6 includes a third operational amplifier A3 , a fifth transistor MN4 and a resistor R2 ; the inverting input terminal of the third operational amplifier A3 serves as the first input terminal of the charging current adjusting unit 6 , the forward input terminal is used as the second input terminal of the charging current adjustment unit 6, and is connected to the reference voltage REF; its output terminal is connected to the gate terminal of the fifth transistor MN4; the source terminal of the fifth transistor MN4 is grounded, and its drain terminal The second resistor R2 is connected to the external charge pump output voltage PUMPC; the common terminal PUMP of the fifth transistor MN4 and the resistor R2 is used as the output terminal of the charging current adjustment unit 6; the drain terminal of the fifth transistor MN4 is used as the output of the unit to connect to the charging current The gate terminal of the unit 1 is used to adjust the constant current charging current of the charging unit 1 .

The digital control unit 7 of the present invention is used to generate a logic control signal for controlling the on and off of the linkage switch in the switched capacitor circuit unit 3, so as to change the working state of the circuit. The work control sequence of controlling the switched capacitor circuit unit 3 is shown in FIG. 7 , wherein a high level represents the switch is closed, and a low level represents the switch is off.

8 is a schematic diagram of the overall operation of the constant current charging circuit with high precision and high stability of the present invention, assuming that the offset voltages of the first operational amplifier A1 and the second operational amplifier A2 are V _OS1 and V _OS2 , respectively.

In conjunction with Fig. 7 and Fig. 8, the concrete working process of the present invention is described as follows:

It is defined that when the linkage switches S1-S6 are closed and the linkage switches S7-S12 are disconnected, the overall circuit is in the stage T1, and its equivalent structure is shown in Figure 9a. At this time, the second operational amplifier A2 is connected in the form of deep negative feedback, and the positive and negative input terminal voltages are equal, then there is a voltage V _C2 =V _OS2 across the second capacitor C2 , which means that the offset voltage V of the second operational amplifier A2 _OS2 is stored in the second capacitor C2;

It is defined that when the linkage switches S7-S12 are closed and the linkage switches S1-S6 are disconnected, the overall circuit is in the stage T2, and its equivalent structure is shown in Figure 9b. At this time, the second operational amplifier A2, the second capacitor C2, the voltage compensation unit 4 and the signal conversion unit 5 form a voltage signal loop, then the voltages of the positive and negative input terminals of the second operational amplifier A2 are equal, that is, BFETH-V _C2 =BAT- V _OS2 , since in the T1 stage, the voltage V _C2 across the second capacitor C2 is equal to the offset voltage V _OS2 , and the capacitor voltage cannot be abruptly changed, then BFETH=BAT, which eliminates the influence of the offset voltage V _OS2 on the circuit accuracy (BAT in the traditional circuit -V _OS =BFETH, and BAT=BFETH+V _OS ).

Similarly, the offset voltage V _OS1 of the first operational amplifier A1 can also be eliminated. In the T2 stage, the first capacitor C1 records the offset voltage V _OS1 of the first operational amplifier A1. In the T1 state, the offset voltage V _OS1 and the first capacitor C1 The voltages V _C1 at both ends cancel each other out, and BFETH is still equal to BAT.

Because the T1 and T2 stages represent a complete signal acquisition and amplification processing cycle, BFETH is always equal to BAT in the entire cycle, so the offset voltage of the operational amplifier will not affect the current sampling accuracy.

The above description is only a specific example of the present invention, and does not constitute any limitation to the present invention. Obviously, for those skilled in the art, after understanding the content and principle of the present invention, they may not deviate from the principle and structure of the present invention. Under certain circumstances, various corrections and changes in form and details are made, but these corrections and changes based on the idea of the present invention are still within the scope of protection of the claims of the present invention.

Claims (6)

1. A constant current charger circuit, comprising: a charging unit (1), a sampling unit (2), a signal conversion unit (5), a charging current adjustment unit (6), and a digital control unit (7), characterized in that It comprises: a switched capacitor circuit unit (3), a voltage compensation unit (4); The charging unit (1) is used for constant current charging of the lithium battery; it is provided with two inputs, the first input is connected to the output voltage SYS of the front-stage DC-DC (direct current-direct current) converter, the second The input end is connected to the output of the charging current adjustment unit (6); the output end is connected to the anode BAT of the lithium battery to be charged; The sampling unit (2) is provided with two inputs for proportional sampling of the charging current; the first input end is connected to the output voltage SYS of the DC-DC (direct current-direct current) converter, and the second input end is connected to the The input end of the charging unit (1); the output end BFETH outputs the sampling current signal I _BFETH ; The switched capacitor circuit unit (3) is provided with four inputs, the first input end is connected to the anode BAT of the lithium battery to be charged, the second input end is connected to the output end BFETH of the sampling unit (2), and the third input end The terminal is connected to the output control signal P1 of the digital control unit (7), and the fourth input terminal is connected to the output control signal P1X of the digital control unit (7); its output terminal is connected to the input terminal of the voltage compensation unit (4); The switched capacitor circuit unit (3) includes a first operational amplifier A1, a second operational amplifier A2, a first capacitor C1, a second capacitor C2 and 12 linkage switches S1-S12; wherein the first linkage switch S1 is connected to the sampling unit (2 ) output terminal BFETH and the upper plate of the first capacitor C1, the lower plate of the first capacitor C1 is connected to the reverse input end of the first operational amplifier A1; the second linkage switch S2 is connected to the anode BAT of the lithium battery and the first The forward input terminal of the operational amplifier A1; the third linkage switch S3 is connected to the output terminal of the first operational amplifier A1 and the input terminal A of the voltage compensation unit (4); the fourth linkage switch S4 is connected across the reverse direction of the second operational amplifier A2 Input terminal and output terminal; the fifth linkage switch S5 is connected to the upper plate of the second capacitor C2 and the forward input terminal of the second operational amplifier A2; the sixth linkage switch S6 is connected to the forward input terminal of the second operational amplifier A2 and the voltage The input terminal A of the compensation unit (4); the seventh linkage switch S7 is connected across the reverse input terminal and the output terminal of the first operational amplifier A1; the eighth linkage switch S8 is connected to the upper plate of the first capacitor C1 and the first operational amplifier The forward input terminal of A1; the ninth linkage switch S9 is connected to the forward input terminal of the first operational amplifier A1 and the input terminal A of the voltage compensation unit (4); the tenth linkage switch S10 is connected to the output terminal BFETH of the sampling unit (2) and the upper plate of the second capacitor C2, the lower plate of the second capacitor C2 is connected to the reverse input end of the second operational amplifier A2; the eleventh linkage switch S11 is connected to the anode BAT of the lithium battery and the second operational amplifier A2. Forward input terminal; the twelfth linkage switch S12 is connected to the output terminal of the second operational amplifier A2 and the input terminal A of the voltage compensation unit (4); The voltage compensation unit (4), the output end of which is connected to the first input end of the signal conversion unit (5), is used for compensating the output voltage of the switched capacitor circuit unit (3) and reducing the sampling accuracy error; The signal conversion unit (5) is provided with two inputs, the second input terminal of which is connected to the sampling current signal I _BFETH ; the switched capacitor circuit unit (3), the voltage compensation unit (4) and the signal conversion unit (5) are formed a feedback loop to ensure that the sampling unit (2) can accurately sample the charging current in proportion; the output end is connected to the first input end of the charging current adjusting unit (6); The charging current adjustment unit (6) is provided with two inputs, the second input terminal is connected to the reference voltage REF, and the output terminal is connected to the input terminal of the signal sampling unit (2) and the input terminal of the charging unit (1), It is used to adjust the charging current to ensure that the charging current is constant; The digital control unit (7) is used to generate two logic control signals P1 and P1X, wherein the first logic control signal P1 controls the linkage switches S1-S6 in the switched capacitor circuit unit (3), and the second logic control signal P1X The linkage switches S7-S12 in the switched capacitor circuit unit (3) are controlled. 2 . The constant current charger circuit according to claim 1 , wherein the charging unit ( 1 ) is composed of a first transistor MN1 , and this transistor is composed of N (N>1) identical NMOSs. 3 . The gate terminal of the first transistor MN1 is connected to the output terminal of the charging current adjustment unit (6), the source terminal is connected to the anode BAT of the lithium battery, the anode voltage is VBAT, and the drain terminal is connected to the front-stage DC-DC The output voltage SYS of the (DC-DC) converter, and SYS=VBAT+△V1, △V1>0V. 3. A constant current charger circuit according to claim 2, characterized in that: the sampling unit (2) is composed of a second transistor MN2, the size of which is exactly the same as the size of the NMOS transistor constituting the first transistor MN1 , the number ratio is 1:N; the drain terminal of the second transistor MN2 is connected to the drain terminal of the first transistor MN1, the gate terminal of the second transistor MN2 is connected to the gate terminal of the first transistor MN1, and the second transistor MN2 is connected to the gate terminal of the first transistor MN1. The source end of MN2 serves as the output end BFETH of the sampling unit (2), and outputs the sampling current signal I _BFETH . 4. A constant current charger circuit according to claim 1, characterized in that: the voltage compensation unit (4) comprises a third transistor MN3 and a constant current source I1; The gate terminal of the third transistor MN3 is used as the input terminal of the voltage compensation unit (4), the drain terminal is connected to the power supply voltage VCC, and the source terminal is used as the output terminal of the voltage compensation unit (4); The current source I1 is connected between the source terminal of the third transistor MN3 and the ground. 5. A constant current charger circuit according to claim 1, characterized in that: the signal conversion unit (5) comprises a fourth transistor MP1 and a first resistor R1; The gate terminal of the fourth transistor MP1 is used as the input terminal of the signal conversion unit (5), the source terminal is connected to the output terminal BFETH of the sampling unit (2), and the drain terminal is used as the output terminal of the signal conversion unit (5); The first resistor R1 is connected between the drain terminal of the fourth transistor MP1 and the ground. 6. A constant current charger circuit according to claim 1, characterized in that: the charging current adjusting unit (6) comprises a third operational amplifier A3, a fifth transistor MN4 and a second resistor R2; The reverse input terminal of the third operational amplifier A3 is used as the first input terminal of the charging current adjustment unit (6), and the forward input terminal is used as the second input terminal of the charging current adjustment unit (6), and is connected to the reference voltage REF; Its output terminal is connected to the gate terminal of the fifth transistor MN4; The source terminal of the fifth transistor MN4 is connected to the ground, and its drain terminal is connected to the external charge pump output voltage PUMPC through the second resistor R2; the common terminal PUMP of the fifth transistor MN4 and the second resistor R2 is used as the charging current adjustment unit (6 ) output.

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