Patent Grant
8569965
This application is based upon and claims the benefit of priority from Japan Patent Application No. 2010-275970, filed on Dec. 10, 2010, and Japan Patent Application No. 2010-274564, filed on Dec. 9, 2010, the entire contents of which are incorporated herein by reference.
The present disclosure relates to a technique of driving a light emitting element.
Recently, a light emitting device using a light emitting element including a light emitting diode (LED) has been used as a backlight of a liquid crystal panel or a lighting system.
Each of the LED strings 1006 includes a plurality of LEDs connected in series. The switching power source 1004 boosts an input voltage Vin and supplies a driving voltage Vout to one end portion of the LED strings 1006_1˜1006—n.
The current driving circuit 1008 includes current sources CS1˜CSn installed at the respective LED strings 1006_1˜1006—n. The respective current sources CS supply a driving current ILED, which is based on target luminance, to the corresponding LED strings 1006.
The switching power source 1004 includes an output circuit 1102 and a control IC 1100. The output circuit 1102 includes an inductor L1, a switching transistor M1, a rectifying diode D1, and an output capacitor C1. The control IC 1100 feedback-controls a duty ratio of ON/OFF operations of the switching transistor M1 such that the lowest one among voltages VLED1˜VLEDn (also called detection voltages) generated from each of cathode terminals of the LED strings 1006_1˜1006—n is close to a target voltage Vref. As a result, an output voltage Vout from the switching power source 1004 is stabilized to (Vref+Vf). In this configuration, Vf indicates a forward voltage (voltage drop) of the LED strings 1006.
In such a light emitting device 1003, to adjust the luminance of the LED strings 1006, the driving current ILED is often pulse width modulation (PWM)-controlled. More specifically, a PWM controller 1009 of the current driving circuit 1008 generates burst dimming pulses PWM1˜PWMn, each having a duty ratio based on luminance, and controls switching of the current sources CS1˜CSn that correspond to the burst dimming pulses PWM1˜PWMn, respectively. Such controlling is also referred to as burst dimming or burst controlling.
Such a light emitting device is generally known to have the following problems.
During a period in which the current source CS is in an OFF state, namely, during a turn-off period of the LED strings 1006, the detection voltage VLED is negated, so it is difficult to perform feedback controlling based on the detection voltage VLED. Thus, the control IC 1100 adjusts the duty ratio of ON/OFF operations of the switching transistor M1 based on the detection voltage VLED during a period in which the current source CS is in an ON state, namely, during a turn-on period of the LED strings 1006.
Further, when the turn-on period of the LED strings 1006 is shortened, the period during which feedback controlling is valid is shortened. When the turn-on period becomes as short as a switching pulse of the switching transistor M1 of the switching power source, feedback by an error amplifier cannot be followed, degrading the driving voltage Vout. Therefore, during the turn-on period, the luminance of the LED strings 1006 is degraded or the LED strings 1006 may not emit light.
The applicant of the present disclosure notes that the above problems are not considered common general knowledge in the field of the present disclosure. In other words, the foregoing discussion was first made by the applicant of the present disclosure.
The present disclosure provides some embodiments of a control circuit capable of restraining a switch in an output voltage when the turn-on time of burst dimming becomes as short as a switching pulse.
According to one embodiment of the present disclosure, there is provided a driving circuit of a light emitting element including a switching power source for supplying a driving voltage to a first terminal of the light emitting element to be driven and a current driver connected to a second terminal of the light emitting element for supplying a driving current to the light emitting element while a burst dimming pulse is being asserted.
The switching power source includes a capacitor in which a potential of one end is fixed and an error amplifier configured to supply a current depending on a difference between a detection voltage generated from the second terminal of the light emitting element and a reference voltage to the capacitor. The switching power source also includes a switch installed between an output terminal of the error amplifier and the capacitor and maintained in an ON state while the burst dimming pulse is being asserted, and a pulse generation unit configured to receive a feedback voltage generated in the capacitor and generate a switching pulse signal having a corresponding duty ratio. A driver of the switching power source is configured to drive a switching element of the switching power source based on the switching pulse signal. And a feedback voltage regulator circuit of the switching power source is configured to be switched between ON and OFF states based on a pulse width of the burst dimming pulse and supply a current to the capacitor when in an ON state.
In one embodiment, the feedback voltage regulator circuit is turned on when the pulse width of the burst dimming pulse is longer than a predetermined threshold value, turned on while the burst dimming pulse is being asserted when the pulse width of the burst dimming pulse is shorter than the threshold value, and then turned off.
In one embodiment, the driving circuit of the light emitting element further includes a short detection comparator configured to generate a short detection signal asserted when the detection voltage is higher than a predetermined threshold voltage. The feedback voltage regulator circuit is turned off when the short detection signal is being asserted at a timing when the burst dimming pulse is negated.
In one embodiment, the feedback voltage regulator circuit includes a flipflop having an input terminal to which the short detection signal is input and a clock terminal to which an inverted signal of the burst dimming pulse is input, and wherein an ON/OFF state of the feedback voltage regulator circuit is switchable depending on an output signal from the corresponding flip-flop.
In one embodiment, the feedback voltage regulator circuit is turned on when the short detection signal is asserted while the burst dimming pulse is being negated.
In one embodiment, the feedback voltage regulator circuit includes an NAND gate configured to receive the burst dimming pulse and an inverted signal of the short detection signal and a flipflop having an input terminal to which the short detection signal is input, a clock terminal to which an inverted signal of the burst dimming pulse is input, and a reset terminal to which an output signal from the NAND gate is input. An ON/OFF state of the feedback voltage regulator circuit is switchable depending on an output signal from the corresponding flip-flop.
In one embodiment, the feedback voltage regulator circuit includes a current source configured to supply a current to the capacitor when in an ON state.
According to another embodiment of the present disclosure, there is provided a light emitting device including a light emitting element and a driving circuit as described above for driving the light emitting element.
According to another embodiment of the present disclosure, there is provided an electronic device including a liquid crystal panel and a light emitting device as described in above as a backlight of the liquid crystal panel.
An embodiment of the present disclosure will now be described in detail based on appropriate embodiments with reference to the drawings. The same reference numerals are used for the same or equivalent components, members, and processing illustrated in respective drawings, and repeated descriptions are aptly omitted. Also, an embodiment of the present disclosure is merely illustrative, rather than limiting the present disclosure, and any features or combination thereof described in the embodiment are not necessarily considered to be essential.
In the present disclosure, a “state in which member A is connected with member B” also includes a case in which member A and member B are indirectly connected through a different member that does not affect an electrical connection state, besides a case in which member A and member B are physically directly connected. Similarly, a “state in which member C is installed between member A and member B” also includes a case in which member C is indirectly connected to member A and member B through a different member that does not affect an electrical connection state, besides a case in which member A and member C or member B and member C are directly connected.
An electronic device 2 is a battery-driven device such as a notebook PC, a digital camera, a digital video camera, a mobile phone terminal, a personal digital assistant (PDA), or the like, and includes a light emitting device 3 and a liquid crystal display (LCD) panel 5. The light emitting device 3 is installed as a backlight of the LCD panel 5.
The light emitting device 3 includes LED strings 6_1˜6—n as light emitting elements, a current driving circuit 8, and a switching power source 4. The current driving circuit 8 and the switching power source 4 constitute a driving circuit of the light emitting strings.
The respective LED strings 6 include a plurality of LEDs connected in series. The switching power source 4, which is a boost type DC/DC converter, boosts an input voltage (e.g., a battery voltage) Vin which is input to an input terminal P1 and outputs an output voltage (driving voltage) Vout from an output terminal P2. One end (anode) of each of the plurality of LED strings 6_1˜6—n is commonly connected to the output terminal P2.
The switching power source 4 includes a control IC 100 and an output circuit 102. The output circuit 102 includes an inductor L1, a rectifying diode D1, a switching transistor M1, and an output capacitor C1. The topology of the output circuit 102 is general, so a description thereof will be omitted. Also, a person skilled in the art will understand that the topology may be variably modified and thus the present disclosure is not limited thereto.
A switching terminal P4 of the control IC 100 is connected to a gate of the switching transistor M1. The control IC 100 adjusts the duty ratio of ON/OFF operations of the switching transistor M1 through feedback such that an output voltage Vout required for turning on the LED strings 6 can be obtained. Also, the switching transistor M1 may be installed in the control IC 100.
The current driving circuit 8 is connected to the other ends (cathodes) of the plurality of LED strings 6_1˜6—n. The current driving circuit 8 supplies an intermittent driving current ILED1˜ILEDn based on target luminance to each of the LED strings 6_1˜6—n, respectively. More specifically, the current driving circuit 8 includes a plurality of current sources CS1˜CSn, installed for each of the LED strings 6_1˜6—n, respectively, and a PWM controller 9. An ith current source CSi is connected to a cathode of a corresponding ith LED string 6—i. The current source CSi is configured to be switched over between an operation (active) state φON in which a driving current ILEDi is output and an off state φOFF in which the driving current ILEDi is stopped, depending on a burst dimming pulse PWMi output from the PWM controller 9. The PWM controller 9 generates burst dimming pulses PWM1˜PWMn, each having a duty ratio based on target luminance, and outputs the generated burst dimming pulses PWM1˜PWMn to the current sources CS1˜CSn, respectively. While the burst dimming pulse PWMi is being asserted (e.g., high level), that is, turn-on period TON, the corresponding current source CSi is in an operational state φON and the LED string 6—i is turned on. While the burst dimming pulse PWMi is being negated (e.g., low level), that is, turn-off period TOFF, the corresponding current source CSi is in an off state φOFF and the LED string 6—i is turned off. By controlling a time ratio between the turn-on period TON and the turn-off period TOFF, an effective value (average value in time base) of the driving current IILEDi flowing across the LED string 6—i is controlled, thus adjusting luminance. The frequency of the PWM driven by the current driving circuit 8 ranges from tens to hundreds Hz. Hereinafter, the burst dimming pulses PWM1˜PWMn are assumed to transition at the same timing and those pulses are generally called burst dimming pulses PWM.
The control IC 100 and the current driving circuit 8 may be integrated in a single semiconductor chip or integrated in separate chips. They may configure a single package (module) or may configure separate packages.
An overall configuration of the light emitting device 3 has been described. A configuration of the control IC 100 will now be described. The control IC 100 includes LED terminals LED1˜LEDn installed at the respective LED strings 6_1˜6—n. Each LED terminal LEDi is connected to a cathode terminal of a corresponding LED string 6—i. Also, a plurality of LED strings may not be provided and instead only one LED string may be provided.
The control IC 100 largely includes an error amplifier 22, a first switch SW10a, a pulse generation unit 20, a driver 28, short detection circuits 601˜60n, and feedback circuits 701˜70n.
A phase compensation resistor R7 and a phase compensation capacitor C3 are installed between an FB terminal and an external fixed voltage terminal (earth terminal).
The feedback circuits 701˜70n are installed at LED terminals (channels) LED1˜LEDn, respectively. An ith feedback circuit 70i outputs a voltage VLED1′ depending on a detection voltage VLEDi from a corresponding LED terminal LEDi to the error amplifier 22. More specifically, the feedback circuit 70i, which is a voltage divider including resistors R11 and R12, divides the detection voltage VLEDi by a division ratio K1. A first switch SW11 is turned on while a burst dimming pulse PWMi of a corresponding channel is being asserted (turn-on period) and turned off while the burst dimming pulse PWMi is being negated (turn-off period). Also, the first switch SW11 of an ith channel is turned off when the channel is excluded from a feedback target. For example, the first switch SW11 is an N channel MOSFET controlled based on the burst dimming pulse PWMi. A second switch SW12 is turned on when the channel should be excluded from the feedback target and pulls up a detection voltage VLEDi′, for example, to a power source voltage VDD. Accordingly, the detection voltage VLEDi′ of the channel can become higher than a detection voltage VLEDj′ (where j≠i) of a different channel, thus being excluded from feedback. Also, dividing of the detection voltage is not a fundamental processing, so in the following description, VLED′ and VLED will not be distinguished if not particularly necessary. For example, the second switch SW12 is a P channel MOSFET controlled based on the burst diming signal PWMi.
The error amplifier 22, which is a so-called gm (transconductance) amplifier, generates a current depending on a difference between the detection voltage VLED and a reference voltage Vref during the turn-on period of the LED string 6 and supplies the generated current to the FB terminal. A feedback voltage VFB is generated based on the difference between the detection voltage VLED and a reference voltage Vref at the FB terminal.
More specifically, the error amplifier 22 includes a plurality of inverting input terminals (−) and one non-inverting input terminal (+). Detection voltages VLED1˜VLEDn are input to the plurality of inverting input terminals, respectively, and the reference voltage is input to the non-inverting input terminal. The error amplifier 22 outputs a current depending on the difference between the lowest detection voltage VLED and the reference voltage Vref.
The first switch SW10a is installed between an output terminal of the error amplifier 22 and the FB terminal. The first switch SW10a is turned on while the burst dimming pulse PWM is being asserted, namely, during a turn-on period TON, and turned off while the burst dimming pulse PWM is being negated, namely, during a turn-off period TOFF. In the case where the phases of the burst diming pulses PWM1-PWMn with respect to the plurality of current sources CS1˜CSn are shifted, the first switch SW10a is turned on while at least one burst dimming pulse PWM is being asserted.
The pulse generation unit 20, which is, for example, a pulse width modulator, receives the voltage VFB generated from the FB terminal and generates a switching pulse signal Spwm having a corresponding duty ratio. More specifically, as the feedback voltage VFB has a higher level, the duty ratio of the switching pulse signal Spwm is increased. The pulse generation unit 20 includes an oscillator 24 and a PWM comparator 26. The oscillator 24 generates a periodic voltage Vosc having a triangular wave or a sawtooth wave.
The PWM comparator 26 compares the feedback voltage with the periodic voltage Vosc and generates a PWM signal Spwm having a level based on the comparison result. Also, a pulse frequency modulator or the like may be used as the pulse generation unit 20. The frequency of the PWM signal Spwm is hundreds of kHz (e.g., 600 kHz), which is sufficiently high in comparison to the frequency of the PWM driven by the current driving circuit 8.
The driver 28 drives the switching transistor M1 of the switching power source 4 based on the switching pulse signal Spwm.
The short detection circuits 601˜60n are installed at every channel of the LED strings 6_1˜6—n, and configured in the same manner. A short detection circuit 60i generates a short detection signal LSPiCH asserted when the detection voltage VLEDi of the LED terminal is higher than a certain threshold value voltage VTH during the turn-on period TON. During the turn-off period TOFF, a short detection is invalidated.
The short detection circuit 60i includes a short detection comparator 62, resistors R1 and R2, and a transistor 63.
The detection voltage VLEDi of the LED terminal is divided by the resistors R1 and R2. When R1=2.4 MΩ, and R2=0.6 MΩ, the division ratio is β=1/5. The transistor 63, which is controlled in synchronization with the burst dimming pulse PWMi, is turned on during the turn-on period TON and turned off during the turn-off period TOFF. The short detection comparator 62 compares the detection voltage VLEDi′ divided by the resistors R1 and R2 with a threshold voltage VTH′ during the turn-on period TON, and outputs a short detection signal LSPiCH having a high level (asserted) when VLEDi′>VTH′. Here, the following equation is established:
VTH′=VTH×β
A feedback voltage regulator circuit 50 is configured to be switched between ON and OFF states depending on a pulse width of the burst dimming pulse PWM, and when the feedback voltage regulator circuit 50 is turned on, it supplies a current IC to the phase compensation capacitor C3, and when the feedback voltage regulator circuit 50 is turned off, it stops current supply to the phase compensation capacitor C3.
When the pulse width of the burst dimming pulse PWM is longer than a certain threshold value, the feedback voltage regulator circuit 50 is turned on during both the turn-on period and turn-off period. Also, when the pulse width of the burst dimming pulse PWM is shorter than the threshold value, the feedback voltage regulator circuit 50 is turned off when the turn-on period is terminated.
The current IC is injected when the feedback voltage regulator circuit 50 is in an ON state, thereby changing the feedback voltage VFB such that the turn-on period of the switching transistor M1 is lengthened. To be more specific, the feedback voltage regulator circuit 50 increases the feedback voltage VFB in an ON state to thus lengthen the turn-on time of the switching transistor M1.
It is desirable that the injection current IC is smaller than a source current or sync current of the error amplifier 22. For example, when the source current or sync current is a maximum 100 μA, the injection current IC of the feedback voltage regulator circuit 50 is preferably about 1 μA.
More specifically, the feedback voltage regulator circuit 50 transitions from an ON state to an OFF state when the following conditions are met. It is assumed that the detection voltage VLEDi of the ith channel is fed back. Here, the feedback voltage regulator circuit 50 is turned off when the short detection signal LSPiCH is asserted at a timing at which the burst dimming pulse PWMi transitions from assertion to negation.
Thereafter, when the short detection signal LSPiCH is asserted while the burst dimming pulse PWMi is being negated, the feedback voltage regulator circuit 50 is turned on.
The current source 56 generates a current IC to be supplied to the phase compensation capacitor C3. The current IC is, for example, about 1 μA. The switch 58 is installed in the path of the current IC, and an ON/OFF operation of the switch 58 corresponds to an ON/OFF operation of the feedback voltage regulator circuit 50. As the current IC is introduced into the phase compensation capacitor C3, the feedback voltage VFB is increased.
The flipflop 52 and the NAND gate 54 are installed at every channel of the LED strings (6). The short detection signal LSPiCH is input to an input terminal D of an ith flipflop 52, and an inverted signal
The NAND gate 54 performs an NAND operation of the burst dimming pulse PWM and the inverted signal of the short detection signal LSPiCH. An output signal from the NAND gate 54 is input to a reset terminal of the flipflop 52.
The OR gate 59 performs an OR operation of output signals Q1˜Qn from the flipflop 52 of the respective channels, and supplies the result obtained through the OR operation. The switch 58 is turned on when an output signal from the OR gate has a low level and turned off when the output signal from the OR gate 59 has a high level.
The configuration of the control IC 100 has been described. An operation of the control IC 100 will now be described.
First, with reference to
Before a time t0, the burst dimming pulse PWM1 has a low level, so the current source CS1 is in an OFF state and the LED string 6_1 is turned off. At this time, since the transistor 63 is turned off, a short detection is invalidated, and since the detection voltage VLED′ has been pulled down to have a low level (ground voltage), LSP1CH has a low level.
When the burst dimming pulse PWM1 transitions to have a high level at the time t0, the current source CS1 is turned on and a driving current starts to flow to the LED string 6_1, and a voltage drop Vf of the LED string 6_1 is gradually increased from zero. The detection voltage VLED1 is supplied as VLED1=Vout−Vf, so it is gradually lowered over time. Immediately after the burst dimming pulse PWM1 transitions to have a high level, the short detection signal LSP1CH has a high level in order to establish VLED1′>VTH′. At a time t1, when the detection voltage VLED1′ is lower than a threshold voltage VTH′, the short detection signal LSP1CH transitions to have a low level, and thereafter, is maintained at the low level.
At a timing when the burst dimming pulse PWM1 transitions to have a low level at a time t2, the inverted short detection signal LSP1CH has a low level, so an output signal Q1 from the flipflop 52 has a low level, and then the output signal Q1 continues to have the low level during a turn-off period TOFF until such time as the burst dimming pulse PWM1 transitions to have a high level at a time t3 (not shown).
The operations of the time t0 to t3 are repeated, and in order to maintain a control signal of the switch 58 at a low level, the switch 58, i.e., the feedback voltage regulator circuit 50, is kept in an ON state, so the injection current IC is continuously supplied to the phase compensation capacitor C3. In this manner, when the pulse width of the burst dimming pulse PWM is relatively long, the feedback voltage regulator circuit 50 is turned on. Since the current capability of the error amplifier 22 is sufficiently greater than the injection current Ic of the feedback voltage regulator circuit 50, it is barely affected by the injection current IC.
With continuing reference to
Here, in order to clarify the effect of the control IC 100 in
When the pulse width of the burst dimming pulse PWM1 is short, a response of the error amplifier 22 is delayed, insufficiently supplying a current to the phase compensation capacitor C3 from the error amplifier 22 to lower the feedback voltage VFB. As a result, the ON time duration of the switching pulse signal Spwm is shortened to lower the driving voltage Vout. When the driving voltage Vout is lowered, the LED string 6 does not emit light.
An operation with the feedback voltage regulator circuit 50 will now be described. Although the response of the error amplifier 22 is delayed and a current supply to the phase compensation capacitor C3 from the error amplifier 22 is insufficient, since the injection current IC is supplied to the phase compensation capacitor C3 from the feedback voltage regulator circuit 50, restraining the feedback voltage VFB from being lowered or increasing the feedback voltage VFB, the ON time duration of the switching pulse signal Spwm is lengthened. As a result, lowering of the driving voltage Vout can be restrained, so the LED string 6 can emit light.
In this respect, however, during the turn-off period TOFF thereafter, when the current IC is continuously supplied to the phase compensation capacitor C3, the feedback voltage VFB is continuously increased resulting in an excessively high output voltage Vout. Thus, when the pulse width of the burst dimming pulse PWM is short, the current Ic is interrupted upon transitioning to the turn-off period TOFF, thereby restraining the output voltage Vout from being increased.
In this manner, in the control IC 100 according to this embodiment, lowering of the output voltage due to a delay in the response speed of the error amplifier 22 can be restrained, and thus, the LED string 6 can emit light.
So far, the present disclosure has been described based on the embodiment. The embodiment is merely illustrative and there may be various modifications in the respective components, respective processes, and combinations thereof. Hereinafter, such modifications will be described.
In the embodiment, the non-insulating type switching power source using an inductor has been described, but the present disclosure can also be applicable to an insulating type switching power source using a transformer.
In the embodiment, the electronic device has been described as an application of the light emitting device 3, but the purpose thereof is not particularly limited but may be applicable for lighting purposes or the like.
Also, in the present embodiment, the setting of the high level, low level, assert, and negate logical signals are taken as an example, and those may be appropriately inverted by an inverter or the like, so as to be freely switched.
According to the present disclosure in some embodiments, it is possible to stabilize an output voltage when a turn-on time of burst dimming is short.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosures. Indeed, the novel methods and apparatuses described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and switches in the form of the embodiments described herein may be made without departing from the spirit of the disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosures.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-274564 | Dec 2010 | JP | national |
| 2010-275970 | Dec 2010 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 7723922 | Lee et al. | May 2010 | B2 |
| 8058810 | Chen et al. | Nov 2011 | B2 |
| 20100156319 | Melanson | Jun 2010 | A1 |
| Number | Date | Country |
|---|---|---|
| 2006-114324 | Apr 2006 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20120146531 A1 | Jun 2012 | US |
