A novel DC and PWM dual-mode dimming circuit for the WLED driver

LETTER IEICE Electronics Express, Vol.10, No.19, 1 6 A novel DC and PWM dual-mode dimming circuit for the WLED driver Lianxi Liu 1, 2a), Yue Niu 1, Jiao Zou 1, and Zhangming Zhu 1, 2 1 School of Microelectronics Xidian University Xi an 710071 P R China 2 Key Laboratory of Wide Band Gap Semiconductor Materials and Devices Xi an 710071 P R China a) lxliu mail xidian edu cn or adam79416 126 com Abstract: It is very important for a light source that the brightness could be adjusted conveniently. A novel DC and PWM dual-mode dimming circuit for a monolithic WLED driver is proposed for the first time in this paper. It can alter flexibly the control mode between the analog (DC) and digital (PWM) dimming according to the input control signal. In the DC dimming mode, when the input DC control voltage is adjusted from 0.5 V to 2.5 V, the average output current can be changed from 20% to 100% of the nominal current value. While in the PWM dimming mode, the output current is proportional to the duty cycle of the input PWM signal and changed from about 0% to 100% of the nominal current value. The WLED driver with the proposed dual-mode dimming circuit has been verified in a 0.5 μm HVCMOS process, and the nominal value could be set more than 1 A by the external resistors. Keywords: WLED driver, dual-mode, DC dimming mode, PWM dimming mode Classification: Integrated circuits References [1] J.-H. Park and H.-D. Yoon: IEICE Electron. Express 8 [8] (2011) 556. [2] Y.-T. Hsieh, B.-D. Liu, J.-F. Wu, C.-L. Fang, H.-H. Tsai and Y.-Z. Juang: IEEE Trans. Power Electron. 27 [11] (2012) 4562. [3] K. I. Hwu and W. C. Tu: IET Power Electronics 5 [1] (2012) 59. [4] T. Zhang, Y. Yang, Z. H. Song and Y. B. Fan: Proc. Int. Conf. Computer Science and Information Processing (2012) 1129. [5] Y. Yang, Z. H. Song and Y. Gao: Symp. on Photonics and Optoelectronics (2009) 1. [6] H. J. Chiu and S. J. Cheng: IEEE Trans. Ind. Electron. 54 [5] (2007) 2751. 1 Introduction As white light emitting diodes (WLEDs) are becoming more prevalent in a wide variety of applications and it is considered as a general trend that the WLED will replace the traditional light sources. It is very important for a light source that the luminance can be adjusted conveniently [1]. 1

There are two common ways used in dimming luminance of WLED, one is analog dimming (DC), and the other is digital dimming (PWM or PFM) [2]. The analog dimmer is a technology of adjusting LED brightness by changing the forward current of the LED. It has the advantages of the simple structure and low cost. Since the light intensity of a LED is proportional to the forward current, the DC dimming has a very good linearity. But when the forward current is decreased, the wavelength of the main component will increase in the spectrum, which causes the color change. So the DC dimming is unsuitable for the application in which the color is strictly required, such as automobile headlights, LCD backlights, or traffic signals [3]. In digital dimming, the light intensity is adjusted by controlling the duty cycle of a logic signal. In this control mode, the current of the LED alters between 0 and the nominal current value. So dimming in this manner makes the intensity independent of color. A dual-mode dimming technology based switching converters has been proposed in [4] and [5], which can automatically switch between PWM and PFM modes according to the input logic signals. But the nature of PWM and PFM dimming is achieved by changing the duty ratio of the input logic signal. So it is only a digital dimming mode, not a real dual-mode dimming. Compared with the DC dimming, the digital dimming requires more external devices, complicated structure and higher cost. A novel dual-mode dimming circuit for the WLED driver is proposed in this paper. According to the input control signal, it can alter flexibly the dimming mode between analog and digital dimming. Compared with the single-mode (analog or digital) dimming, it has the advantages of the lower cost and broader applications. 2 Principle of the driver with dual-mode dimming circuit The block diagram of the WLED driver with the proposed dual-mode dimming circuit is shown in Fig. 1. The proposed dual-mode dimming circuit mainly consists of the dimming current generation circuit (DCG) and current-controlled current-sense circuit (CSC). Because the current mode is suitable for the high-power WLED driver system with wide voltage supply rang [1], the driver applies a hysteretic-current-control mode. Fig. 1. The diagram of the WLED driver with the proposed dual-mode dimming circuit 2

When the input voltage V IN is first applied, the initial current in the inductor L and resistor R S is zero. The switch M 2 is turned off and there is no current from V IN to ground through R 1,R 5 and R 6. In this condition, the (-) input to the comparator is shorted to ground and its output is high. Then the power switch MN is turned on, which cause the current flow from V IN to ground, via R S, L and the WLED. The current rises at a rate determined by V IN and L to produce a voltage ramp (V IN -V CSN ) across R S. This ramp voltage is forced across the resistor R 1 by the CSC and produces a proportional current flowing through the internal resistors R 5 and R 6. This produces a ground referred rising voltage at the (-) input of the comparator. When this voltage reaches the high threshold voltage (V max ), the comparator output switches low and then MN is turned off. The comparator output also drives another NMOS switch M 1, which bypasses the internal resistor R 6 to provide a controlled amount of hysteresis. When MN is off, the current in L continues to flow via the external diode D and the WLED back to V IN. The current decays at a rate determined by the WLED and the diode D forward voltages to produce a falling voltage at the input of the comparator. When this voltage decays to low threshold voltage (V min ), the comparator output switches high again. This process occurs periodically, so that the average current through the WLED keeps constant. According to the control signal applied to DIM pin, the driver alters the DC or PWM dimming mode. In the DC dimming mode, when the input DC voltage of the DIM pin varies from 0.5 V to 2.5 V, the value of I con changes proportionally with it. It will result in the average current through WLED proportional to V DIM. In the PWM dimming mode, the input of the DIM pin is a logic signal and its pulse width can be changed. When the pulse level is low (<0.5 V), the CSC sets the output current through WLED to zero. When the pulse level is high (>2.5 V), the output current is set to the nominal value. Dimming in this manner makes the average value of the output current proportional to the duty cycle of the PWM signal. 3 Novel dual-mode dimming circuit for the WLED driver The detailed schematic of the dual-mode dimming circuit is shown in Fig. 2. The devices marked by red color are the HVMOS (High Voltage MOS). The voltage across DIM pin is compared with an internal reference to generate the dimming-control-current Icon. Assume V REF > V DIM,R b1 = R b2 =R, V C is higher than V B. Then the Equ. (1) can be obtained. V C V B R As I 1 =I 2,I 3 is given by the Equ. (2). ¼ I e I d (1) I 3 ¼ I b þ I e I 2 ¼ I e I d (2) From (1) and (2), I con can be calculated by Equ. (3). I con ¼ I 3 ¼ V REF V DIM (3) R If V DIM V REF,I con is 0, the output current of the driver is the nominal 3

Fig. 2. The schematic of the novel dual-mode dimming circuit current and the dimming circuit does not work. Only when V DIM <V REF,I con is greater than 0, and the system has dimming function. If R 1 =R 2, the cascade current source can be generated by M 18 ~M 21 to keep I R1 = I R2. Since the bias currents are the same in the two branches, the cascade current mirror consisted of M 14 ~M 17 shows the effect of voltage mirror [6]. This will make the nodes E and F in the same potential, so it can be expressed as Equ. (4). V IN ði R1 þ I R3 ÞR 1 ¼ V CSN I R2 R 2 (4) Suppose I con =0 at first, the voltage V sense is given by Equ. (5). V sense ¼ I R3 R 3 ¼ V IN V CSN R 3 (5) R 1 The threshold voltages of the hysteretic comparator is V max and V min, the nominal current is given by the Equ. (6). ð I OUTnom ¼ V max þ V min ÞR 1 (6) 2R 3 R s The charge and discharge processes of the external inductor L 1 make V CSN change rapidly. So V E needs to be adjusted quickly to ensure that its value equals V F. This can be achieved by using a negative feedback structure consisted of M 22 ~M 25. When there is a small fluctuation in V CSN,V F will be amplified by the two-stage common-gate amplifier consisted of M 15 and M 17, which results in the change of V G. The feedback will make V E vary with V G, and thus make the voltages of node E and F be equal. The gain and bandwidth of the loop directly affect the accuracy and sensitivity of the entire current sensor. As a zero-pole compensation device, the zeroregulator resistor R 4 and compensation capacitor C C are introduced into the loop to achieve two poles. A zero at node G can ensure the high unitgain bandwidth and good stability. 3.1 DC voltage dimming mode If V DIM <V REF,I con is larger than 0. The V sense is expressed by Equ. (7). V sense ¼ ði R3 þ I con ÞR 3 ¼ V IN V CSN R 3 þ V REF V DIM R 3 R 1 R (7) Therefore, the average output current of this driver can be expressed by 4

Equ. (8). I avg ¼ R 1 ðv max þ V min Þ R 1 ðv REF V DIM Þ 2R 3 R s RR (8) s Set R 3 /R = (V max +V min )/(2 V REF ), and Equ. (8) can be further simplified as Equ. (9). I avg ¼ V DIM ð V max þ V min ÞR 1 ¼ V DIM I OUTnom (9) V REF 2R 3 R s V REF When V DIM varies from 0.5 V to V REF (2.5 V), the output current (brightness) changes linearly from 20% to 100% of the nominal value. 3.2 PWM dimming mode When a logic signal is applied to the DIM pin, the driver operates in PWM dimming mode. If V DIM is low, the power switch MN is turned off, and the output current is 0, otherwise V DIM is high, the output current is equal to the nominal value. So the average output current in this mode can be derived as Equ. (10). I avg ¼ T DIM on I T OUTnom ¼ D I OUT nom (10) DIM Where T DIM is the period of the PWM signal, T DIM-on is the time of the high duration, and D is the duty cycle. As shown in Equ. (10), the average current is controlled by the duty cycle of the PWM signal, and it can be changed from about 0 to 100% of the nominal current value I OUTnom. 4 Experiment results The WLED driver with the proposed dual-mode dimming circuit has been verified in a 0.5 μm HVCMOS process. Fig. 3 shows the whole WLED driver chip microphotograph, and the driver die size is about 1.0 0.7 mm 2. The size of the novel dual-mode dimming circuit constituted by the DCG and CSC is about 0.2 0.1 mm 2. The value of the resistor R S can be chosen according to the current requirement by Equ. (6). The value of the inductor L can be chosen according to the supply voltage range and the ripple current requirement. Fig. 3. The Photo of the WLED Driver Die 5

Fig. 4a shows that the average current varies with the DC voltage applied on DIM pin. When the V DIM varies from 0.5 V to 2.5 V, the average current can be changed linearly from 20% to 100% of the nominal current. The nominal current is determined mainly by the sensing resistor R S. Once V DIM is greater than 2.5 V, the output current will reach the nominal value and cannot be adjusted in this condition. When V DIM is smaller than 0.5 V, the output current is 0. The test results shown in Fig. 4a are consistent with the expression of Equation (9). Fig. 4. Output current vs. the input signal applied on DIM pin a) in the DC dimming mode b) in the PWM dimming mode Fig. 4 b shows the average output current versus the duty cycle of V DIM with different frequency. The values of R S and L are designed to be 0.0825 Ω and 27 μh. When the input of DIM pin is PWM signal, the output current is adjusted with the duty cycle of the PWM signal, but almost independents to its frequency. The range of the intensity is about 0% to 100% of the nominal current. The test results shown in Fig. 4 b are consistent with the expression of Equation (10). 5 Conclusion This paper presents a new dual-mode dimming circuit for the WLED driver for the first time. The WLED driver has been implemented in a 0.5 μm HVCMOS process. The WLED driver die size is about 1.0 0.7 mm 2, and the dimming circuit size is about 0.2 0.1 mm 2. The WLED driver with the dual-mode dimming circuit proposed can alter flexibly the dimming mode according to the input control signal. In the DC dimming mode, the output current will be adjusted from 20% to 100% of I OUTnom. And in the PWM dimming mode, the range of the output current is about from 0% to 100%. The mathematical model of the dimming circuit is derived, and the test results are consistent with it. This proposed dual-mode dimming circuit is verified to be effective in reducing the costs and expanding the range of applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 61234002, 61006028, 61204044), and the National High-tech Program of China (No. 2012AA012302) 6