Driver for delivering current to an LED load

Abstract

The driver has a current control loop for regulating the output current, which has a controllable configuration comprising an electrical value that influences the conversion function of the converter. A regulation circuit is provided to selectively deliver or not deliver the output current of the converter to the load and to efficiently regulate the load seen by the driver between a first level and a second level. In addition to the output current regulation implemented by the current control loop, this regulation enables for example the PWM control of the output current. Before the regulation circuit changes the load seen by the driver from the first level to the second level, the electrical value of the current control loop is saved; and when the regulation circuit changes the load seen by the driver from the second level back to the first level, the saved electrical value is reapplied to the current control loop. The response time of the current control loop is reduced when controlling the desired output current of the converter. The electrical value is the voltage of a capacitor in the compensation circuit of the error amplifier of the current control loop.

Description

TECHNICAL FIELD

[0001] The present application relates to a driver for delivering current to an LED load. BACKGROUND

[0002] Lighting drivers typically use a switched mode power converter for delivering current to a lighting load. It can be desirable to be able to control the effective current level, for example, in order to implement dimming control.

[0003] For some applications, it can be desirable to have a large possible dimming range. For example, for light projector display applications, a wide dimming range enables enhanced display contrast, and enables improved colour fidelity.

[0004] Dimming can be achieved by adjusting the analog output current, in other words, adjusting the steady amplitude of the steady output current delivered to the LED load (analog dimming), or delivering a constant peak amplitude current but with a duty cycle based on pulse width modulation (PWM). This provides digital dimming. One implementation of PWM control involves shorting the load when the load is to be turned off, but continuing to output current by the driver for a future fast turn-on of the load when the load is no longer shorted.

[0005] It is also known to combine these two approaches in order to extend the dimming range. Thus, analog dimming can be used for a first range of dimming levels, and this range can be extended using PWM control without the need to increase the range of adjustment that needs to be enabled by analog dimming.

[0006] The present application is primarily concerned with PWM control of dimming. When the load is not shorted, the load as seen by the driver has a substantial value, but when the load is shorted, the load as seen by the driver is no longer at a substantial level, but is almost zero. The reason for using PWM control is that the load as seen by the power converter is different in the different PWM states. As a result, when switching between different PWM states, the control loop that adjusts the output current (for analog dimming control) will adjust the converter operating point. This results in the control loop building up a delay each time the PWM state changes, and this can result in inaccuracies in the brightness level. This problem is described in more detail below.

[0007] There is a need for an improved driver that can provide fast current adjustment when coping with changes in the load (for example, caused by PWM control).

[0008] US20090322234A1 discloses an LED driver having multiple feedback loops, in which the feedback control loop comprises an amplifier having a frequency compensation network formed by a capacitor and a resistor. SUMMARY

[0009] The invention is defined by the claims.

[0010] The inventors have found that when the converter is providing a desired output at a certain load condition, the control loop has at least one electrical value formed by the control loop, and this electrical value can be retrieved by the control loop to control the converter to maintain the output at a certain load condition. When the load condition changes, the control loop needs to change the electrical value, and this change creates the delay mentioned above. Therefore, the basic idea of the invention is to save the electrical value when the driver is loaded / not loaded short-circuited; change the load for the purpose of PWM control; and directly re-apply the saved electrical value to the control loop when the load is changed back. Thus, the control loop retrieves the previously saved electrical value, which is exactly suitable for the load condition to provide the desired output. The electrical value is thus retrieved at a faster speed (compared to the normal control loop regulation speed), so the control loop can control the driver to output the desired output faster. More specifically, the concept of the invention is to provide a driver with a current regulated converter. The current control loop has a controllable configuration, providing different current control options. The regulation circuit regulates the total load seen by the driver between a first level and a second level, and this is also used to achieve current regulation in addition to the current control loop. The configuration of the current control loop is saved before the regulation circuit changes the load seen by the driver from the first level to the second level. The saved configuration of the current control loop is re-applied when the regulation circuit changes the load seen by the driver from the second level back to the first level. In this way, the response speed of the current control loop is increased when switching between different load levels, e.g. created by PWM dimming control. More specifically, the electrical value is the voltage of a capacitor in a compensation circuit coupled to the output of an error amplifier in the control loop.

[0011] According to an example consistent with an aspect of the invention, there is provided a driver, comprising:

[0012] an input for receiving input power,

[0013] a converter for converting the input power to an output current;

[0014] a current control loop for controlling the converter and regulating the output current, the current control loop having an error amplifier and a first compensation circuit provided to the output of the error amplifier, said first compensation circuit comprising a first capacitor;

[0015] an output for outputting said output current to a load;

[0016] a regulation circuit for selectively delivering or not delivering the output current to the load and regulating the load seen by the driver between a first level and a second level, and

[0017] a controller for controlling the regulation circuit and the current control loop, wherein the controller is adapted to:

[0018] before the regulation circuit changes the load seen by the driver from the first level to the second level, by disconnecting the first capacitor from the output of the error amplifier

[0019] save the voltage of the first capacitor; and

[0020] when the regulation circuit changes the load seen by the driver from the second level back to the first level, reconnect the first capacitor with the saved voltage to the output of the error amplifier of the current control loop.

[0021] The driver has a current control loop for maintaining a stable amplitude of the output current given a variable load state. Furthermore, the load seen by the driver is regulated, e.g. the current control loop can be switched to short circuit to prevent current to reach the load. This enables PWM output current control, as the current to the load can be reduced to zero. A first level of the load corresponds e.g. to a load-in mode in PWM control, and the output current is delivered to the load; while a second level corresponds to a load-out mode in PWM control, and the output current, although generated by the driver, is not delivered to the load. The control loop output is different for different loads: if the LED is present, the control loop output needs to output a relatively high control signal to control the driver to work at e.g. 75% duty cycle in a switched mode power converter to provide a certain (desired) current; however, if the LED is shorted, the control loop output needs to output a relatively low control signal to control the driver to work at e.g. 10% duty cycle to provide the same current.

[0022] The internal state of the current control loop (e.g. voltage across opamp / error amplifier) depends on the load and influences the control loop output. If the load changes, conventionally the control loop can change slowly, e.g. by changing the internal state to reach the same output of the opamp again, so the response speed is slow. The invention proposes that the state (in form of an electrical value) is saved when the load returns, and directly reapplied, so that the control loop avoids adjusting its internal state by itself. Thus, the response time to return to normal state is faster.

[0023] The current control loop is e.g. for regulating a stable peak amplitude of the output current to a desired value, the error amplifier is adapted to compare the output current with a reference current, and the regulation circuit is for regulating the effective current amplitude to the load by applying a duty cycle in which the output current with the stable peak amplitude is delivered to the load.

[0024] Thus, the driver has a stable peak amplitude regulation (e.g. analog current level) and in addition an effective amplitude regulation by PWM duty cycle control. This can be used to extend the current regulation range, e.g. the dimming range in case of an LED driver. During PWM duty cycle control, different current control loop characteristics are required when the output of the driver is shorted compared to when the output of the driver is connected to a load.

[0025] The controller is for example adapted to apply a PWM control signal to the regulation circuit. This provides a digital PWM current control, such as PWM digital dimming of an LED load.

[0026] The regulation circuit can comprise a circuit for selectively:

[0027] shorting to conduct the output current and bypass the load, so as to set the load seen by the driver at a second level, e.g. a zero load level.

[0028] shorting to conduct the output current and bypass the load, so as to set the load seen by the driver at a second level, e.g. a zero load level.

[0029] By bypassing the load, the load is turned off, and this provides PWM duty cycle control of the current control.

[0030] The regulation circuit for example comprises a FET for connection in parallel with the load.

[0031] The control loop can be adapted to retrieve a voltage level or a charge level as an electrical value to control the converter regulating output current, the controller being adapted to:

[0032] configure the current control loop to include the first capacitor when the LED load is not shorted and the charge level or voltage level of the first capacitor is retrieved by the control loop to control the converter;

[0033] decouple the first capacitor from the current control loop before the load is shorted, thereby preserving a value of the charge level or voltage level of the first capacitor;

[0034] short the load; and

[0035] reconfigure the current control loop to couple the first capacitor to the current control loop and reconnect the load again substantially simultaneously, so that the current control loop is able to retrieve the preserved charge level or voltage level of the first capacitor immediately.

[0036] When the current control loop is in the load-in (not bypassed) mode, the first capacitor determines the behavior of the current control loop. By disconnecting the first capacitor from the circuit, there is no path for the components to change their charge / voltage, so the components can still hold their charge or voltage during the time the current control loop is configured in the load-out bypass mode. And, by reconnecting this first capacitor back to the control loop, the control loop can retrieve the same previous value of charge / voltage and can quickly control the driver output to output the same current as was output in the last load period.

[0037] The control loop is for example adapted to output a control output in dependence on the voltage of the first compensation circuit to regulate the converted power of the converter, wherein the control loop is adapted to:

[0038] when the load is at the first level, form and retrieve the voltage of the first capacitor as an electrical value in order to output a high output to control the converter to output a high power, and

[0039] when the load is at the second level, form and retrieve a changed electrical value in order to output a low output, lower than the high output, to control the converter to output a low power.

[0040] The controller can configure the current control loop to replace the first capacitor with a second capacitor when the load is shorted; and to decouple the second capacitor from the current control loop and replace the second capacitor with the first capacitor when the load is reconnected again.

[0041] When the current control loop is in the bypass mode, the second capacitor determines the behavior of the current control loop.

[0042] The current control loop can comprise a current sensing component adapted to sense the output current of the driver, the first compensation circuit being located between the output and the inverting input of the error amplifier.

[0043] wherein the second capacitor is in a second compensation circuit of the error amplifier, and wherein the controller is adapted to control a switch selecting one of the first compensation circuit and the second compensation circuit.

[0044] Thus, the current control loop sets the integration time and gain of the integrating amplifier in the current sensing circuit. In this topology, the output of the opamp depends on the voltage on the first or second capacitor of the feedback circuit. Thus, by saving and reapplying the voltage of the first capacitor, the control loop can quickly recover to its previous state, which was the state for the previous load, and thus the control loop can quickly control the driver to output the same output current in the same load state.

[0045] The first and second compensation circuits each further comprise a resistor for forming a series resistor-capacitor circuit, with the first and second feedback circuits being connected alternately. This enables PWM digital control of the output current.

[0046] The error amplifier can be adapted to receive the sensed output current at its inverting input and to receive a reference current at its non-inverting input, and to output a voltage substantially equal to the sum of the voltage at the inverting input and the voltage across the capacitor in the first or second compensation circuit.

[0047] Thus, by preserving the voltage of the first capacitor and re-applying this voltage to the control loop, the output of the error amplifier can quickly recover to its previous value for the previous load state, and thus the control loop can quickly control the driver to output the same output current for the same load state.

[0048] The driver can comprise a lighting driver, with the controller having a dimming signal input for setting a dimmed output current of the driver, wherein the configuration of the current control loop and the adjustment circuit are selected in dependence on the dimming signal input.

[0049] The present application also provides a lighting system comprising:

[0050] a driver as defined above; and

[0051] a lighting load driven by the driver.

[0052] The lighting load comprises, for example, an LED device. The present application also provides an LED projector comprising the above lighting system.

[0053] These and other aspects of the present application will become apparent from the embodiments described below. BRIEF DESCRIPTION OF DRAWINGS

[0054] For a better understanding of the present application and to show how it can be put into effect, there will now be described by way of example only, reference being made to the accompanying drawings in which

[0055] Figure 1 A known driver implementing analog current regulation as well as PWM dimming control is shown;

[0056] Figure 2 An example of a circuit configuration according to the present application is shown;

[0057] Figure 3 An example of a current control loop using an I-type opamp configuration is shown in a state connected to the first compensation circuit;

[0058] Figure 4 An example of a current control loop using an I-type opamp configuration is shown in a state connected to the second compensation circuit;Figure 3 the circuit in a state of connection of the second compensation circuit;

[0059] Figure 5 shows Figure 1 the dimming signal and output current for a single compensation circuit of Figure 2 the dimming signal and output current for two compensation circuits of

[0060] Figure 6 shows one example of a circuit configuration according to the present invention using a type II opamp configuration; and

[0061] Figure 7 shows one example of a circuit configuration according to the present invention using a type III opamp configuration. DETAILED DESCRIPTION

[0062] The present invention is described with reference to the accompanying drawings.

[0063] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of apparatuses, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the present invention. These and other features, aspects, and advantages of the apparatuses, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the drawings are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the drawings for like or similar items.

[0064] The present invention provides a driver having a current control loop for regulating an output current, the current control loop having a controllable configuration including electrical values that affect the switching function of the converter. A regulation circuit is provided to selectively deliver or not deliver the output current to a load and to efficiently regulate the load seen by the driver between a first level and a second level. In addition to the output current regulation achieved by the current control loop, such regulation also enables, for example, PWM control of the output current. Prior to the regulation circuit changing the load seen by the driver from the first level to the second level, the electrical values of the current control loop are saved; and when the regulation circuit changes the load seen by the driver from the second level back to the first level, the saved electrical values are reapplied to the current control loop.

[0065] Figure 1 A known driver 10 is shown that implements analog current regulation as well as PWM dimming control.

[0066] The driver 10 comprises an input "DC input" to receive input power. The converter 20 converts the input power to an output current. The converter is for example a switched mode power converter, such as a buck converter or a buck-boost converter. Any suitable converter topology can be used. The switched mode converter has a main power switch which controls whether input power is delivered to the load or whether energy cycled (typically from an inductor) is delivered to the load. The main switch of the power converter is thus controlled using a high frequency switching signal and the duty cycle of the switching determines the operating point of the converter 20, i.e. the output current.

[0067] The output current is delivered to a load 12 in the form of an LED arrangement. The output current is sensed by a current sensor, in particular a current sense resistor 21.

[0068] The sensed current is provided to an error amplifier 24, in which the sensed current is compared to a reference current. Within a control loop, the output of the error amplifier 24 is provided to a compensation circuit 26. The error amplifier and compensation circuit 26 combine to implement a current control loop for controlling the converter 20 and regulating the output current.

[0069] More specifically, the output of the current control loop is provided to a main control circuit 28 of the power converter 20. The converter 20 is controlled, for example by the main control circuit 28, in a known manner by controlling the switching timing of the main power switch of the switched mode power converter as mentioned above. More specifically, there is feedback control to regulate the duty cycle of the high frequency switching, thereby regulating the output current, in particular to regulate the steady peak amplitude of the output current at a desired reference value.

[0070] The feedback system described above provides regulation of the peak current, i.e. analogue current control. In addition, a regulation circuit 22 is provided for PWM control of the load, in other words for in- load and out-of-load control, to regulate the effective current to the load. By regulating the in- load configuration and the out-of-load configuration, the regulation circuit 22 efficiently regulates the load seen by the driver between a first level and a second level. In the example shown, the regulation circuit 22 is a shorting transistor (FET). When turned on, the output current to the load is bypassed and the load seen by the driver is close to zero; whereas when turned off, the output current flows through the load and the load seen by the driver is the load 12 of the LED arrangement. When turned on, the current to the LED load drops to zero to enable PWM duty cycle based dimming of the LED load.

[0071] The control circuit receives a dimming signal ("Dim signal") as input and it implements control of the regulation circuit 22. The dimming signal determines for example the dimming achieved by the PWM control.

[0072] The control regulation circuit 22 is implemented, for example, using a PWM dimming signal. When the PWM dimming signal controls the regulation circuit to produce a short circuit, this can be considered as the load external mode. When the PWM dimming signal controls the regulation circuit to produce an open circuit, this can be considered as the load internal mode. The alternation of the load external mode and the load internal mode determines the effective current of the load over time, thus implementing the desired dimming level.

[0073] When the load changes due to PWM dimming, the current control loop slowly changes its internal state to reach again the desired output of the error amplifier and the response speed is slow.

[0074] Because the same current control loop is used for the load internal mode and the load external mode, a problem arises with this circuit. When the circuit switches between these modes, especially when the circuit switches from the load external mode to the load internal mode, the current regulation will adapt the settings of the converter 20 because different converter settings are needed to maintain the same analog current due to the different load applied. This means that, when the load internal mode starts, the converter 20 cannot immediately output the same / desired output current, but it needs a certain time to output this current, this delay affects the response of the LED load and can be a problem in high frequency applications like projectors.

[0075] To describe this problem in more detail, from the voltage point of view, under lossless conditions, the duty cycle used to control the main switch of a switched mode power converter is proportional to the output voltage and the input voltage (for example, for a buck converter, the duty cycle D = Vo / Vin; for a boost converter D = (Vo-Vin) / Vo). Therefore, when the output voltage changes, the duty cycle changes accordingly. When the load is shorted, the output voltage becomes very small and the required duty cycle becomes very low.

[0076] A typical PWM generator is a comparator used to compare the output signal of the error amplifier with a sawtooth signal. A low feedback voltage will generate a low duty cycle PWM signal and vice versa. This technique of PWM generator is well known in the art and therefore this description will not give further details.

[0077] When in the load external mode, when the dimming FET 22 is on, the output acts as a short circuit. The voltage gain drops to a very low level and the main switch of the converter 20 needs a low duty cycle signal to maintain the output current at the same level. This requires the error amplifier 24 to step its output to a low value. An RC time delay arises in the compensation circuit 26, so the error amplifier 24 takes time to charge the capacitor of the compensation circuit before its output voltage drops accordingly.

[0078] When the dimming FET 22 is turned off, the converter operates at the load in output voltage, so the voltage gain recovers from a very low level. Therefore, the main switch of the switch mode power converter 20 requires a high duty cycle signal to match its operation and the output of the error amplifier 24 ramps to a high value. Likewise, the capacitor of the compensation circuit 26 needs to be discharged to make the error amplifier output voltage rise to a high value.

[0079] During PWM dimming, the charging and discharging times of the compensation circuit capacitor can cause unwanted settling time / delay time.

[0080] The present invention provides an adaptive compensation circuit for this type of PWM dimming application. As explained above, the circuit of the present invention provides appropriate compensation parameters for the load in and load out modes to provide an improved independent loop response.

[0081] Figure 2 An example of a circuit configuration according to the present invention is shown.

[0082] As in Figure 1 The same components are given the same reference numerals.

[0083] The compensation circuit 26 is replaced by a first compensation circuit 30 and a second compensation circuit 32 and a switch 25 for selecting one or the other of the compensation circuits.

[0084] The current control loop can thus comprise one or the other of the compensation circuits 30, 32. In this way, the current control loop has a controllable configuration comprising electrical values (e.g. voltages, charges or currents associated with components of the compensation circuits) that affect the conversion function of the converter 20.

[0085] The electrical value of the current control loop is saved before the regulation circuit 22 changes the load seen by the driver from a first level to a second level (i.e. when switching from open circuit load in mode to short circuit load out mode). Thus, the electrical set point (voltage or current or charge) of the first compensation circuit is saved. When the regulation circuit changes the load seen by the driver from the second level back to the first level, the saved electrical value is returned to the current control loop. Most importantly, the operation of saving and returning the electrical value means that the electrical value is maintained and directly re-applied to the current control loop, unlike the gradual change in charge stored on the control loop capacitor of the compensation circuit when switching between load in and load out modes, as in the prior art as described above.

[0086] Figure 3One example of a current control loop is shown. Error amplifier 24 comprises an opamp which receives a reference current Iref at the non-inverting input. Compensation circuits 30, 32 each comprise a negative feedback path of an opamp. In the example shown, each negative feedback path comprises a series RC circuit. The first compensation circuit comprises resistor Rl and capacitor Cl, and the second compensation circuit comprises resistor R2 and capacitor C2.

[0087] The opamp circuit receives the sensed current Isen at the inverting input of the opamp, and receives a reference current Iref at the non-inverting input of the opamp. The output of the opamp is a voltage substantially equal to the sum of the voltage at the inverting input and the voltage on capacitor Cl, C2 in the first or second feedback circuit (when stable, no current flows, so there is no voltage drop across resistors Rl, R2). Note that the polarity of the voltage on capacitor Cl / C2 can result in a negative value when calculating the sum.

[0088] Thus, the stored voltage across the capacitor in the feedback path influences and sets the control implemented by the main controller 28.

[0089] Switch 25 selects which of the two compensation circuits is connected in the feedback path. When one circuit is disconnected, it becomes an open circuit, the result of which is that the charge (and hence voltage) stored on the capacitor is fixed (i.e. saved or held). In this open circuit state, the electrical value of the current control loop (in this case, the charge, and hence the voltage stored on the capacitor) is therefore fixed when switch 25 instead connects the second compensation circuit 32 (and hence changes the load seen by the driver from the first level to the second level). Likewise, when switch 25 reconnects the first compensation circuit 30 (and hence changes the load seen by the driver from the second level back to the first level), the saved electrical value (i.e. the fixed charge and voltage on Cl) is returned to the current control loop.

[0090] Thus, the normal current control loop of the driver has a current control loop for analogue output current control. In addition, control of the short-circuit transistor 22 provides PWM output current control. Using two compensation circuits (and hence the entire current control loop with an adjustable configuration) means that the control loop adjusts its internal state on the fly, rather than having to rely on slow adjustment through the analogue current regulation loop. Thus, the response time to return to the normal state is faster.

[0091] Figure 3 Switch 25 is shown in a state in which the first compensation circuit is connected.

[0092] Figure 4The circuit is shown in which the switch 25 is in a state connecting the second compensation circuit.

[0093] The switching of the switch 25 is controlled by a "dim signal", which is the same signal as used for the PWM switching to short the transistor 22. This signal comes for example from the main board of an LED projector and is connected to the light source driver to control its dimming behavior.

[0094] When the dim signal is low, the shorting transistor 22 is off and the first compensation circuit 30 is selected, and the control loop outputs a high control signal to control the driver switch in an inner current regulation mode with high duty cycle for the load. When the dim signal is high, the shorting transistor 22 is on, and the output current is immediately bypassed by the shorting transistor, and the control loop outputs a low control signal to control the driver switch in an outer current regulation mode with low duty cycle for the load. The second compensation circuit 32 is selected to better match the dimming mode (i.e. shorting). Because the first compensation circuit remains open, the charge of the capacitor is maintained almost constant. When the dim signal is low again, the shorting transistor is off, the first compensation circuit 30 is selected (while the second compensation circuit 32 is disconnected), and the control loop jumps to output the previous high control signal, so it can control the driver to jump to switch in an inner current regulation mode with high duty cycle for the load fast, so a reduced settling time is achieved.

[0095] Figure 5 The dim signal is shown as top plot, and the output current Iout1 with a single compensation circuit as in Figure 1 the output current Iout2 with two compensation circuits as in Figure 2 It can be clearly seen that the build-up time of the output current to the desired current is shortened.

[0096] The example above is based on the voltage level stored on the capacitor of the negative feedback path of the error amplifier. However, the invention more generally requires storing any electrical value, and this electrical value is an electrical value that influences the function of the converter when regulating the output current. The example of a capacitor is only one possible example. For example, if it is a digitalized driver, the electrical value can be stored in a digital memory, and this electrical value is read out into the current control loop in order to make the current control loop jump to the previous state of controlling the converter to output the desired current in the condition of the load.

[0097] The driver is for example a lighting driver as explained above. The dim signal input to the controller is used to implement a PWM dimming mode.

[0098] The invention also provides a lighting system, such as an LED projector comprising a driver and a lighting load 12 driven by the driver. The LED projector has for example an LED projection light source and an LCD display module.

[0099] The above description describes embodiments of the invention using a type I topology of op-amps. It will be appreciated that the idea of the invention can also be applied to type II and III topologies of op-amps, which are illustrated in Figure 6 and Figure 7 respectively.

[0100] In the type II configuration, the negative feedback paths comprise a series RC circuit in parallel with the second capacitor. Thus, one feedback path is R1, C1 in parallel with C3, and the other feedback path is R2, C2 in parallel with C4.

[0101] In the type III configuration, there is also a reactive (RC) circuit present at the inverting input of the opamp.

[0102] The operation of embodiments of the invention in type II and III op-amps is similar to the operation in type I op-amps as described above. Therefore, the description does not give more details.

[0103] Variations to the disclosed embodiments can become apparent to those of ordinary skill in the art from the disclosure and the accompanying claims and fall within the scope of the invention. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

[0104] The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0105] If the term "adapted" is used in the claims or the description, it should be noted that the term "adapted" is intended to be equivalent to the term "configured to".

[0106] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A driver comprising: an input for receiving an input power, a converter (20) for converting the input power into an output current; a current control loop (24, 25, 30, 32) for controlling the converter and regulating the output current, the current control loop having an error amplifier (24) and a first compensation circuit (30) provided to an output of the error amplifier (24), the first compensation circuit (30) comprising a first capacitor (Cl); an output for outputting the output current to a load LED; a regulation circuit (22) for selectively delivering or not delivering the output current to the load and efficiently regulating the load seen by the driver between a first level and a second level, and a controller (28) for controlling the regulation circuit (22) and the current control loop (24, 25, 30, 32), wherein the controller is adapted to: save a voltage of the first capacitor (Cl) of the current control loop by disconnecting the first capacitor (Cl) from the output of the error amplifier (24) before the regulation circuit changes the load seen by the driver from the first level to the second level; and reconnect the first capacitor (Cl) with the saved voltage to the output of the error amplifier (24) of the current control loop when the regulation circuit changes the load seen by the driver from the second level back to the first level.

2. The driver according to claim 1, wherein the current control loop is for regulating a stable peak amplitude of the output current at a desired value, the error amplifier (24) is adapted to compare the output current with a reference current, and the regulation circuit (22) is for regulating an effective current amplitude of the load by applying a duty cycle in which the output current with a stable peak amplitude is delivered to the load.

3. The driver according to claim 2, wherein the controller (28) is adapted to apply a PWM control signal to the regulation circuit.

4. The driver according to any of claims 1 to 3, wherein the regulation circuit (22) comprises a circuit for selectively: breaking to allow the output current to pass through the load LED in order to set a load seen by the driver at the first level; and shorting to conduct the output current and bypass the load LED in order to set the load seen by the driver at the second level.

5. The driver according to claim 4, wherein the regulation circuit (22) comprises a FET for connecting in parallel with the load, the second level being a zero load level.

6. The driver according to any of claims 1 to 3, wherein the control loop is adapted to retrieve a voltage level or a charge level of the first capacitor to control the converter to regulate the output current, the controller is adapted to: ​ configuring the current control loop to include the first capacitor (C1) when the load LED is not shorted and the charge level or voltage level of the first capacitor (C1) is retrieved by the control loop to control the converter; decoupling the first capacitor (C1) from the current control loop before the load is shorted, thereby saving the value of the charge level or voltage level of the first capacitor; shorting the load; and reconfiguring the current control loop to couple the first capacitor (C1) to the current control loop and simultaneously reconnecting the load again, so that the current control loop can immediately retrieve the saved charge level or voltage level of the first capacitor (C1) when the load is reconnected again.

7. The driver of claim 6, wherein the control loop is adapted to control the output according to the voltage output of the first compensation circuit to regulate the converted power of the converter, wherein the control loop is adapted to: form and retrieve the voltage of the first capacitor (C1) when the load is the first level, so that a high output is outputted to control the converter to output a high power, and form and retrieve a changed voltage when the load is the second level, so that a low output is outputted to control the converter to output a low power.

8. The driver of claim 7, wherein the controller is adapted to configure the current control loop to replace the first capacitor (C1) with a second capacitor (C2) when the load is shorted; and decouple the second capacitor (C2) from the current control loop and replace the second capacitor (C2) with the first capacitor (C1) when the load is reconnected again.

9. The driver of claim 8, wherein the current control loop includes a current sense component (Rs) adapted to sense the output current of the driver, and the first compensation circuit is between the output and inverting input of the error amplifier, wherein the second capacitor (C2) is in a second compensation circuit of the error amplifier, and wherein the controller is adapted to control a switch that selects one of the first and second compensation circuits.

10. The driver of claim 9, wherein each of the first and second compensation circuits further includes a resistor for forming a series resistor-capacitor circuit (R1, C1; R2, C2), wherein the first and second compensation circuits are connected alternately, and wherein optionally, the error amplifier is any one of a type I topology, a type II topology, or a type III topology.

11. The driver of claim 9 or 10, wherein the error amplifier is adapted to receive the sensed output current at the inverting input of the error amplifier and a reference current at the non-inverting input of the error amplifier; and to output a voltage equal to the sum of the voltage at the inverting input and the voltage across the capacitor in the first or second compensation circuit.

12. The driver of any one of claims 1 to 3, comprising a lighting driver, wherein the controller has a dimming signal input for setting a dimmed output current of the driver, wherein the configuration of the regulation circuit and the current control loop is selected in dependence on the dimming signal input.

13. A lighting system comprising: a driver according to any one of claims 1 to 3; and a lighting load driven by the driver.

14. The lighting system of claim 13, wherein the lighting load comprises an LED arrangement.

15. An LED projector comprising the lighting system of claim 14. ​

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