Sampling Circuit and Driving Circuit for Power Switch

Abstract

The present invention provides a sampling circuit and a driving circuit for a power switch. A first input terminal is coupled to a first sampling module, a second sampling module, and a first end of a sampling resistor. A second input terminal is coupled to a second end of the sampling resistor and the first sampling module. Output terminals of the first sampling module and the second sampling module are coupled to a driving control circuit. When the voltage at the first end of the sampling resistor is higher than a second reference voltage, the first sampling module operates independently. When the voltage at the first end of the sampling resistor is lower than or equal to the second reference voltage, the first sampling module and the second sampling module cooperate, and the feedback voltage provided by the second sampling module is not lower than the feedback voltage provided by the first sampling module when it operates independently. Thus, throughout the entire process of power on and off, the feedback voltage received by the driving control circuit will not be too low, and then the inductor current will never exceed the value during normal operation, avoiding the problem of overshoot of the output current provided to the load during the power on and off process.

Description

Technical Field

[0001] The present invention relates to the technical field of drive circuits, and particularly to a sampling circuit and a drive circuit for a power switch. Background Art

[0002] A low-side drive circuit is a circuit that controls the conduction of a load to the ground, and is often used in fields such as coil ignition, relay control, valve control, LED control, etc.

[0003] Figure 1 Shown is a common low-side buck-type LED constant current drive circuit, whose load is an LED. A sampling resistor Rcs is placed at the cathode of the LED string. The sampling circuit 10 provides a feedback voltage Vfb to the drive control circuit U0 according to the voltages Vcsp and Vcsn at both ends of the sampling resistor Rcs, so as to feedback the current changes of the inductor L and the load to the drive control circuit U0. The drive control circuit U0 compares and amplifies Vfb with the reference voltage Vref, and outputs a corresponding drive signal to control the conduction state of the power switch Q0. The conduction and cut-off of Q0 can control the inductor L to store and release energy, thereby controlling the output current flowing through the LED to reach a preset value.

[0004] During the startup and shutdown processes, when the input voltage Vin is close to the output voltage Vout, the voltages Vcsp and Vcsn at both ends of the sampling resistor Rcs are close to the ground potential. At this time, the feedback voltage Vfb provided by the sampling circuit 10 to the drive control circuit U0 cannot correctly reflect the magnitude of the current of the inductor L, and may always be less than the reference voltage Vref. Since the inductor current control loop of this buck-type LED constant current drive circuit adopts a negative feedback control mechanism, in this case, the current of the inductor L may overshoot, and the output current provided to the LED will also correspondingly overshoot. Summary of the Invention

[0005] The purpose of the present invention is to provide a sampling circuit and a drive circuit for a power switch, which can solve the problem of output current overshoot during the startup and shutdown processes by limiting the minimum value of the output feedback signal.

[0006] To achieve the above object, the present invention provides a sampling circuit, including: a first input terminal, coupled to a first sampling module, a second sampling module, and a first end of a sampling resistor; a second input terminal, coupled to a second end of the sampling resistor and the first sampling module; wherein, an output terminal of the first sampling module and an output terminal of the second sampling module are coupled to a first input terminal of a drive control circuit, and the second sampling module receives a first reference voltage; when a voltage at the first end of the sampling resistor is higher than a second reference voltage, the first sampling module works alone and provides a corresponding feedback voltage to the drive control circuit; when the voltage at the first end of the sampling resistor is lower than or equal to the second reference voltage, the first sampling module and the second sampling module work together and provide a corresponding feedback voltage to the drive control circuit according to the voltage at the first end of the sampling resistor and the first reference voltage, and at this time, the feedback voltage provided by the second sampling module is not lower than the feedback voltage provided when the first sampling module works alone.

[0007] Optionally, the second reference voltage is set corresponding to the first reference voltage.

[0008] Optionally, the second sampling module includes: an adder, a first input terminal of the adder receives the first reference voltage, a second input terminal of the adder is coupled to the first end of the sampling resistor and receives the voltage at the first end of the sampling resistor, and the adder is configured to perform weighted superposition on the first reference voltage and the voltage at the first end of the sampling resistor and then output; a comparator and a control switch, a first input terminal of the comparator is coupled to an output terminal of the adder, a second input terminal of the comparator is coupled to an output terminal of the first sampling module and the first input terminal of the drive control circuit, an output terminal of the comparator is coupled to a control terminal of the control switch, and when the voltage at the first end of the sampling resistor is lower than or equal to the second reference voltage, the comparator controls the control switch to conduct.

[0009] Optionally, the voltage output by the adder is K1*Vstr–K2*Vcsp, where Vstr is the first reference voltage, Vcsp is the voltage at the first end of the sampling resistor, and K1 and K2 are weighted coefficients greater than 0 respectively.

[0010] Optionally, the first sampling module includes a transconductance amplifier and a pull-down resistor, a first input terminal of the transconductance amplifier is coupled to the first end of the sampling resistor, a second input terminal of the transconductance amplifier is coupled to the second end of the sampling resistor, and an output terminal of the transconductance amplifier is coupled to one end of the pull-down resistor and the first input terminal of the drive control circuit.

[0011] Optionally, the feedback voltage provided when the first sampling module works alone is expressed as Vfb = Gm * (Vcsp - Vcsn) * R, where Gm is the conductance value of the transconductance amplifier, Vcsp is the voltage at the first end of the sampling resistor, Vcsn is the voltage at the second end of the sampling resistor, and R is the resistance value of the pull-down resistor.

[0012] The present invention also provides a drive circuit for a power switch, including a sampling resistor, a drive control circuit, and the sampling circuit as described above; wherein, the sampling resistor is connected in series between the power switch and the load, the first end of the sampling resistor is coupled to one end of the load and the first input end of the sampling circuit, and the second end of the sampling resistor is coupled to the second input end of the sampling circuit; the output end of the drive control circuit is coupled to the control end of the power switch, and the drive control circuit receives the feedback voltage and the reference voltage, and controls the power switch according to the feedback voltage and the reference voltage.

[0013] Compared with the prior art, the technical solution of the present invention has at least the following beneficial effects:

[0014] The first input end of the sampling circuit is coupled to the first sampling module, the second sampling module, and the first end of the sampling resistor, the second input end is coupled to the second end of the sampling resistor and the first sampling module, and the output ends of the first sampling module and the second sampling module are coupled to the first input end of the drive control circuit, and the second sampling module receives the first reference voltage. Among them, when the voltage at the first end of the sampling resistor is higher than the second reference voltage, the first sampling module works alone and provides a corresponding feedback voltage to the drive control circuit; when the voltage at the first end of the sampling resistor is lower than or equal to the second reference voltage, the first sampling module and the second sampling module work together and provide a corresponding feedback voltage to the drive control circuit according to the voltage at the first end of the sampling resistor and the first reference voltage, and at this time, the feedback voltage provided by the second sampling module is not lower than the feedback voltage provided when the first sampling module works alone. Thus, during the entire process of power on and off, the feedback voltage received by the drive control circuit is not lower than the feedback voltage generated when the first sampling module works alone, and then the inductor current is always controlled not to exceed the value during normal operation, thereby avoiding the problem of overshoot of the output current provided to the load during the power on and off process and improving the system reliability. Description of the Drawings

[0015] Those of ordinary skill in the art will understand that the provided drawings are used to better understand the present invention and do not constitute any limitation to the scope of the present invention. Among them:

[0016] Figure 1 is a schematic structural diagram of an existing drive circuit with a sampling circuit.

[0017] Figure 2 It is a schematic structural diagram of a sampling circuit according to a specific embodiment of the present invention and a driving circuit for a power switch where the sampling circuit is located. Specific Embodiments

[0018] In the following description, numerous specific details are given to provide a more thorough understanding of the present invention. However, it will be apparent to one of ordinary skill in the art that the present invention may be practiced without one or more of these details. In other instances, well-known features have not been described in order to avoid obscuring the present invention. It should be understood that the present invention can be implemented in different forms and should not be construed as limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Like reference numerals refer to like elements throughout. It should be understood that when an element is referred to as being "connected to" or "coupled to" another element, it can be directly connected to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly connected to" another element, there are no intervening elements. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It should also be understood that the term "comprises" is used to specify the presence of the stated features, steps, operations, elements, and / or components, but does not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups. As used herein, the term "and / or" includes any and all combinations of the associated listed items.

[0019] The technical solutions proposed by the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The advantages and features of the present invention will be more apparent according to the following description. It should be noted that the drawings are all in a very simplified form and use non-precise scales, only for the purpose of facilitating and clearly assisting in explaining the objectives of the embodiments of the present invention.

[0020] Please refer to Figure 2, an embodiment of the present invention provides a sampling circuit 10, which is used in any suitable driving circuit. It samples and detects the voltages Vcsp (i.e., the voltage at the first end of the sampling resistor Rcs) and Vcsn (i.e., the voltage at the second end of the sampling resistor Rcs) at both ends CSP (i.e., the first end of the sampling resistor Rcs) and CSN of the sampling resistor Rcs coupled to a load (such as an LED), and provides a corresponding feedback voltage Vfb to the driving control circuit U0 of the driving circuit according to the voltages Vcsp and Vcsn, so that the driving control circuit U0 can timely know the change of the output current or output voltage provided to the load in the system by comparing the feedback voltage Vfb with the reference voltage Vref, and then control the conduction and cutoff of the power switch Q0. The conduction and cutoff of Q0 can control the inductor L to store and release energy, and further control the output current provided to the load to reach a preset value, or control the output voltage provided to the load to reach a preset constant voltage value.

[0021] In this embodiment, the sampling circuit 10 includes a first sampling module 11 and a second sampling module 12. The first input terminal of the first sampling module 11 is coupled to the first end CSP of the sampling resistor Rcs, the second input terminal is coupled to the second end CSN of the sampling resistor Rcs, and the output terminal of the first sampling module 11 is coupled to the first input terminal of the driving control circuit U0. The first input terminal of the second sampling module 12 is coupled to the first end CSP of the sampling resistor Rcs, the second input terminal receives the first reference voltage Vstr, and the output terminal of the second sampling module 12 is coupled to the output terminal of the first sampling module 11 and the first input terminal of the driving control circuit U0.

[0022] Wherein, when the voltage Vcsp at the first end of the sampling resistor Rcs is higher than the second reference voltage V2, the first sampling module 11 works alone and provides a corresponding feedback voltage Vfb to the driving control circuit U0; when the voltage Vcsp at the first end of the sampling resistor Rcs is lower than or equal to the second reference voltage V2, the first sampling module 11 and the second sampling module 12 work together, and according to the voltage Vcsp at the first end of the sampling resistor Rcs and the first reference voltage Vstr, provide a corresponding feedback voltage Vfb to the driving control circuit U0, and at this time, the feedback voltage Vfb provided by the second sampling module 12 is not lower than the feedback voltage Vfb provided when the first sampling module 11 works alone.

[0023] The second reference voltage V2 is set corresponding to the first reference voltage Vstr. Optionally, the second reference voltage V2 and the first reference voltage Vstr are related. At this time, the second sampling module 12 can be implemented by any suitable circuit design, as long as it can make the second reference voltage V2 related to the first reference voltage Vstr, and achieve the effect that the feedback voltage Vfb provided when the second sampling module 12 works alone is not lower than the feedback voltage Vfb provided when the first sampling module 11 works alone.

[0024] As an example, the second sampling module 12 of this embodiment includes an adder U2, a comparator U3, and a control switch Q1. Among them, the first input terminal of the adder U2 receives the first reference voltage Vstr, the second input terminal of the adder U2 is coupled to the first terminal CSP of the sampling resistor Rcs to receive the voltage Vcsp at the first terminal of the sampling resistor Rcs, and the output terminal of the adder U2 is coupled to the first input terminal “+” of the comparator U3. The adder U2 is used to perform weighted addition on the first reference voltage Vstr and the voltage Vcsp at the first terminal of the sampling resistor Rcs and then output. For example, the output voltage V1 of the adder U2 = K1*Vstr – K2*Vcsp, where K1 and K2 are respectively weighting factors greater than 0.

[0025] The control switch Q1 can be any suitable switching element such as a MOS transistor or a triode, or any suitable switching logic circuit such as a logic gate. Taking Q1 as an NMOS transistor as an example, the second input terminal “-” of the comparator U3 is coupled to the source electrode of the control switch Q1, the output terminal of the first sampling module 11, and the first input terminal of the drive control circuit U0. The output terminal of the comparator U3 is coupled to the gate electrode (i.e., the control terminal of Q1) of the control switch Q1, and the drain electrode of the control switch Q1 receives the power supply voltage VDD. Only when the voltage Vcsp at the first terminal of the sampling resistor Rcs is lower than or equal to the second reference voltage V2, the comparator U3 controls the control switch Q1 to conduct.

[0026] The first sampling module 11 and the drive control circuit U0 can be implemented by any suitable existing circuit structure.

[0027] As an example, the first sampling module 11 includes a transconductance amplifier U1 and a pull-down resistor R0. The first input terminal “+” of the transconductance amplifier U1 is coupled to the first terminal CSP of the sampling resistor Rcs to receive the voltage Vcsp at the first terminal of the sampling resistor Rcs. The second input terminal “-” of the transconductance amplifier U1 is coupled to the second terminal CSN of the sampling resistor Rcs to receive the voltage Vcsn at the second terminal of the sampling resistor Rcs. The output terminal of the transconductance amplifier U1 is coupled to one end of the pull-down resistor R0 and the first input terminal of the drive control circuit U0. When the first sampling module 11 operates independently, the feedback voltage Vfb = Gm * (Vcsp - Vcsn) * R, where Gm is the conductance value of the transconductance amplifier U1, Vcsp is the voltage at the first terminal of the sampling resistor Rcs, Vcsn is the voltage at the second terminal of the sampling resistor Rcs, and R is the resistance value of the pull-down resistor R0.

[0028] Taking the case where the second input terminal of the drive control circuit U0 receives the reference voltage Vref, the second reference voltage V2 is K1 / K2 * Vstr, the load is an LED, the conductance value of the transconductance amplifier U1 is Gm, the resistance value of the pull-down resistor R0 is R, and the values of K1, K2, and Vstr are reasonably selected as an example, the working principle of the sampling circuit 10 of this embodiment will be described in detail as follows:

[0029] When the voltage Vcsp at the first terminal of the sampling resistor Rcs is very low, for example, lower than or equal to the third voltage V3, and V3 is lower than V2 (i.e., Vcsp ≤ V3 < K1 / K2 * Vstr), the second sampling module 12 operates, and the V1 output by its adder U2 is K1 * Vstr - K2 * Vcsp. At this time, the Vfb received by the drive control circuit U0 is K1 * Vstr - K2 * Vcsp, and Vfb > Vref. It controls the power switch Q0 to cut off, and the output current provided to the load LED is 0.

[0030] When the voltage Vcsp at the first terminal of the sampling resistor Rcs gradually increases and is less than V2 (i.e., Vcsp < K1 / K2 * Vstr), and Gm * (Vcsp - Vcsn) * R < V2, the second sampling module 12 operates, and the V1 output by its adder U2 is K1 * Vstr - K2 * Vcsp. At this time, the Vfb received by the drive control circuit U0 is K1 * Vstr - K2 * Vcsp, Vcsp = (K1 * Vstr - Vref) / K2, and Vfb < Vref. The drive control circuit U0 controls the power switch Q0 to turn on, and the output current provided to the load LED is not 0 and will not overshoot.

[0031] When Vcsp gradually increases and is higher than V2 (i.e., Vcsp > K1 / K2 * Vstr), the second sampling module 12 does not work, and the first sampling module 11 works alone. At this time, the Vfb received by the drive control circuit U0 is solely determined by the first sampling module 11. Specifically, the Vfb received by the drive control circuit U0 is Vfb = Gm * (Vcsp - Vcsn) * R. In this stage, the drive control circuit U0 compares the magnitudes of Vfb and Vref, and controls the conduction and cut-off of the power switch Q0 according to the comparison result to adjust the magnitude of the output current provided to the load LED.

[0032] Obviously, by reasonably selecting the values of K1, K2, and Vstr, during the entire process of power-on and power-off (i.e., startup and shutdown), the Vfb voltage is not lower than the Vfb value generated when the first sampling module 11 works alone. Therefore, the current IL of the inductor L will never exceed the value during normal system operation, thus avoiding the problem of output current overshoot during power-on and power-off, and improving system reliability. Therefore, neither Vstr nor the ratio K1 / K2 can be too large or too small. Assuming that the minimum Vcsp voltage required at the first end CSP of the sampling resistor Rcs when the sampling circuit 10 works normally is Vcsp_min, the selection of K1, K2, and Vstr needs to satisfy: K1 / K2 * Vstr > K1 / K2 * Vstr - Vref / K2 > Vcsp_min > 0.

[0033] The sampling circuit of this embodiment, compared with Figure 1 the shown sampling circuit, actually adds a second sampling module on the basis of the first sampling module, and enables the second sampling module to cooperate with the first sampling module only when the voltage at the first end of the sampling resistor is lower than the second reference voltage (such as K1 / K2 * Vstr), and at this time, the feedback voltage provided by the second sampling module is not lower than the feedback voltage value generated when the first sampling module works alone. Thus, during the entire process of power-on and power-off, the feedback voltage received by the drive control circuit is not lower than the feedback voltage generated when the first sampling module works alone normally, and then controls the inductor current to never exceed the value during normal operation, thereby avoiding the problem of output current overshoot provided to the load during power-on and power-off, and improving system reliability.

[0034] Please refer to Figure 2, based on the same inventive concept, an embodiment of the present invention further provides a driving circuit for a power switch, which includes a driving control circuit U0 and a sampling circuit 10 as described in any embodiment of the present invention. Wherein, the first end CSP of the sampling resistor Rcs is coupled to one end of the load LED, the first input end of the first sampling module 11 in the sampling circuit 10, and the second input end of the second sampling module 12; the second end CSN of the sampling resistor Rcs is coupled to the second input end of the first sampling module 11 in the sampling circuit 10 and one end of the inductor L; the output end of the driving control circuit U0 is coupled to the control end of the power switch Q0 (for example, the gate of the MOS transistor), and the second input end of the driving control circuit U0 receives a reference voltage Vref; the first end of the power switch Q0 (for example, the source of the MOS transistor) is grounded to GND, and the second end of the power switch Q0 (for example, the drain of the MOS transistor) is coupled to the other end of the inductor L.

[0035] The sampling circuit 10 provides a feedback voltage Vfb to the driving control circuit U0 according to the voltage Vcsp at the first end of the sampling resistor Rcs, the voltage Vcsn at the second end, and the received first reference voltage Vstr, so as to feedback the current change of the inductor L to the driving control circuit U0. The driving control circuit U0 compares and amplifies Vfb with the reference voltage Vref, and outputs a corresponding driving signal to control the conduction and cut-off of the power switch Q0. The conduction and cut-off of Q0 can control the inductor L to store and release energy, and further control the output current flowing through the load LED to reach a preset constant current value, or control the output voltage provided to the load LED to reach a preset constant voltage value.

[0036] As an example, the driving circuit can be manufactured as a low-side driving controller chip, which includes a driving control circuit U0 and a sampling circuit 10 as described in any embodiment of the present invention. The sampling resistor Rcs and the power switch Q0 are both peripheral circuits and control objects of the low-side driving controller chip.

[0037] As another example, when allowed by chip design, one or both of the sampling resistor Rcs and the power switch Q0, and at least part of the circuits in the sampling circuit 10 can be integrated with the driving control circuit U0 into a low-side driving controller chip to serve as the driving circuit.

[0038] The above description is only a description of the preferred embodiments of the present invention, and does not limit the scope of the present invention in any way. Any changes and modifications made by those of ordinary skill in the art of the present invention based on the above disclosure belong to the protection scope of the technical solution of the present invention.

Claims

1. A sampling circuit, characterized in that, Including: A first input terminal, coupled to a first sampling module, a second sampling module, and a first end of a sampling resistor; A second input terminal, coupled to a second end of the sampling resistor and the first sampling module; Wherein, an output terminal of the first sampling module and an output terminal of the second sampling module are coupled to a first input terminal of a drive control circuit, and the second sampling module receives a first reference voltage; When a voltage at the first end of the sampling resistor is higher than a second reference voltage, the first sampling module works alone and provides a corresponding feedback voltage to the drive control circuit; When the voltage at the first end of the sampling resistor is lower than or equal to the second reference voltage, the first sampling module and the second sampling module work together, and according to the voltage at the first end of the sampling resistor and the first reference voltage, provide a corresponding feedback voltage Vfb = K1*Vstr - K2*Vcsp to the drive control circuit, where Vstr is the first reference voltage, Vcsp is the voltage at the first end, and K1 and K2 are weighting factors greater than 0, and at this time, the feedback voltage provided by the second sampling module is not lower than the feedback voltage provided when the first sampling module works alone.

2. The sampling circuit according to claim 1, wherein The second reference voltage is set corresponding to the first reference voltage.

3. The sampling circuit according to claim 1, wherein The second sampling module includes: An adder, a first input terminal of the adder receives the first reference voltage, a second input terminal of the adder is coupled to the first end of the sampling resistor and receives the voltage at the first end of the sampling resistor, and the adder is configured to perform weighted superposition on the first reference voltage and the voltage at the first end of the sampling resistor and then output; A comparator and a control switch, a first input terminal of the comparator is coupled to an output terminal of the adder, a second input terminal of the comparator is coupled to an output terminal of the first sampling module and the first input terminal of the drive control circuit, an output terminal of the comparator is coupled to a control terminal of the control switch, and when the voltage at the first end of the sampling resistor is lower than or equal to the second reference voltage, the comparator controls the control switch to conduct.

4. The sampling circuit according to claim 3, wherein The voltage output by the adder is K1*Vstr – K2*Vcsp, where Vstr is the first reference voltage, Vcsp is the voltage at the first end of the sampling resistor, and K1 and K2 are weighting factors greater than 0.

5. The sampling circuit according to any one of claims 1-4, characterized in that, The first sampling module includes a transconductance amplifier and a pull-down resistor, a first input terminal of the transconductance amplifier is coupled to the first end of the sampling resistor, a second input terminal of the transconductance amplifier is coupled to the second end of the sampling resistor, and an output terminal of the transconductance amplifier is coupled to one end of the pull-down resistor and the first input terminal of the drive control circuit.

6. The sampling circuit according to claim 5, wherein The feedback voltage provided when the first sampling module works alone is expressed as Vfb = Gm*(Vcsp - Vcsn)*R, where Gm is the conductance value of the transconductance amplifier, Vcsp is the voltage at the first end of the sampling resistor, Vcsn is the voltage at the second end of the sampling resistor, and R is the resistance value of the pull-down resistor.

7. A driving circuit for a power switch, characterized in that, It includes a drive control circuit and a sampling circuit as described in any one of claims 1-6; wherein, the sampling circuit is coupled to a sampling resistor, the sampling resistor is connected in series between the power switch and the load, a first end of the sampling resistor is coupled to one end of the load and a first input end of the sampling circuit, and a second end of the sampling resistor is coupled to a second input end of the sampling circuit; an output end of the drive control circuit is coupled to a control end of the power switch, the drive control circuit receives a feedback voltage and a reference voltage, and controls the power switch according to the feedback voltage and the reference voltage.

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