US008319444B2 (12) United States Patent (10) Patent N0.: US 8,319,444 B2 Liao et a]. (45) Date of Patent: Nov. 27, 2012 (54) TAIL-LESS LED CONTROL CIRCUIT (56) References Cited (75) Inventors: ChiaWei Liao, San Jose, CA (US); U.S. PATENT DOCUMENTS Jing-Meng Liu, Zhubei (TW); 7,898,187 B1* 3/2011 Mei et a1. .................... .. 315/247 Leng-Nien Hsiu, Zhubei (TW) 2009/0179575 A1* 7/2009 Mednik et a1. 315/193 2010/0289424 A1* 11/2010 Chang et a1. 315/250 (73) Assignee: Richtek Technology Corporation, 2011/0248640 A1* 10/2011 Welten ........................ .. 315/210 R.O.C., Hsinchu (TW) * cited by examiner * Notice: Sub'ectJ to any disclaimer, the term of this patent is extended or adjusted under 35 Primary Examiner * Tung X Le U.S.C. 154(b) by 287 days. (74) Attorney, Agent, or Firm * Tung & Associates (21) Appl. N0.: 12/787,599 (57) ABSTRACT (22) Filed: May 26, 2010 The present invention discloses a tail-less LED control cir cuit, Which includes: a poWer supply stage having an output (65) Prior Publication Data terminal Which provides electrical poWer to an LED circuit; US 2010/0301761 A1 Dec. 2,2010 an output capacitor coupled to the output terminal; an LED driver circuit coupled to the poWer supply stage for control Related US. Application Data ling the poWer supply stage to provide the electrical poWer to (60) Provisional application No. 61/183,380, ?led on Jun. the LED circuit, the LED driver circuit receiving a PWM 2, 2009. dimming signal for adjusting brightness of the LED circuit; and a MOSFET sWitch coupled to the output capacitor in (51) Int. Cl. series, the MOSFET sWitch switching synchronously With H05B 37/00 (2006.01) the PWM dimming signal to alleviate LED afterglow, Wherein the MOSFET sWitch includes a body diode having an US. Cl. ....................... .. 315/186; 315/193; 315/224 (52) anode-cathode direction against the discharge direction of the (58) Field of Classi?cation Search ............ .. 315/185 R, output capacitor. 315/186, 193, 224*226, 291, 294, 297, 307, 315/312 See application ?le for complete search history. 6 Claims, 4 Drawing Sheets _ Power Supply v'n Stage Q l | 20 VI SW 8 ISP M dimming [SN control PWM (EN or DCTL ~ “A10 dimming signal 0" ACTL) US. Patent Nov. 27, 2012 Sheet 1 M4 US 8,319,444 B2 PWIVI ACTL ‘\/\ 10 dimming signal Fig. 1 (Prior Art) _ Power Supply Vout V'n _ Stage@ 1 | 20 \/| SW Cout g ISP : : ‘ M dimming [SN : : control Q1 dimming signal or ACTL) US. Patent Nov. 27, 2012 Sheet 2 M4 US 8,319,444 B2 Vin Vin Vin Vout Vout Vout 77/7/ Fig. 3A Fig. 38 Fig. 3C Vin Vin J_ Vout Vout 77/77 Fig. 3D Fig. 3E Vin Vin Vout i _l_ Vout (-) Mg; Fig. 3F Fig. 3G US. Patent Nov. 27, 2012 Sheet 3 of4 US 8,319,444 B2 M dimming control PWM (ENA‘gTEiCTL W10 dimming signal or ) .- Vout m dimming control PWM (EN or DCTL 45m dimming signal WAC-FL) 7777 US. Patent Nov. 27, 2012 Sheet 4 M4 US 8,319,444 B2 Vin dimming control (EN or DCTL or ACTL) 10 Fig. 6 US 8,319,444 B2 1 2 TAIL-LESS LED CONTROL CIRCUIT When the PWM dimming signal turns OFF the LED circuit during its loW state period (referred to as “PWM loW period”), CROSS REFERENCE the present invention also turns OFF the MOSFET sWitch to isolate the output capacitor, such that the output capacitor The present invention claims priority to US. provisional Cout is not discharged via the LED circuit; thus, the LEDs are application No. 61/183,380, ?led on Jun. 2, 2009. turned OFF sharply (tail-less). In this PWM loW period, only the magnetiZing current of the inductor Will light up the LED BACKGROUND OF THE INVENTION for a very short While. As the inductor magnetiZing current reaches Zero, the LED stays off for the rest of the PWM loW 1. Field of Invention period. Therefore, a high contrast PWM dimming can be The present invention relates to a tail-less light emitting achieved by the added MOSFET sWitch. And since the output diode (LED) control circuit; particularly, it relates to a tail capacitor Cout is not discharged during the OFF period, the less LED control circuit Which connects and disconnects an output capacitor in synchronization With a pulse Width modu appropriate voltage level is Well kept across the output capaci lation (PWM) dimming signal, to alleviate LED aftergloW tor Cout. Thus, no re-charging process is required and there is (tail). no settling time issue in the next cycle. 2. Description of Related Art The aforementioned MOSFET sWitch can be provided FIG. 1 shoWs a schematic circuit diagram of a conventional external to or integrated in the LED driver circuit. LED control circuit. As shoWn in FIG. 1, an LED driver The MOSFET sWitch may be a P-type metal oxide semi circuit 10 controls the inductor current in a poWer supply 20 conductor ?eld effect transistor (PMOSFET) or an N-type stage 60 via a sWitching signal pin SW, such that the poWer metal oxide semiconductor ?eld effect transistor (NMOS supply stage 60 provides electrical poWer to an LED circuit FET). An MOSFET inherently includes a body diode (para 30. The LED driver circuit 10 receives a PWM dimming sitic diode). Preferably, the body diode of the MOSFET has signal 20; depending on different designs, such PWM dim an anode-cathode direction against the discharge direction of ming signal 20 may be received via an enable pin EN, a digital 25 the output capacitor Cout, such that When the MOSFET is control pin DCTL, or an analog control pin ACTL. The LED turned OFF, the output capacitor Cout is not discharged driver circuit 10 turns ON/ OFF the LED circuit 30 according through the parasitic diode of the MOSFET. to the PWM dimming signal 20, Whereby the brightness of the The aforementioned LED circuit may include one or mul LED circuit 30 is adjusted according to the duty ratio of the tiple LED strings, and each LED string may include one or PWM dimming signal 20. 30 more LEDs. The aforementioned prior art has the folloWing drawbacks. By the present invention, higher dimming contrast, or tail When the LED circuit 30 is turned OFF, due to the residual less ON/OFF, can be achieved in an existing loW dimming charges in the output capacitor Cout, the LEDs are still poW contrast circuit With the use of an MOSFET of the present ered for a short While; that is, the LEDs Will still shine for a invention. short While after they have been turned OFF, Which is an 35 The present invention is applicable to buck, boost, buck undesired “aftergloW” or “tail”. And since the output capaci boost, inverting, and ?yback poWer converters. tor Cout is discharged during the OFF period, it Will need to The objectives, technical details, features, and effects of be re-charged to the appropriate voltage level in the next the present invention Will be better understood With regard to cycle, requiring more settling time. Thus, the actual bright the detailed description of the embodiments beloW. ness of the LED circuit 30 does not exactly folloW the duty 40 ratio of the PWM dimming signal 20, resulting in loW dim BRIEF DESCRIPTION OF THE DRAWINGS ming contrast and reducing the effectiveness of the PWM dimming signal 20. FIG. 1 illustrates a schematic circuit diagram shoWing an In vieW of the foregoing, the present invention provides a LED driver circuit, an LED circuit, and a poWer supply stage tail-less LED control circuit Which connects and disconnects 45 in a prior art LED control circuit. the output capacitor in synchronization With a pulse Width FIG. 2 illustrates an embodiment of the present invention. modulation (PWM) dimming signal, to alleviating the fore FIG. 3A-3G illustrates several examples of the poWer sup going problems. The present invention does not need a com ply stage. plicated circuit, so there is no area penalty. FIG. 4-6 shoWs three other embodiments of the present 50 invention. SUMMARY OF THE INVENTION DESCRIPTION OF THE PREFERRED The objective of the present invention is to provide a tail EMBODIMENTS less LED control circuit. To achieve the objective mentioned above, the present 55 FIG. 2 shoWs a ?rst embodiment of the present invention. invention provides a tail-less LED control circuit. The tail An LED driver circuit 10 controls a poWer supply stage circuit less LED control circuit comprises: a poWer supply stage 60 to convert an input voltage Vin to an output voltage Vout, having an output terminal Which provides electrical poWer to Which is supplied to an LED circuit 30. The poWer stage an LED circuit; an output capacitor coupled to the output circuit 60 can be (but not limited to) anyone of buck, boost, terminal; an LED driver circuit coupled to the poWer supply 60 buck-boost, inverter, and ?yback poWer converters shoWn in stage for controlling the poWer supply stage to provide the FIGS. 3A-3G. In some cases, the poWer transistors in FIGS. electrical poWer to the LED circuit, the LED driver circuit 3A-3G are integrated into the circuit 10; in other cases, the receiving a PWM dimming signal for adjusting brightness of poWer transistors are outside of the circuit 10. Referring to the LED circuit; and a MOSFET sWitch coupled to the output FIG. 2, the LED driver circuit 10 receives a PWM dimming capacitor in series, the MOSFET sWitch sWitching synchro 65 signal 20 and turns ON/OFF the LED circuit 30 accordingly nously With the PWM dimming signal to alleviate LED after to adjust the brightness of the LED circuit 30.