GB2544185A

GB2544185A - Light-source systems and controllers thereof

Info

Publication number: GB2544185A
Application number: GB1616795.9A
Authority: GB
Inventors: Putra Ade; Badami Kais Kaizer; Chieh Su Ying
Original assignee: O2Micro Inc
Current assignee: O2Micro Inc
Priority date: 2016-10-03
Filing date: 2016-10-03
Publication date: 2017-05-10
Anticipated expiration: 2036-10-03
Also published as: GB2544185B; GB201616795D0

Abstract

A light source 304 is powered by a light-source current ILED from an output path 320. A sense terminal 318, coupled to the light source, provides a sense signal indicative of the light-source current. A capacitor CFF passes an AC component IFF from the output path to a node 322. A resistor RFF passes the AC component from the node to the sense terminal and provides, at the node, an indication signal VFF indicative of a combination of the AC component and the sense signal. Driving circuitry generates, on the output path, an output current ISW including the AC component and the light-source current, detects a variation in a ripple amplitude of the output current by detecting a level of the indication signal, and controls a duty cycle of a switch 308 so as to reduce an increasing rate of the ripple amplitude if the level of the indication signal increases, and to reduce a decreasing rate of the ripple amplitude if the level of the indication signal decreases.

Description

LIGHT-SOURCE SYSTEMS AND CONTROLLERS THEREOF BACKGROUND FIG. 1A illustrates a conventional light-source system 100. In the system 100, a switching converter 102 generates a ripple output current Iout by alternately turning on a switch 108 in the converter 102. An output capacitor Cout filters out the ripples of the current Iout and provides a light-source current Iled to the light source 104. The converter 102 can increase the current Iled if a feedback voltage Vfb indicates that the current Iled is less than a target level, and reduce the current Iled if the feedback voltage Vfb indicates that the current Iled is greater than the target level, thereby adjusting the current Iled to the target level. However, if the system 100 is used to convert an AC input with a lower frequency (e.g., 220VAC/50Hz, 120VAC/60Hz, or the like from an electric supply) to the light-source current Iled, the system 100 has a shortcoming which is described in combination with FIG. IB. FIG. IB illustrates examples of waveforms for signals Iout, Iled and Vfb in the system 100. As shown in FIG. IB, the output current Iout includes ripples because the switch 108 is turned on and off alternately. The ripple amplitude of the current Iout increases and decreases periodically because the input power VIN of the system 100 is AC power. The waveform for the light-source current Iled is smoothened because the output capacitor Cout filters out the ripples of the current Iout However, there is a relatively large phase delay Δφΐ from the output current Iout to the light-source current Iled This is because the capacitance of the output capacitor Cout is relatively high. If a peak of the output current Iout appears at time Ta, then the capacitor Cout delays the appearance of a corresponding peak of the light-source current Iled to time Tb (Tb =ΤΑ+Δφ1). Because the feedback voltage Vfb is in phase with the light-source current Iled. the feedback voltage Vfb is also phase-delayed relative to the output current Iout in a similar manner. At time Ta, the converter 102 detects that the feedback voltage Vfb has increased to a voltage reference, e.g., indicating that the light-source current Iled has increased to a target level. Hence, the converter 102 reduces a duty cycle of the switch 108 to reduce the ripple amplitude of the output current Iout- However, the decrease of the ripple amplitude of the output current Iout does not result in an immediate decrease of the light-source current Iled due to the phase delay Δφΐ.

Consequently, if high current ringing appears in the output current Iout, the converter 102 is unable to detect it immediately and protect the light source 104 from an over-shoot/over-current condition. FIG. 2A illustrates another conventional light-source system 200. In the system 200, a capacitor 218 passes an AC component I218 of the output current Iout to the sense resistor Rs. Hence the feedback voltage V’fb represents a sum of the light-source current ILed and the AC component I2i8- Thus, as shown in FIG. 2B, the phase delay Δφ2 from the output current Iout to the feedback voltage V’fb is less than the phase delay Δφΐ from the output current lou r to the light-source current Ii .m. In other words, the phase of the feedback voltage V’fb is advanced ahead of the phase of the light-source current ILed- If high current ringing appears in the output current Iout, then the converter 202 can detect it earlier and respond faster, compared with the converter 102 in FIG. 1 A.

However, in order to advance the phase of the voltage V’fb to be close enough to the phase of the current Iout, the capacitor 218 should have a relatively large capacitance. In some situations, there is a limitation of a PCB size for the system 200. Therefore, the size and capacitance of the capacitor 218 cannot be too large. Thus, the phase delay Δφ2 from the current Iout to the voltage V’fb may still be relatively large. Solutions that address these shortcomings would be beneficial.

SUMMARY

In a system in one embodiment, a light source is powered by a light-source current from an output path. A sense terminal, coupled to the light source, provides a sense signal indicative of the light-source current. A capacitor, coupled to the output path, passes an AC component from the output path to a node. A resistor, coupled between the node and the sense terminal, passes the AC component from the node to the sense terminal and provides, at the node, an indication signal indicative of a combination of the AC component and the sense signal. Driving circuitry generates, on the output path, an output current including the AC component and the light-source current, detects a variation in a ripple amplitude of the output current by detecting a level of the indication signal, reduces an increasing rate of the ripple amplitude if the level of the indication signal increases, and reduces a decreasing rate of the ripple amplitude if the level of the indication signal decreases.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the claimed subject matter will become apparent as the following detailed description proceeds, and upon reference to the drawings, wherein like numerals depict like parts, and in which: FIG. 1A illustrates a conventional light-source system. FIG. IB illustrates examples of signal waveforms of the system in FIG. 1 A. FIG. 2A illustrates another conventional light-source system. FIG. 2B illustrates examples of signal waveforms of the system in FIG. 2A. FIG. 3 A illustrates an example of a light-source system, in an embodiment of the present invention. FIG. 3B illustrates examples of signal waveforms of the system in FIG. 3 A, in an embodiment of the present invention. FIG. 4 illustrates an example of a light-source system, in an embodiment of the present invention. FIG. 5A illustrates a block diagram of an example of a controller, in an embodiment of the present invention. FIG. 5B illustrates examples of signal waveforms of the controller in FIG. 5 A, in an embodiment according to the present invention. FIG. 6A illustrates a block diagram of an example of a current regulator, in an embodiment of the present invention. FIG. 6B illustrates examples of signal waveforms of the current regulator in FIG. 6A, in an embodiment of the present invention. FIG. 7 illustrates a flowchart of examples of operations performed by a light-source system, in an embodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the embodiments of the present invention. While the invention will be described in conjunction with these embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims.

Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. FIG. 3A illustrates an example of a light-source system 300, in an embodiment of the present invention. The system 300 includes a light source 304, e.g., one or more LEDs, and driving circuitry, e.g., including an AC/DC converter 302 and an output capacitor Cout, for driving/powering the light source 304. The AC/DC converter 302 can include a switching converter.

In one embodiment, the converter 302 includes an input path 306 that receives power from an AC power source AC-in, an output path 320 that provides power to the light source 304, and a switch 308 that selectively enables/disables power transfer from the input path 306 to the output path 320. When the switch 108 is in a first state, e.g., on (or off), the power transfer is enabled, and an output current Isw generated on the output path 320 increases. When the switch 108 is in a second state, e.g., off (or on), the power transfer is disabled, and the current Isw decreases. The converter 302 can generate a control signal 310 to alternately turn on and off the switch 308, which causes the output current Isw to ramp up and down. In one embodiment, the output current Isw includes ripples. The output capacitor Cout filters out the ripples of the current Isw and therefore a substantially direct-current (DC) portion of the current Isw can flow through the light source 304. As used herein, “substantially direct-current” means that a current may vary and the variation in the current during a predetermined time interval is relatively small and can be neglected. The substantially DC portion of the current Isw can be referred to as a light-source current Iled· A feedback circuit, e.g., including a feed-forward capacitor Cff, a feed-forward resistor RFf (e.g., referred to as a first resistor), and a sense resistor Rs2 (e.g., referred to as a second resistor), can provide an indication signal Vff, indicative of the light-source current ILED, to the converter 302. The converter 302 can generate a control signal 310 to control the switch 308 according to the indication signal Vff· For instance, if the indication signal Vff indicates that the current Iled is less than a target level, then the converter 302 controls the signal 310 to increase the current Iled; or if the signal Vff indicates that the current Iled is greater than the target level, then the converter 302 controls the signal 310 to decrease the current Iled· As a result, the light-source current ILed can be adjusted to the target level.

In one embodiment, the sense resistor Rs2 is coupled in series to the light source 304 via a sense terminal 318, the feed-forward capacitor Cff is coupled between the output path 320 and the feed-forward resistor RFf, and the resistor Rff is coupled between the capacitor Cff and the sense resistor Rs2. When the output current Isw is generated on the path 320, the capacitor Cff passes an AC component Iff of the current Isw from the path 320 to the connection node 322 of the capacitor Cff and the resistor Rff, and the resistor Rff passes the AC component IFf from the node 322 to the sense terminal 318. Because both the currents Iff and Iled flow through the resistor Rs2, a voltage Vrs2 across the resistor RS2 can be given by: Vrs2 = Iled*Rs2 + Iff*Rs2· The voltage component Iled*Rs2 is caused by the light-source current Iled flowing through the resistor Rs2 and can be referred to as a first voltage component Viled, and the voltage component Iff*Rs2 is caused by the AC component Iff flowing through the resistor Rs2 and can be referred to as a second voltage component Vswi· Hence, the resistor Rs2 can provide at the terminal 318 a sense signal V318 indicative of the sum of the voltage components Viled and Vswi· In one embodiment, an average level of the AC component Iff can be equal to zero amperes, and therefore an average level of the second voltage component Vswi can be equal to zero volts. Thus, an average level of the sense signal V318 can represent/indicate the first voltage component Viled and the light-source current It.fd.

Additionally, in one embodiment, the AC component Iff flowing through the feed-forward resistor Rff causes the resistor Rff to have a voltage drop of Iff*Rff- The voltage drop Iff*Rff can be referred to as a third voltage component Vsw2 Hence, the indication signal Vff at the node 322 can have a voltage level Vff given by: Vff = V318+VSW2 = Vπ F.D+Vsw 1 + Vsw2. · The indication signal Vff includes the sum of the voltage components Viled, Vswi and Vsw2· Thus, the indication signal Vff represents/indicates a combination of the AC component Iff and the sense signal V318. The indication signal Vff also represents/indicates a combination of the AC component Iff and the light-source current Ιτ.ρ,π.

In one embodiment, the indication signal Vff includes an AC voltage component representing the AC current component Iff, and a DC voltage component representing the light-source current Ιτ.ρ,π. Advantageously, the feed-forward resistor Rff can increase a ratio of the AC voltage component to the DC voltage component. Thus, the phase of the indication signal Vff can be further advanced ahead of the phase of the light-source current Iled, compared with that of the feedback voltage V’fb in the conventional light-source system 200. If high current ringing appears in the output current Isw, the converter 302 can detect it earlier and protect the light source 304 from an over-shoot/over-current condition. Additionally, the feed-forward capacitor Cff can have a lower capacitance and a smaller size compared with the capacitor 218 in the conventional light-source system 200, which reduces the cost and PCB size of the light-source system 300. FIG. 3B illustrates examples of signal waveforms of the light-source system 300, in an embodiment of the present invention. As shown in FIG. 3B, the phase delay Δφ3 from the output current Isw to the indication signal Vff is relatively small, e g., less than the phase delay Acp2 in FIG. 2B. In other words, the phase of the indication signal Vff is relatively close to the phase of the output current Isw· Thus, if high current ringing appears in the output current Isw, the converter 302 can detect it earlier. FIG. 4 illustrates an example of a light-source system 400, in an embodiment of the present invention. The light-source system 400 can be an embodiment of the light source system 300. In the system 400 in one embodiment, a rectifier circuit (e.g., a diode bridge D1-D4) receives AC power ACin, e.g., 220VAC/50Hz, 120VAC/60Hz, or the like from an electric supply, and outputs rectified power Vrec to the input path 306. A transformer 424 includes a primary winding P424 coupled to the input path 306, and a secondary winding S424 coupled to the output path 320. The switch 308 can selectively connect the primary winding P424 to ground, thereby enabling/disabling the power transfer from the input path 306 to the output path 320. For example, the power transfer is enabled when the switch 308 is on, and disabled when the switch 308 is off. A controller 428 can turn on/off the switch 308 according to the indication signal Vff· For example, the controller 428 includes a first pin SW that provides a control signal 310 to selectively turn on the switch 308, a second pin SEN2 that receives the indication signal Vff, and control circuitry, coupled to the first pin SW and the second pin SEN2, that generates the control signal 310 according to the indication signal Vff· More details regarding the control circuitry will be described in combination with FIG. 5 A, FIG. 5B, FIG. 6A and FIG. 6B.

Additionally, in one embodiment, the feed-forward resistor Rff includes a variable resistor, and the abovementioned ratio of the AC voltage component to the DC voltage component of the indication signal Vff is adjustable based on the resistance of the variable resistor, and so is the phase delay Δφ3 from the output current Isw to the indication signal Vff· For example, the phase delay Δφ3 can be reduced by increasing the resistance Rff or increased by reducing the resistance Rff· Advantageously, the phase delay Δφ3 can be set by setting the resistance Rff according to some practical situations/conditions, e.g., system requirements. FIG. 5A illustrates a block diagram of an example of the controller 428 in FIG. 4, in an embodiment of the present invention. The controller 428 can include a filter circuit 532, a current regulator 536, a switch driver 538, and a line ripple detector 534.

The filter circuit 532, e g., including a low-pass filter, can receive the indication signal Vff and generate an output signal 526 indicative of an average voltage level of the indication signal Vff· As mentioned above, an average level of the AC component Iff can be equal to zero amperes. Therefore, an average level of the voltage component Vswi and an average level of the voltage component Vsw2 can be equal to zero volts. Hence, the average level of the indication signal Vff can represent/indicate an average level of the voltage component Viled· Thus, the output signal 526 can represent/indicate the voltage component Viled and the light-source current ILed·

The current regulator 536 can include a pulse-width-modulation (PWM) generator (not shown) that generates a PWM signal according to the output signal 526. The PWM signal controls the switch driver 538 to control the switch 308, thereby controlling the output current Isw and the light-source current ILed- For example, if the output signal 526, e.g., representing an average voltage level of the indication signal Vff, is less than a predetermined reference Vref, then the current regulator 536 increases a duty cycle of the PWM signal to increase an average level of the output current Isw, thereby increasing the light-source current Iled. If the output signal 526 is greater than the reference Vref, then the current regulator 536 decreases the duty cycle to decrease the average level of the output current Isw, thereby decreasing the light-source current Iled As a result, the light-source current Iled can be adjusted to a target level determined by the reference Vref·

In one embodiment, referring to FIG. 4, the input path 306 receives an input voltage Vrec that can increase or decrease. For example, the input voltage Vrec may derive from an AC power source ACin, e.g., 220VAC/50Hz, 120VAC/60Hz, or the like from an electric supply, and therefore can increase/decrease according to the AC power. The increase/decrease of the input voltage Vrec can cause a ripple amplitude of the output current Isw to increase/decrease, which reduces the stability of the light-source current Iled· Advantageously, in one embodiment, the detector 534 in FIG 5A can detect a variation in the ripple amplitude of the output current Isw by detecting a specific level of the indication signal Vff, and provides a phase signal Vph indicative of the variation in the ripple amplitude of the output current Isw· The current regulator 536 can reduce a changing rate of the ripple amplitude of the output current Isw according to the phase signal Vph· As a result, the stability of the light-source current ILed can be enhanced. FIG. 5B illustrates examples of signal waveforms of the controller 428, in an embodiment according to the present invention. As shown in FIG. 5B, the output signal 526, represented by the dashed line in plot 512, is indicative of the average level of the indication signal Vff· The output signal 526 is also indicative of the light-source current ILed, and the phase of the output signal 526 is advanced ahead of the phase of the light-source current ILed· Additionally, as shown in FIG. 5B, a variation in the phase signal Vph can represent/indicate a variation in the indication signal Vff, thereby representing/indicating a variation in the AC component Iff·

In one embodiment, the indication signal Vff includes ripples caused by alternately switching on and off the switch 308. The detector 534 can generate the phase signal Vph by sampling the indication signal Vff at the same predetermined phase angle (e.g., 90°, 60°, etc.) in each switching cycle of the switch 308 (or in each two or more switching cycles of the switch 308). In other words, the phase signal VPH can represent a specific level of the indication signal Vff at a predetermined phase angle. Thus, a variation in the phase signal VPH can represent a variation in the ripple amplitude of the output current Isw· In another embodiment, the detector 534 may generate the signal Vph by monitoring a specific level such as an average level of the indication signal Vff For example, the detector 534 may include a low-pass filter that generates an output signal Vph indicative of an average level of the indication signal Vff· A variation in such a signal Vph can also represent a variation in the ripple amplitude of the output current Isw· In one such example, the filter circuit 532 may be included in the detector 534, and the signal VPh includes the output signal 525. In yet another embodiment, the detector 534 can generate the signal VPH to indicate a variation in the ripple amplitude of the output current Isw in another manner. FIG. 6A illustrates a block diagram of an example of the current regulator 536 in FIG. 4, in an embodiment of the present invention. The current regulator 536 can include a ramp signal generator 640, an operational transconductance amplifier 642, a comparator 644, a clock signal generator 648, and a logic circuit (e.g., an RS latch 646).

In one embodiment, the amplifier 642 receives the output signal 526, e.g., indicative of the light-source current It fh as mentioned in relation to FIG. 5B, and compares the output signal 526 with a predetermined reference Vref- The amplifier 642 can increase its output voltage Vcomp (e.g., a voltage on the compensation capacitor Ccomp) if the signal 526 is greater than the reference Vref, and reduce its output voltage Vcomp if the signal 526 is less than the reference Vref- The output voltage Vcomp can be a reference signal provided to the comparator 644. The comparator 644 compares the reference signal Vcomp with a ramp signal Vramp, and generates a comparison result 650 accordingly. The RS latch 646 generates a PWM signal to control the switch 308, and adjusts a duty cycle of the PWM signal according to the comparison result 650. For example, the RS latch 646 receives a clock signal CLK at its set terminal, and sets the PWM signal to be logic high when a pulse is presented in the clock signal CLK. The RS latch 646 also receives the comparison result 650 at its reset terminal, and resets the PWM signal to be logic low when a pulse is presented in the comparison result 650. Hence, the duty cycle of the PWM signal can increase if the reference signal Vcomp increases, and decrease if the reference signal Vcomp decreases. As a result, the current regulator 536 can adjust the output voltage 526 to the predetermined reference Vref, thereby adjusting the light-source current ILed to a target level determined by the reference Vref·

In one embodiment, the signal generator 640 generates the ramp signal Vramp and adjusts a ramping slope of the ramp signal Vramp according to the phase signal Vph. For example, the signal generator 640 can increase the ramping slope if the phase signal Vph increases, e.g., indicating that an abovementioned specific level of the indication signal Vff increases, and reduce the ramping slope if the phase signal Vph decreases, e.g., indicating that the specific level of the indication signal Vff decreases. Hence, if the specific level of the indication signal Vff increases, then the current regulator 536 can reduce the duty cycle of the PWM signal, thereby reducing an increasing rate of the ripple amplitude of the output current Isw· If the specific level of the indication signal Vff decreases, then the current regulator 536 can increase the duty cycle of the PWM signal, thereby reducing a decreasing rate of the ripple amplitude of the output current Isw As a result, the stability of the light-source current Iled can be enhanced. FIG. 6B illustrates examples of signal waveforms of the current regulator 536, in an embodiment of the present invention. FIG. 6B is described in combination with FIG. 6A. As shown in FIG. 6B, if the reference signal Vcomp increases (e.g., at time t2), then the duty cycle of the PWM signal increases. Similarly, if the reference signal Vcomp decreases (not shown), then the duty cycle of the PWM signal decreases. Hence, the light-source current ILed can be adjusted to a target level determined by the predetermined reference Vref· Additionally, if the phase signal Vph increases (e.g., at time Ϊ3), then the ramping slope of the ramp signal Vramp increases thereby reducing the duty cycle of the PWM signal. Similarly, if the phase signal Vph decreases (not shown), then the ramping slope of the ramp signal Vramp decreases thereby increasing the duty cycle of the PWM signal. Hence, a changing rate of the output current Isw can be reduced. The stability of the light-source current Iled can be enhanced. FIG. 7 illustrates a flowchart 700 of examples of operations performed by a light-source system, in an embodiment of the present invention. FIG. 7 is described in combination with FIG. 3 A, FIG. 3B, FIG. 4, FIG. 5A, FIG. 5B, FIG. 6A and FIG. 6B.

In block 702, the light-source system (e.g., 300 or 400) generates an output current Isw on the output path 320 coupled to the light source 304. The output current Isw includes an AC component Iff and a light-source current Iled flowing through the light source 304.

In block 704, the sense resistor Rs2 generates a sense signal V318 indicative of the light-source current Iled, at the sense terminal 318 coupled to the light source 304.

In block 706, the feed-forward capacitor Cff passes the AC component Iff from the output path 320 to the node 322.

In block 708, the feed-forward resistor RFf passes the AC component Iff from the node 322 to the sense terminal 318.

In block 710, the feedback circuit, e.g., including the capacitor Cff and the resistors Rff and Rs2, provides an indication signal Vff, at the node 322, indicative of a combination of the AC component Iff and the sense signal V318·

In block 712, the controller 428 controls the output current Isw according to the indication signal Vff·

While the foregoing description and drawings represent embodiments of the present invention, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope of the principles of the present invention as defined in the accompanying claims. One skilled in the art will appreciate that the invention may be used with many modifications of form, structure, arrangement, proportions, materials, elements, and components and otherwise, used in the practice of the invention, which are particularly adapted to specific environments and operative requirements without departing from the principles of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims and their legal equivalents, and not limited to the foregoing description.

Claims

WE CLAIM:

1. A system comprising: a light source powered by a light-source current from an output path, said light-source current flowing through said light source; a sense terminal, coupled to said light source, that provides a sense signal indicative of said light-source current; a capacitor, coupled to said output path, that passes an alternating-current (AC) component from said output path to a node; a first resistor, coupled between said node and said sense terminal, that passes said AC component from said node to said sense terminal and provides, at said node, an indication signal indicative of a combination of said AC component and said sense signal; and driving circuitry that generates, on said output path, an output current comprising said AC component and said light-source current, that detects a variation in a ripple amplitude of said output current by detecting a first level of said indication signal, that reduces an increasing rate of said ripple amplitude if said first level increases, and that reduces a decreasing rate of said ripple amplitude if said first level decreases.

2. The system of claim 1, further comprising a second resistor, coupled to said light source via said sense terminal, that provides said sense signal if said light-source current flows through said second resistor.

3. The system of claim 2, wherein said indication signal comprises the sum of a first voltage component, a second voltage component, and a third voltage component, and wherein said first voltage component is caused by said light-source current flowing through said second resistor, said second voltage component is caused by said AC component flowing through said second resistor, and said third voltage component is caused by said AC component flowing through said first resistor.

4. The system of any of claims 1 to 3, wherein said first resistor comprises a variable resistor, and a phase delay from said output current to said indication signal is adjustable based on the resistance of said variable resistor.

5. The system of any of claims 1 to 4, wherein said driving circuitry increases a level of said output current if a second level of said indication signal is less than a predetermined reference, and decreases said level of said output current if said second level is greater than said predetermined reference.

6. The system of any of claims 1 to 5, wherein said driving circuitry comprises: an input path that receives power; a switch, coupled to said input path, that selectively enables power transfer from said input path to said output path; and a controller that controls said switch according to said indication signal.

7. The system of claim 6, wherein said driving circuitry comprises: a signal generator that generates a ramp signal and adjusts a ramping slope of said ramp signal according to said first level; a comparator, coupled to said signal generator, that compares said ramp signal with a reference signal to generate a comparison result; and a logic circuit, coupled to said comparator, that generates a pulse-width-modulation (PWM) signal to control said switch and adjusts a duty cycle of said PWM signal according to said comparison result.

8. The system of any of claims 1 to 7, wherein said system receives AC power that causes said ripple amplitude to increase or decrease.

9. The system of any of claims 1 to 8, further comprising an output capacitor, coupled to said light source, that filters out ripples of said output current.

10. A method for controlling a light source, said method comprising: generating an output current on an output path coupled to said light source, wherein said output current comprises an AC component and comprises a light-source current flowing through said light source; generating a sense signal indicative of said light-source current, at a sense terminal coupled to said light source; passing, using a capacitor, said AC component from said output path to a node; passing, using a first resistor, said AC component from said node to said sense terminal; providing, at said node, an indication signal indicative of a combination of said AC component and said sense signal; detecting a variation in a ripple amplitude of said output current by detecting a first level of said indication signal; reducing an increasing rate of said ripple amplitude if said first level increases; and reducing a decreasing rate of said ripple amplitude if said first level decreases.

11. The method of claim 10, wherein said generating said sense signal comprises: controlling said light-source current to flow through a second resistor couple to said light source.

12. The method of claim 11, wherein said indication signal comprises the sum of a first voltage component, a second voltage component, and a third voltage component, and wherein said first voltage component is caused by said light-source current flowing through said second resistor, said second voltage component is caused by said AC component flowing through said second resistor, and said third voltage component is caused by said AC component flowing through said first resistor.

13. The method of any of claims 10 to 12, wherein said controlling said output current comprises: increasing a level of said output current if a second level of said indication signal is less than a predetermined reference; and decreasing said level of said output current if said second level is greater than said predetermined reference.

14. The method of any of claims 10 to 13, further comprising; selectively enabling, using a switch, power transfer from an input path to said output path; and controlling said switch according to said indication signal.

15. The method of claim 14, further comprising: adjusting a ramping slope of a ramp signal according to said first level; comparing said ramp signal with a reference signal to generate a comparison result; generating a PWM signal to control said switch; and adjusting a duty cycle of said PWM signal according to said comparison result.

16. The method of any of claims 10 to 15, wherein said first resistor comprises a variable resistor, and a phase delay from said output current to said indication signal is adjustable based on the resistance of said variable resistor.

17. A controller for controlling a light source, said controller comprising: a first pin that provides a control signal to selectively turn on a switch to enable power transfer from an input path to an output path such that an output current is generated on said output path, wherein said output current comprises an AC component and a light-source current that flows through said light source; a second pin that receives an indication signal, indicative of a combination of said AC component and said light-source current, from a connection node of a capacitor and a first resistor, wherein said capacitor passes said AC component from said output path to said connection node, wherein said first resistor passes said AC component from said connection node to a sense terminal, and wherein said sense terminal provides a sense signal indicative of said light-source current to said connection node via said first resistor; and control circuitry, coupled to said first and second pins, that generates said control signal according to said indication signal, that detects a variation in a ripple amplitude of said output current by detecting a first level of said indication signal, that reduces an increasing rate of said ripple amplitude if said first level increases, and that reduces a decreasing rate of said ripple amplitude if said first level decreases.

18. The controller of claim 17, wherein said indication signal comprises the sum of a first voltage component, a second voltage component, and a third voltage component, and wherein said first voltage component is caused by said light-source current flowing through a second resistor coupled to said sense terminal, said second voltage component is caused by said AC component flowing through said second resistor, and said third voltage component is caused by said AC component flowing through said first resistor.

19. The controller of claim 17 or claim 18, wherein said control circuitry controls said switch to increase a level of said output current if a second level of said indication signal is less than a predetermined reference, and decrease said level of said output current if said second level is greater than said predetermined reference.

20. The controller of any of claims 17 to 19, wherein said control circuitry comprises: a signal generator that generates a ramp signal and adjusts a ramping slope of said ramp signal according to said first level; a comparator, coupled to said signal generator, that compares said ramp signal with a reference signal to generate a comparison result; and a logic circuit, coupled to said comparator, that generates a PWM signal to control said switch and adjusts a duty cycle of said PWM signal according to said comparison result.