CN203645895U - Load sampling circuit of non-isolated LED driving circuit - Google Patents

Abstract

The utility model relates to a load sampling circuit of a non-isolated LED driving circuit, belonging to the technical field of LED illumination driving circuits. The load sampling circuit of the non-isolated LED driving circuit comprises the non-isolated LED driving circuit and the sampling circuit, the sampling circuit is connected with the non-isolated LED driving circuit, and comprises a first resistor R1, a second resistor R2 and a first capacitor C1, one end of the first resistor R1 is used as an input end of the sampling circuit and connected with the non-isolated LED driving circuit, the other end of the first resistor R1 is connected with one end of the first capacitor C1, the other end of the first capacitor C1 and one end of the second resistor R2 are connected to a driving chip U1 together and used as the output end of the sampling circuit, and the other end of the second resistor R2 is grounded. The load sampling circuit of the non-isolated LED driving circuit has the advantages that circuit connection is simple, precision requirements for components are low, the circuit is easy to realize, the sampling stability and precision are better, and the load sampling circuit can be used for driving circuits in a continuous/interrupted current mode.

Description

The load sample circuit of non-isolated LED driving circuit

Technical field

The utility model belongs to LED illumination driving circuit technical field, is specifically related to a kind of load sample circuit of non-isolated LED driving circuit.

Background technology

Traditional LED drive circuit or DC/DC electric power management circuit adopt the voltage at electric resistance partial pressure method sensing lead two ends conventionally, then by driving chip to calculate the voltage difference at load two ends according to sampled load voltage.As shown in Figure 3, traditional LED drive circuit comprises driving chip U1, inductance L 1, the first metal-oxide-semiconductor M1, the first resistance R 1, the second resistance R 2, the 3rd resistance R 3, the 4th resistance R 4, the 5th resistance R 5, the 6th resistance R 6, the second capacitor C 2, the 3rd capacitor C 3, the 4th capacitor C 4, the first diode D1, the second diode D2, the 3rd diode D3, the 4th diode D4, the 5th diode D5 and LED lamp string DL, wherein, the first diode D1, the second diode D2, the 3rd diode D3, the 4th diode D4 forms a rectifier bridge, connect external communication power supply.C point in the drawings, the voltage of LED lamp string DL anode inputs to and drives chip U1 by the 5th resistance R 5 and the 6th resistance R 6 dividing potential drop, after scaled, and at B point, the voltage of LED lamp string DL negative terminal inputs to and drives chip U1 by the first resistance R 1 and the second resistance R 2 dividing potential drop, after scaled, drive chip U1 by comparing the input voltage at B point and C point place, calculate the voltage V at LED lamp string DL two ends _lED.In this scheme, not only need described driving chip U1 that two extra pins are provided, and owing to there being two input circuits, therefore require higher to the accuracy of resistance.

In view of above-mentioned prior art, be necessary to be improved, for this reason, the applicant has done useful design, and technical scheme described below produces under this background.

Summary of the invention

The purpose of this utility model is to provide the load sample circuit of non-isolated LED driving circuit, circuit connects, and stability and precision simple, low to the required precision of element, that be easy to realization, sampling are better, and can be used for the drive circuit of continuous current mode (CCM) or interrupted (discontinuous) current-mode (DCM).

The purpose of this utility model reaches like this, a kind of load sample circuit of non-isolated LED driving circuit, comprise non-isolated LED driving circuit and sample circuit, described sample circuit is connected with non-isolated LED driving circuit, described non-isolated LED driving circuit comprises driving chip U1, inductance L 1, the first metal-oxide-semiconductor M1, the 3rd resistance R 3, the 4th resistance R 4, the second capacitor C 2, the 3rd capacitor C 3, the 4th capacitor C 4, the first diode D1, the second diode D2, the 3rd diode D3, the 4th diode D4, the 5th diode D5 and LED lamp string DL, the negative pole of the first described diode D1 and the negative pole of the second diode D2, one end of the 3rd resistance R 3, one end of the 3rd capacitor C 3, the negative pole of the 5th diode D5, the positive terminal of one end of the 4th capacitor C 4 and LED lamp string DL connects, the positive pole of the first diode D1 and the negative pole of the 3rd diode D3 are connected one end of external ac power source jointly, the positive pole of the second diode D2 and the negative pole of the 4th diode D4 are connected the other end of external ac power source jointly, the other end of the 3rd resistance R 3 is connected with one end of the second capacitor C 2, and be connected to and drive chip U1, the positive pole of the 5th diode D5 is connected with the drain electrode of the first metal-oxide-semiconductor M1 and one end of inductance L 1, and be connected with an input of sample circuit, the grid of the first metal-oxide-semiconductor M1 is connected with the one end that drives chip U1, the source electrode of the first metal-oxide-semiconductor M1 is connected with one end of the 4th resistance R 4, and be connected to and drive chip U1, the other end of inductance L 1 is connected with the other end of the 4th capacitor C 4 and the negative pole end of LED lamp string DL, the positive pole of the 3rd diode D3, the positive pole of the 4th diode D4, the other end of the second capacitor C 2, the other end common ground of the other end of the 3rd capacitor C 3 and the 4th resistance R 4, it is characterized in that: described sample circuit comprises the first resistance R 1, the second resistance R 2 and the first capacitor C 1, one end of the first resistance R 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first resistance R 1 is connected with one end of the first capacitor C 1, one end of the other end of the first capacitor C 1 and the second resistance R 2 is jointly connected to and drives chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R 2.

In a specific embodiment of the present utility model, described sample circuit comprises the first resistance R 1, the second resistance R 2 and the first capacitor C 1, one end of the first capacitor C 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first capacitor C 1 is connected with one end of the first resistance R 1, one end of the other end of the first resistance R 1 and the second resistance R 2 is jointly connected to and drives chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R 2.

In another specific embodiment of the present utility model, described sample circuit comprises the first resistance R 1, the second resistance R 2, the first capacitor C 1 and the second metal-oxide-semiconductor M2, one end of the first resistance R 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first resistance R 1 is connected with one end of the first capacitor C 1, the other end of the first capacitor C 1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects and drives chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end and the driving chip U1 of the second resistance R 2, and become the output of sample circuit, the other end ground connection of the second resistance R 2.

In another specific embodiment of the present utility model, described sample circuit comprises the first resistance R 1, the second resistance R 2, the first capacitor C 1 and the second metal-oxide-semiconductor M2, one end of the first capacitor C 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first capacitor C 1 is connected with one end of the first resistance R 1, the other end of the first resistance R 1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects and drives chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end and the driving chip U1 of the second resistance R 2, and become the output of sample circuit, the other end ground connection of the second resistance R 2.

The utility model is owing to having adopted said structure, compared with prior art, the beneficial effect having is: reduced the connection pin that drives chip U1, the syndeton of circuit is simplified, the required precision of the electric circuit constitute element is reduced, this circuit is easy to realize, and the load voltage precision collecting is higher.This sample circuit can be sampled to the load voltage of non-isolated LED driving circuit at the state of continuous current mode, in addition, by increasing by the second metal-oxide-semiconductor M2, also can sample to the load voltage of the non-isolated LED driving circuit of discontinuous current mode.

Brief description of the drawings

Fig. 1 is the electrical schematic diagram of an embodiment of the present utility model.

Fig. 2 is the current-voltage waveform figure of an embodiment of the present utility model under stable state.

Fig. 3 is the electrical schematic diagram of another embodiment of the present utility model.

Fig. 4 is the current-voltage waveform figure of another embodiment of the present utility model under stable state.

Fig. 5 is the electrical schematic diagram of the load sample circuit of non-isolated LED driving circuit of the prior art.

Embodiment

In order to make the public can fully understand technical spirit of the present utility model and beneficial effect; applicant will describe in detail embodiment of the present utility model below by reference to the accompanying drawings; but applicant is not the restriction to technical scheme to the description of embodiment, anyly changes in the form rather than substance and all should be considered as protection range of the present utility model according to the utility model design.

Embodiment 1:

Refer to Fig. 1, a kind of load sample circuit of non-isolated LED driving circuit, comprises non-isolated LED driving circuit and sample circuit, and described sample circuit is connected with non-isolated LED driving circuit.Described non-isolated LED driving circuit comprises driving chip U1, inductance L 1, the first metal-oxide-semiconductor M1, the 3rd resistance R 3, the 4th resistance R 4, the second capacitor C 2, the 3rd capacitor C 3, the 4th capacitor C 4, the first diode D1, the second diode D2, the 3rd diode D3, the 4th diode D4, the 5th diode D5 and LED lamp string DL, wherein, the first diode D1, the second diode D2, the 3rd diode D3 and the 4th diode D4 form a rectifier bridge.The negative pole of the first described diode D1 and the negative pole of the second diode D2, one end of the 3rd resistance R 3, one end of the 3rd capacitor C 3, the negative pole of the 5th diode D5, the positive terminal of one end of the 4th capacitor C 4 and LED lamp string DL connects, the positive pole of the first diode D1 and the negative pole of the 3rd diode D3 are connected one end of external ac power source jointly, the positive pole of the second diode D2 and the negative pole of the 4th diode D4 are connected the other end of external ac power source jointly, the other end of the 3rd resistance R 3 is connected with one end of the second capacitor C 2, and be connected to and drive chip U1, the positive pole of the 5th diode D5 is connected with the drain electrode of the first metal-oxide-semiconductor M1 and one end of inductance L 1, and be connected with an input of sample circuit, the grid of the first metal-oxide-semiconductor M1 is connected with the one end that drives chip U1, the source electrode of the first metal-oxide-semiconductor M1 is connected with one end of the 4th resistance R 4, and be connected to and drive chip U1, the other end of inductance L 1 is connected with the other end of the 4th capacitor C 4 and the negative pole end of LED lamp string DL, the positive pole of the 3rd diode D3, the positive pole of the 4th diode D4, the other end of the second capacitor C 2, the other end common ground of the other end of the 3rd capacitor C 3 and the 4th resistance R 4.Described sample circuit comprises the first resistance R 1, the second resistance R 2 and the first capacitor C 1, one end of the first resistance R 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first resistance R 1 is connected with one end of the first capacitor C 1, one end of the other end of the first capacitor C 1 and the second resistance R 2 is jointly connected to and drives chip U1, and become the output (sampled point of illustrating in figure) of sample circuit, the other end ground connection of the second resistance R 2.

Please continue to refer to Fig. 1 and in conjunction with Fig. 2, the operation principle of the present embodiment is described.In the present embodiment, describe as an example of the state of continuous current mode example.Fig. 2 is flow through under the stable state current waveform figure of inductance L 1 and the voltage oscillogram at A point place of circuit.Suppose that it is T that the first metal-oxide-semiconductor M1 keeps the time durations of conducting ₁, the time durations of the first metal-oxide-semiconductor M1 remain off is T ₂, the peak current of inductance L 1 is I _pEAK.When the electric current of the inductance L 1 of flowing through reaches peak I _pEAKtime, the first metal-oxide-semiconductor M1 cut-off, in the time that the electric current of the inductance L 1 of flowing through reduces to zero, the first metal-oxide-semiconductor M1 conducting.Especially, circuit working, in the critical condition of continuous current mode and discontinuous current mode, is also applicable to the present embodiment.The size of establishing again inductance L 1 is L, and the DC input voitage of external ac power source after rectifier bridge voltage stabilizing is V _iN, the voltage at the first resistance R 1 two ends is V _r1, the voltage at the second resistance R 2 two ends is V _r2, the voltage at the first capacitor C 1 two ends is V _c1, the output voltage values of LED lamp string DL is V _lED, obtain

$T_1=\frac{L\·I_{\mathrm{PEAK}}}{V_{\mathrm{IN}}-V_{\mathrm{LED}}}---\left(1\right)$

${\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="HDA0000452404470000031.png"\; state="\{\{state\}\}">T</figure-callout>}}_2=\frac{L\&CenterDot;I_{\mathrm{PEAK}}}{V_{\mathrm{LED}}}---\left(2\right)$

By formula (1) and formula (2), can derive

$\frac{T_2}{T_1+{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="HDA0000452404470000031.png"\; state="\{\{state\}\}">T</figure-callout>}}_2}=\frac{V_{\mathrm{IN}}-V_{\mathrm{LED}}}{V_{\mathrm{IN}}}---\left(3\right)$

At T ₁during this time, the first metal-oxide-semiconductor M1 conducting, in figure, A point voltage is 0; And at T ₂during this time, the 5th diode D5 conducting, now A point voltage is identical with the voltage in the 3rd capacitor C 3, and A point voltage is V _iN, suppose that RC time constant (R1+R2) C1 of described sample circuit is far longer than the switching frequency of the first metal-oxide-semiconductor M1, the voltage V in the first capacitor C 1 when circuit stable state _c1stabilize to

$V_{C1}=\frac{T_2}{T_1+{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="HDA0000452404470000031.png"\; state="\{\{state\}\}">T</figure-callout>}}_2}\&CenterDot;V_{\mathrm{IN}}=\frac{V_{\mathrm{IN}}-V_{\mathrm{LED}}}{V_{\mathrm{IN}}}\&CenterDot;V_{\mathrm{IN}}=V_{\mathrm{IN}}-V_{\mathrm{LED}}---\left(4\right)$

Now, the magnitude of voltage that the first resistance R 1 and the second resistance R 2 two ends add up to is

V _R1+V _R2=V _IN-(V _IN-V _LED)=V _LED (5)

Because the voltage on the first resistance R 1 and second voltage R2 is according to the proportional distribution of their resistance value, the voltage that therefore can obtain in the second resistance R 2 is

$V_{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="HDA0000452404470000031.png"\; state="\{\{state\}\}">R</figure-callout>}2}=\frac{R_2}{R_1+{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="HDA0000452404470000031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}\&CenterDot;V_{\mathrm{LED}}---\left(6\right)$

As shown from the above formula, described sampled voltage, i.e. the voltage V at the second resistance R 2 two ends _r2voltage V with load LED lamp string DL two ends _lEDlinear, the coefficient of this linear relationship is only relevant with the resistance of the first resistance R 1 and the second resistance R 2, therefore can realize the accurate collection of load end output voltage.

Embodiment 2:

Refer to Fig. 3, described sample circuit comprises the first resistance R 1, the second resistance R 2, the first capacitor C 1 and the second metal-oxide-semiconductor M2, one end of the first resistance R 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first resistance R 1 is connected with one end of the first capacitor C 1, the other end of the first capacitor C 1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects and drives chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end and the driving chip U1 of the second resistance R 2, and become the output (sampled point shown in figure) of sample circuit, the other end ground connection of the second resistance R 2.

Please continue to refer to Fig. 3 and in conjunction with Fig. 4, the operation principle of the present embodiment is described.Fig. 4 is flow through under the stable state current waveform figure of inductance L 1 and the voltage oscillogram at A point place of circuit.In this embodiment, can sample to the load voltage of non-isolated LED driving circuit at the state of discontinuous current mode by setting up the second metal-oxide-semiconductor M2.At described LED drive circuit in stable state, in the time that the electric current of inductance L 1 load-side reduces to zero, not conducting immediately of the first metal-oxide-semiconductor M1, but through conducting after a period of time.In the time of the first metal-oxide-semiconductor M1 normally and cut-off, suppose that it is T that the first metal-oxide-semiconductor M1 keeps the time durations of conducting ₃, the time durations of the first metal-oxide-semiconductor M1 remain off is T ₄, the time durations of reducing to 0 to first metal-oxide-semiconductor M1 conducting from the electric current of inductance L 1 load-side is T ₅, in addition, with embodiment 1, suppose that by the peak current of inductance L 1 be I _pEAK, the size of inductance L 1 is L, the DC input voitage of external ac power source after rectifier bridge voltage stabilizing is V _iN, the voltage at the first resistance R 1 two ends is V _r1, the voltage at the second resistance R 2 two ends is V _r2, the voltage at the first capacitor C 1 two ends is V _c1, the output voltage values of LED lamp string DL is V _lED, obtain

$T_3=\frac{L\&CenterDot;I_{\mathrm{PEAK}}}{V_{\mathrm{IN}}-V_{\mathrm{LED}}}---\left(7\right)$

$T_4=\frac{L\&CenterDot;I_{\mathrm{PEAK}}}{V_{\mathrm{LED}}}---\left(8\right)$

By formula (7) and formula (8), can derive

$\frac{T_4}{T_3+T_4}=\frac{V_{\mathrm{IN}}-V_{\mathrm{LED}}}{V_{\mathrm{IN}}}---\left(9\right)$

If at T ₅during this time, the grid voltage of the second metal-oxide-semiconductor M2 becomes low level, at T ₄voltage V during this time in the first capacitor C 1 _c1for

$V_{C1}=\frac{T_4}{T_3+T_4}\&CenterDot;V_{\mathrm{IN}}=\frac{V_{\mathrm{IN}}-V_{\mathrm{LED}}}{V_{\mathrm{IN}}}\&CenterDot;V_{\mathrm{IN}}=V_{\mathrm{IN}}-V_{\mathrm{LED}}---\left(10\right)$

With the deriving analysis in embodiment 1, can be at T ₄during this time,

V _R1+V _R2=V _LED（11）

$V_{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="HDA0000452404470000031.png"\; state="\{\{state\}\}">R</figure-callout>}2}=\frac{R_2}{R_1+{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="HDA0000452404470000031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}\&CenterDot;V_{\mathrm{LED}}---\left(12\right)$

As shown from the above formula, identical with embodiment 1, described sampled voltage, i.e. the voltage V at the second resistance R 2 two ends _r2voltage V with load LED lamp string DL two ends _lEDlinear, the coefficient of this linear relationship is only relevant with the resistance of the first resistance R 1 and the second resistance R 2, therefore can realize the accurate collection of load end output voltage.

Embodiment 3:

In the present embodiment (not shown), described sample circuit comprises the first resistance R 1, the second resistance R 2 and the first capacitor C 1, one end of the first capacitor C 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first capacitor C 1 is connected with one end of the first resistance R 1, one end of the other end of the first resistance R 1 and the second resistance R 2 is jointly connected to and drives chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R 2.All the other all, with the description of embodiment 1, repeat no more herein.

Embodiment 4:

In the present embodiment (not shown), described sample circuit comprises the first resistance R 1, the second resistance R 2, the first capacitor C 1 and the second metal-oxide-semiconductor M2, one end of the first capacitor C 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first capacitor C 1 is connected with one end of the first resistance R 1, the other end of the first resistance R 1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects and drives chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end and the driving chip U1 of the second resistance R 2, and become the output of sample circuit, the other end ground connection of the second resistance R 2.All the other all, with the description of embodiment 2, repeat no more herein.

Claims (4)

1. the load sample circuit of a non-isolated LED driving circuit, comprise non-isolated LED driving circuit and sample circuit, described sample circuit is connected with non-isolated LED driving circuit, described non-isolated LED driving circuit comprises driving chip U1, inductance L 1, the first metal-oxide-semiconductor M1, the 3rd resistance R 3, the 4th resistance R 4, the second capacitor C 2, the 3rd capacitor C 3, the 4th capacitor C 4, the first diode D1, the second diode D2, the 3rd diode D3, the 4th diode D4, the 5th diode D5 and LED lamp string DL, the negative pole of the first described diode D1 and the negative pole of the second diode D2, one end of the 3rd resistance R 3, one end of the 3rd capacitor C 3, the negative pole of the 5th diode D5, the positive terminal of one end of the 4th capacitor C 4 and LED lamp string DL connects, the positive pole of the first diode D1 and the negative pole of the 3rd diode D3 are connected one end of external ac power source jointly, the positive pole of the second diode D2 and the negative pole of the 4th diode D4 are connected the other end of external ac power source jointly, the other end of the 3rd resistance R 3 is connected with one end of the second capacitor C 2, and be connected to and drive chip U1, the positive pole of the 5th diode D5 is connected with the drain electrode of the first metal-oxide-semiconductor M1 and one end of inductance L 1, and be connected with an input of sample circuit, the grid of the first metal-oxide-semiconductor M1 is connected with driving chip U1, the source electrode of the first metal-oxide-semiconductor M1 is connected with one end of the 4th resistance R 4, and be connected to and drive chip U1, the other end of inductance L 1 is connected with the other end of the 4th capacitor C 4 and the negative pole end of LED lamp string DL, the positive pole of the 3rd diode D3, the positive pole of the 4th diode D4, the other end of the second capacitor C 2, the other end common ground of the other end of the 3rd capacitor C 3 and the 4th resistance R 4, it is characterized in that: described sample circuit comprises the first resistance R 1, the second resistance R 2 and the first capacitor C 1, one end of the first resistance R 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first resistance R 1 is connected with one end of the first capacitor C 1, one end of the other end of the first capacitor C 1 and the second resistance R 2 is jointly connected to and drives chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R 2.

2. the load sample circuit of non-isolated LED driving circuit according to claim 1, it is characterized in that described sample circuit comprises the first resistance R 1, the second resistance R 2 and the first capacitor C 1, one end of the first capacitor C 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first capacitor C 1 is connected with one end of the first resistance R 1, one end of the other end of the first resistance R 1 and the second resistance R 2 is jointly connected to and drives chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R 2.

3. the load sample circuit of non-isolated LED driving circuit according to claim 1, it is characterized in that described sample circuit comprises the first resistance R 1, the second resistance R 2, the first capacitor C 1 and the second metal-oxide-semiconductor M2, one end of the first resistance R 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first resistance R 1 is connected with one end of the first capacitor C 1, the other end of the first capacitor C 1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects and drives chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end and the driving chip U1 of the second resistance R 2, and become the output of sample circuit, the other end ground connection of the second resistance R 2.

4. the load sample circuit of non-isolated LED driving circuit according to claim 1, it is characterized in that described sample circuit comprises the first resistance R 1, the second resistance R 2, the first capacitor C 1 and the second metal-oxide-semiconductor M2, one end of the first capacitor C 1 connects described non-isolated LED driving circuit as an input of sample circuit, the other end of the first capacitor C 1 is connected with one end of the first resistance R 1, the other end of the first resistance R 1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects and drives chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end and the driving chip U1 of the second resistance R 2, and become the output of sample circuit, the other end ground connection of the second resistance R 2.

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