Circuit for providing supply current and method thereof

Abstract

The present disclosure relates to a circuit (10) for providing a supply current, the circuit comprising a current generator (100) for generating a constant current (Iref) and a ramp generator (110) for generating a current ramp, the current ramp being subtracted from the constant current (Iref) to generate the supply current. The circuit (10) further comprises a comparison circuit (120) for comparing the supply current to a first current value (Ires), a clamp switch (130) configured to close the ramp generator (110) when the supply current is less than or equal to the first current value (Ires), and a supply circuit (140) for supplying the supply current to the load (150).

Description

Background Art

[0001] Circuits for generating current pulses can be used in various applications, for example, in the case of LEDs, the circuit provides a current pulsed primarily with slow pulse width modulation (PWM). In general, PWM signals are applied to dim LEDs, control motors and various other electronic devices. Thus, control of the average current and voltage delivered to a load (e.g., an LED) is achieved by quickly turning on and off the switch between the load and the source. However, such modulation implies a repetitive interference pattern. In order to solve the problem, many methods based on reducing the slope of the current pulses have been considered, wherein a constant slope is preferred. That is, the key is to generate current pulses with a linear profile that rises from a low level to a high level and falls from a high level to a low level.

[0002] A common circuit used to achieve this is a ramp generator. A ramp generator creates an output that rises or falls linearly over time. However, generating a linear ramp requires a lot of development work. Accuracy, distortion, and responsiveness are the main challenges to be faced: the ramp should occur in a short time, typically a few tenths of a nanosecond, and the allowed tolerance for regulating the amplitude of the current pulse is very tight (about 1% to 2% with minimal thermal drift). Incorporating the trimming structure into the ramp generator makes the solution more complex and can be burdened by parasitics.

[0003] Furthermore, the presence of a switch for switching the current / voltage on and off to a load such as an LED may cause additional side effects.

[0004] Furthermore, to avoid current swings when starting the ramp procedure, a clamping circuit is implemented. However, combining the concepts sets high limits on the matching requirements of the individual circuit elements that affect the overall performance and the output current slope.

[0005] It is therefore envisaged to seek a circuit which occupies a small area and is able to generate a current pulse with a defined slope.

[0006] It is an object of the present invention to provide an improved circuit. Summary of the invention

[0007] According to an embodiment, a circuit for providing a supply current includes: a current generator for generating a constant current; a ramp generator for generating a current ramp, the current ramp being subtracted from the constant current to generate the supply current; a comparison circuit for comparing the supply current with a first current value; a clamp switch configured to turn off the ramp generator when the supply current is less than or equal to the first current value, and a supply circuit for supplying the supply current to a load.

[0008] The configuration enables the supply current in the circuit to be controlled by sensing the value of the supply current via a comparison circuit and clamping the peak value of the ramp provided by the ramp generator to produce a small supply current or residual current. Leaving this current in the circuit minimizes the dead time between the start and rise of the current ramp (e.g., the start of the ramp program and the change of the output current). This results in an improved circuit that can prevent current distortion, save area and design effort.

[0009] In this context, the circuit may also include a clamping circuit for determining a lower limit of the current ramp.

[0010] Furthermore, the comparison circuit may include a sensing mirror structure including a buffer transistor and a buffer mirror transistor. The buffer transistor may be connected in series to the power supply circuit, and the buffer mirror transistor may be connected to a current source supplying a current of the first current value via a connection node.

[0011] The clamp switch may include a clamp transistor having a gate connected to the connection node.

[0012] A comparison circuit comprising a buffer transistor and a buffer mirror transistor may allow the supply current to be measured. The first current value may be set to a few μA with high accuracy.

[0013] When the ramp generator is turned off, the circuit may supply a supply current of a first current value.

[0014] The power supply circuit may further include a power supply switch element configured to shut off the power supply current when the power supply current is less than or equal to a first current value.

[0015] In one embodiment, the circuit may further include a driving element for driving the power switch element. In this configuration, the driving element is coupled to the clamp switch.

[0016] According to an embodiment, a method for providing a supply current includes the following: generating a constant current, generating a current ramp, subtracting the current ramp from the constant current to generate the supply current, comparing the supply current with a first current value, turning off a ramp generator when the supply current is less than or equal to the first current value, and supplying the supply current to a load.

[0017] Furthermore, a lower limit for the current ramp can be determined.

[0018] Furthermore, when the ramp generator is turned off, a supply current of a first current value may be supplied. The first current value may be set to several μA with high accuracy.

[0019] According to an embodiment, a lighting device includes a light emitting element and the above-mentioned circuit for providing a power supply current to the light emitting element. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings are included to provide a further understanding of embodiments of the present invention and are incorporated into and constitute a part of this specification. These drawings illustrate embodiments of the present invention and are used to explain the principles together with the description. Other embodiments of the present invention and many expected advantages will become more easily understood by reference to the detailed description below. The elements in the accompanying drawings are not necessarily drawn to scale with each other. The same reference numerals indicate corresponding similar parts.

[0021] Figure 1 is a schematic diagram showing a circuit for providing a power supply current according to an embodiment.

[0022] Figure 2A Circuit components for providing the supply current are shown in more detail.

[0023] Figure 2B Examples of waveforms generated by circuit components according to embodiments are shown.

[0024] Figure 3 is a diagram showing components of a circuit for providing a supply current according to an embodiment.

[0025] Figure 4 is a schematic diagram illustrating a method for providing a power supply current according to an embodiment.

[0026] Figure 5 is a schematic diagram showing an electronic device according to an embodiment. DETAILED DESCRIPTION

[0027] In the detailed description below, reference is made to the accompanying drawings, which form a part of this specification and show, by way of example, specific embodiments in which the invention may be implemented. In this regard, directional terms such as "top", "bottom", "front", "rear", "above", "on", "over", "head", "tail", etc. are used with reference to the orientation of the drawings being described. Since the components of embodiments of the invention can be positioned in many different orientations, the directional terms are used for illustrative purposes and not as limitations. It should be understood that other embodiments may be employed and structural or logical changes may be made without departing from the scope defined by the claims.

[0028] The description of these embodiments is non-limiting. In particular, elements of the embodiments described below may be combined with elements of different embodiments.

[0029] The terms "coupled" and / or "electrically coupled" as used herein do not mean that elements must be directly coupled together, but intermediate elements may be provided between the "coupled" or "electrically coupled" elements. The term "electrically connected" is intended to describe a low-ohmic electrical connection between elements that are electrically connected together.

[0030] As used herein, the terms "having", "containing", "including", "comprising" and equivalents are open terms, which are used to indicate the presence of the elements or features, but do not exclude the presence of other elements or features. Unless the context clearly indicates otherwise, the elements with undefined quantity are intended to include the plural and singular.

[0031] Figure 1 2 is a schematic diagram showing a general arrangement of a circuit for providing a supply current 10 according to an embodiment. The circuit 10 includes a current generator 100, a ramp generator 110, a comparison circuit 120, a clamp switch 130, a clamp circuit 135, and a supply circuit 140. The circuit 10 is also used to provide a current pulse to a load 150 such as an LED, for example. FIG. 2 shows the components of the circuit 10 in more detail, which will be described below.

[0032] The current generator 100 may include a current source generating a constant (reference) current Iref, indicated by the corresponding arrow in Figure 2. This current Iref is provided to one end of a transistor TMR, which is also part of the circuit 10. For example, Iref can be set to 5 mA and provided to the source of transistor TMR, which may be an NMOS transistor.

[0033] As shown in FIG. 2 , the ramp generator 110 may include two current sources generating currents Iup and Idn, two corresponding switches S2, S3, and a capacitor C. The two switches S2, S3 alternately conduct currents Iup and Idn in opposite directions to generate a (voltage-) ramp at the gate of the NMOS transistor TMR. The (voltage-) ramp may be provided to a voltage-current converter (e.g., formed by an NMOS transistor TMR connected to a resistor R). In more detail, the transistor TMR acts together with the resistor R coupled in series to the transistor TMR as a generator that subtracts a ramp current from a constant input current Iref to generate a supply current. More specifically, the (voltage) ramp generated at the gate of the NMOS transistor TMR may shape the current subtracted from the current mirror structure TS1, TS2, which may be part of the supply circuit 140 described in detail below.

[0034] This configuration can ensure that when the switches S2 and S3 are alternately turned on, the current supplied to the load 150 has a linear distribution.

[0035] For example, as described above, switches S2, S3 are complementary (i.e., alternately turned on and off). Ideally, a rising transition at switch S2 (e.g., when S2 is turned on) will cause the (voltage) ramp to start rising and may start to reduce the supply current. At the same time, at a falling transition at switch S2 (e.g., when S2 is turned off), a complementary transition at switch S3 can reduce the current flowing through the NMOS transistor TMR, and the supply current will start to increase.

[0036] Since the slopes of the positive (voltage) ramp and the negative (voltage) ramp are respectively given by Iup / C and Idn / C, the conversion slopes of the supply current provided to the load 150 are respectively given by Iup / (C*R) and Idn / (C*R).

[0037] However, this approach may cause some problems, as described below. In this case, Figure 2B An example of a waveform is shown.

[0038] For example, when the gate voltage VTMR_gate approaches the threshold of the NMOS transistor TMR, the gate terminal of the NMOS transistor TMR may start driving the current ITS1 at a voltage slightly greater than zero. This means that if the (voltage) ramp at the gate terminal of the NMOS transistor TMR starts from GND, the supply current flowing through the transistor TS1 will be delayed and reduced due to the time required for the (voltage) ramp to reach the threshold of the NMOS transistor TMR (called the turn-off dead time). This delay time can be expressed by C*Vth / Iup.

[0039] Furthermore, when the ramp voltage is at a given value V1, the current flowing through the NMOS transistor TMR takes the value Iref, the value of which is determined by the size of the NMOS transistor TMR, the resistor R and the temperature. In the case where the (voltage) ramp starts from a different value Vmax, the current in the load 150 starts to rise from zero after a time C*(Vmax-V1) / Idn (called the on-dead time) has passed.

[0040] For the supply current, if the same rise / fall time (ie, Idn=Iup) is used, the effective duration of the pulse will change due to the value of [C*(Vmax+V1)-C*Vth]Iup.

[0041] As will be explained below, this error is reduced due to the functionality of the comparison circuit 120. Thus, the accuracy of the definition of the pulse duration can be improved. Thus, the initial voltage of the (voltage) ramp at Vth and V1, respectively, can be accurately set to eliminate the dead time value.

[0042] As will be explained below, the turn-on dead time can be systematically improved by providing or reserving a small supply current (or residual current) for the off-state current mirror structures TS1 and TS2.

[0043] According to an embodiment, this may be achieved by directly setting the supply current (residual current) using the comparison circuit 120. As will be explained, the circuit 120 may sense the supply current (residual current) and accordingly adopt the current flowing through the NMOS transistor TMR as explained below.

[0044] The comparison circuit 120 compares the supply current with a first current value Ires. The first current value Ires may be set to several μA with high accuracy. In a non-limiting example, the first current value may be set to 5 μA.

[0045] When the supply current is less than or equal to the first current value Ires, the clamp switch 130 suppresses the slope, which causes the supply current to be generated and regulated to be close to the first current value Ires. Figure 1 The clamp switch 130 shown in simplified form in FIG. 1 may include a clamp transistor TCL. A gate of the clamp transistor TCL may be connected to a connection node, and the clamp transistor TCL will be described in detail below.

[0046] Regulation or control of the supply current by the upper limit clamp described above allows a small supply current or residual current to be provided to the supply circuit 140. This current can minimize the dead time when restarting the ramp program.

[0047] The power supply circuit 140 supplies the power supply current to the load 150. In detail, the power supply circuit 140, as the output current branch of the circuit 10, may also include a common source and common gate structure, which is formed by a transistor TMcasc, a switch S1 used as a power supply switch element, an operational amplifier, and a current mirror structure in the form of two transistors TS1 and TS2. For example, the transistors TMcasc, TS1, and TS2 may be NMOS transistors. The input of the operational amplifier may be as follows: Figure 2A and Figure 3 As shown in the figure, the same voltage can be applied to the drains of the current mirror structures TS1 and TS2. Therefore, the accuracy can be improved.

[0048] The output current can be turned on and off in the output current branch via the common source and common gate structure to generate current pulses. As discussed above, due to the presence of the above-mentioned small residual current in the output current branch, the dead time between the start of the ramp program and the output current change is shortened. By sensing the current value and clamping it accordingly in the manner described above, the current can be kept at a small value.

[0049] The circuit 10 may further include a clamping circuit 135 to determine the lower limit of the current ramp. The circuit 135 may include a current generator for generating the current Ierr and transistors TFB1 and TFB2. In more detail, the transistors TFB1 and TFB2 clamp the current ramp at the gate of the transistor TMR at a low level, that is, the transistors TFB1 and TFB2 perform low-level clamping. This ensures that a minimum current always flows through TMR, thereby further minimizing the dead time.

[0050] As described above, the current ramp controls the gate of transistor TMR. If the current through BMT is large, the clamp transistor TCL is turned off (off state), and no clamping occurs.

[0051] As shown in FIG. 2 , the comparison circuit 120 may include a sensing mirror structure, which includes a buffer transistor BT and a buffer mirror transistor BMT. The buffer transistor BT may be connected in series to the power supply circuit 140. The buffer mirror transistor BMT may be connected in parallel to a current source that supplies a current of a first current value Ires. Both transistors BT and BMT may be implemented as PMOS transistors. A scaled copy of the BT current flows through the buffer mirror transistor BMT. When the current drops below the first current value Ires, the clamp transistor TCL may be turned on, that is, the clamp transistor becomes conductive. This stops the ramping of the ramp generator 110 and adjusts the supply current to a scaled current copy of Ires of the transistor TS1 passing through the power supply circuit 140.

[0052] That is, when the ramp generator 110 is turned off, the power supply current of the first current value Ires is supplied to the power supply circuit 140 .

[0053] In particular, the comparison circuit 120 allows the supply current to be measured or sensed so as to adjust the current of the transistor TMR accordingly via the clamp switch 130 , thereby ensuring that the residual current is sufficiently small.

[0054] 2 turns off the supply current in the power supply circuit 140. For example, the switch S1 is turned off to prevent the supply current from reaching the load 150 (eg, LED).

[0055] One aspect of the present invention is to directly measure and set the residual current flowing through the buffer transistor BT. In contrast, the conventional method measures and sets the current absorbed by the ramp generator through the TMR transistor - this current is almost equal to Iref, and the residual current is obtained by their difference. Since any error caused by mismatch will affect the percentage of the measured parameter, the accuracy of the residual current will be greatly improved even with small devices and resistors of minimum width R. This saves a lot of area.

[0056] Furthermore, only the setting accuracy of the first current value Ires and the offsets of the buffer transistor BT and the mirror buffer transistor BMT determine the error affecting the residual current. The inaccuracies associated with the transistor TMR and the resistor R are eliminated. This results in only a few percent error compared to the first current value Ires, even with small devices and resistors of minimum width R. This configuration also saves area, since no additional resistors are required as is required for common clamping circuits.

[0057] Figure 3 Another embodiment of the present invention is shown. Figure 3 The circuit 10 shown in FIG. 2 may include the same components as the circuit 10 shown in FIG. 2 . In addition, the circuit 10 includes a driving element DE for driving the switch element S1. The driving element DE is coupled to the clamp switch 130. In one example, the switch element S1 may also include other components. For example, the switch S1 may include the circuit 10 shown in FIG. 2 and FIG. Figure 3 That is, the switch S1 and the amplifier can form an operational amplifier as shown. Figure 3 The blocks are indicated within the dashed rectangle.

[0058] Typically, the on-time of S1 is set after the ramp at the gate of transistor TMR is completed. Otherwise, the current supplied to the load 150 (e.g., an LED) will drop to zero very quickly from a value that is too high. This further complicates controlling the slope of the current pulse. To ensure a safe on-time, a dedicated ramp must be used to set the on-time of S1, which matches the ramp of transistor TMR and is slightly slower. However, this means that additional area is required and a lot of design work is required.

[0059] Given as Figure 3 The circuit 10 shown, it can be simply observed as follows: once the upper clamp is reached, i.e. when the clamp switch 130 turns off the ramp generator 110 (i.e. when the clamp transistor TCL starts to be traversed by the current and becomes conductive), the residual current flowing through the transistor TS1 is small enough (a few μA) to enable the switch S1 to be turned on. In more detail, this current is compared with an arbitrarily small threshold Ithr, defining the time to turn on the switch S1. This allows the matching ramp to be removed while utilizing existing circuits.

[0060] This embodiment Figure 3 As soon as the current through the clamp transistor TCL exceeds the threshold Ithr, the simple comparator of the driver element DE detects this event. The comparator sends a signal to the logic part ( Figure 3 (not shown) provides a signal to turn on switch S1. Figure 3 The presence of a logic part also ensures the switching off of S1, which can be realized with simple sequential gates, which can be realized in a known manner.

[0061] The present invention also aims to Figure 4 The method for generating current pulses is shown to solve the problem.

[0062] In step S100 , a constant current Iref is generated.

[0063] In step S110 , a current ramp is generated and the current ramp is subtracted from the constant current Iref to generate a supply current.

[0064] In step S120 , the supply current is compared with the first current value Ires.

[0065] In step S130 , when the supply current is less than or equal to the first current value Iref, the ramp generator is turned off.

[0066] In another step, a supply current is supplied to a load 150 such as an LED.

[0067] In a further step, a lower limit of the current ramp can be determined.

[0068] In another step, when the ramp generator 110 is turned off, a supply current of the first current value Ires may be supplied. As described above, the first current value Ires may be set to several μA with high accuracy.

[0069] Figure 5 A schematic diagram of an electronic device 1 according to an embodiment is shown. The device comprises a light emitting element 15 and a circuit 10 for generating current pulses according to the above description, the circuit 10 being used to supply the light emitting element 15 with current.

[0070] In summary, as shown in Figures 2 and Figure 3 The illustrated circuit 10 can minimize the dead time and prevent distortion (eg, sudden jump) of the output current when the current output branch is turned on.

[0071] Furthermore, through the configuration, the area and design workload are substantially saved.

[0072] Although embodiments of the present invention have been described above, it is apparent that there are more embodiments that can be implemented. For example, additional embodiments may include any subcombination of the features described in the claims, or any subcombination of the elements described in the examples above. Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein.

[0073] Reference numerals list

[0074] 1 Lighting equipment

[0075] 10 Circuit for generating current pulses

[0076] 15 Light-emitting element

[0077] 100 Current Generator

[0078] 110 Ramp Generator

[0079] 120 Comparison Circuit

[0080] 130 Clamp switch

[0081] 135 Clamping Circuit

[0082] 140 Power supply circuit

[0083] 150 Load

[0084] Iref constant current

[0085] Ires First current value

[0086] Iup current

[0087] Idn Current

[0088] Ierr Current

[0089] Ithr Current threshold

[0090] ITS1 Current

[0091] V1 Voltage

[0092] Vth voltage threshold

[0093] VTMR_gate voltage

[0094] Vmax Voltage

[0095] S1 switch

[0096] S2 switch

[0097] S3 switch

[0098] R Resistor

[0099] C Capacitor

[0100] TMR transistors

[0101] TCL Clamp Transistor

[0102] TMcasc transistor

[0103] TFB1 transistor

[0104] TFB2 transistor

[0105] TS1 transistor

[0106] TS2 transistor

[0107] BT Buffer Transistor

[0108] BMT Buffer Mirror Transistor

[0109] S100 generates constant current

[0110] S110 generates current ramp

[0111] S120 compares the supply current with the first current value

[0112] S130 When the supply current is less than or equal to the first current value, turn off the ramp generator

[0113] S140 supplies power current to the load (150)

Claims

1. A circuit (10) for providing a supply current, comprising: A current generator (100) for generating a constant current (Iref); A ramp generator (110) for generating a current ramp, wherein the current ramp is subtracted from the constant current (Iref) to generate a supply current; A comparison circuit (120) for comparing the supply current with a first current value (Ires); a clamp switch (130) configured to turn off the ramp generator (110) when the supply current is less than or equal to the first current value (Ires); as well as The power supply circuit (140) is used to supply the power supply current to the load (150).

2. The circuit (10) of claim 1, further comprising a clamping circuit (135) for determining a lower limit of the current ramp.

3. The circuit (10) according to claim 1 or 2, wherein: The comparison circuit (120) includes a sensing mirror structure, the sensing mirror structure includes a buffer transistor (BT) and a buffer mirror transistor (BMT), the buffer transistor (BT) is connected in series to the power supply circuit (140), and the buffer mirror transistor (BMT) is connected to a current source supplying a current of the first current value via a connection node.

4. The circuit (10) according to claim 3, wherein: The clamp switch (130) includes a clamp transistor (TCL) having a gate connected to the connection node.

5. The circuit (10) according to any one of the preceding claims, configured to supply a supply current of the first current value (Ires) when the ramp generator (110) is switched off, wherein: The first current value (Ires) is set to several μA with high accuracy.

6. The circuit (10) according to any one of the preceding claims, wherein: The power supply circuit (140) further comprises a power supply switch element (S1) for shutting off the power supply current when the power supply current is less than or equal to the first current value.

7. The circuit (10) according to claim 6, further comprising a driving element (DE) for driving the power switching element (S1), and the driving element is coupled to the clamping switch (130).

8. A method for providing a supply current, comprising: generating (S100) a constant current; generating (S110) a current ramp, subtracting the current ramp from the constant current to generate a supply current; comparing the supply current with a first current value (S120); When the supply current is less than or equal to the first current value, turning off (S130) the ramp generator; as well as The supply current is supplied (S140) to a load (150).

9. The method according to claim 8, further comprising: A lower limit of the current ramp is determined.

10. The method according to claim 8 or 9, further comprising: When the ramp generator (110) is turned off, a supply current of the first current value (Ires) is supplied, wherein the first current value (Ires) is set to several μA with high accuracy.

11. A lighting device (1), comprising a light emitting element (15) and a circuit (10), wherein the circuit (10) is used to provide a supply current to the light emitting element (15).

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