Youngkook Ahn Inho Jeon Jeongjin Roh

Analog Integr Circ Sig Process (2014) 79:345 354 DOI 10.1007/s10470-014-0269-1 A multiple-output switched-capacitor DC DC converter for individual brightness control of RGB LEDs with time-interleaved control and output current regulation Youngkook Ahn Inho Jeon Jeongjin Roh Received: 17 September 2012 / Accepted: 29 January 2014 / Published online: 18 February 2014 Ó Springer Science+Business Media New York 2014 Abstract In this paper, an integrated multiple-output switched-capacitor (SC) converter with time-interleaved control and output current regulation is presented. The SC converter can reduce the number of passive components and die areas by using only one flying capacitor and by sharing active devices. The proposed converter has three outputs for individual brightness control of red green blue (RGB) LEDs. Each output directly regulates the current due to the V I characteristics of LEDs, which are sensitive to PVT variations. In the proposed converter, the current-sensing technique is used to control the output current, instead of current-regulation elements (resistors or linear regulators). Additionally, in order to reduce the active area, three outputs share one current-sensing circuit. In order to improve the sensing accuracy, bias current compensation is applied to a current-sensing circuit. The proposed converter has been fabricated with a CMOS 0.13-lm 1P6M CMOS process. The input voltage range of the converter is 2.5 3.3 V, and the switching frequency is 200 khz. The peak power efficiency reaches 71.8 % at V IN =2.5 V, I LED1 = 10 ma, I LED2 = 18 ma, and I LED3 = 20 ma. The current variations of individual outputs at different supply voltages are less than 0.89, 0.72, and 0.63 %, respectively. Keywords Switched-capacitor (SC) dc-dc converter RGB LED driver Multiple-output Individual brightness control Time-interleaved control Output current regulation Current-sensing Y. Ahn I. Jeon J. Roh (&) Department of Electrical Engineering, Hanyang University, Ansan 426-791, Korea e-mail: jroh@hanyang.ac.kr 1 Introduction Light-emitting diodes (LEDs), which are solid-state lighting devices, have become very popular due to several key advantages, including their small size, high luminescent efficiency, and long lifetime [1, 2]. LEDs can be divided into two main categories: white LEDs and red green blue (RGB) LEDs. Unlike white LEDs, RGB LEDs can provide various visible colors by mixing RGB lights at different ratios and can also be used in a variety of applications, such as biomedical apparatuses, detector systems, backlighting systems, automotive lighting systems, and general decorative illumination [3 5]. LED brightness is controlled by the forward current of the LED. Figure 1 shows the V I characteristics of typical RGB LEDs [6]. Since the V I characteristics of LEDs are highly sensitive to PVT (process, voltage, temperature) variations, controlling light outputs by regulating the forward voltage is impractical. A small forward voltage variation can lead to a significant forward current change, causing LED color control failure and longevity reduction. Therefore, to obtain predictable and matched luminous intensity and color, directly controlling the forward current is recommended [7]. Recently, diverse current regulation methods have been introduced. The easiest approach is to use a current regulation element (CRE) in series with the LED. In [8], resistors are used as CREs. Although this method can be easily implemented, the forward current of the LED cannot be precisely controlled, which induces the resistive loss. The series current regulator can be used as another CRE. The current regulators are divided into the linear-regulation type and the current-mirror type [9 11]. In the linear-regulation type, the voltage drop across the linear elements results in significant power loss and the current accuracy is influenced

346 Analog Integr Circ Sig Process (2014) 79:345 354 Fig. 1 V I curve characteristics of typical RGB LEDs by parasitic resistance of the ground path. The current-mirror type also incurs significant power loss due to a transistor, which is biased in saturation region, and the current accuracy is worse than in the linear-regulation type due to the mismatch between the transistors of the current mirror and the process variations. Multiple switching converters can also be used to separately regulate the forward current of each LED [12]. Existing topologies for obtaining multiple outputs are based on an inductor-type switching converter, which uses either one inductor for every output [12] or a single inductor for multiple outputs [13, 14]. On the other hand, the switched-capacitor (SC) converter does not use a large off-chip inductor. Thus, the SC converter has smaller board space and lower radiated electromagnetic interference (EMI) than the inductor-type converter [15]. However, the multiple SC converters, which are required for control of individual LED brightness, suffer from numerous off-chip components, large board space, and high cost. In [16], the concept of a multiple-output SC converter is presented for low power applications. In [17], a single mode buck/boost charge pump, which has dual outputs, is introduced. Since both [16, 17] regulate the output voltages, series resistors or current regulators are required to regulate the output current for RGB LEDs. In this paper, the structure of a multiple-output SC converter, which can generate multiple outputs using only one flying capacitor to control the individual brightness of RGB LEDs, is presented. Compared with multiple SC converters, the multiple-output converter can significantly reduce both the on-chip active area and the number of offchip components. The proposed converter uses the timeinterleaved control and the output current regulation to individually control the brightness of RGB LEDs. Current regulation of each output is accomplished by using the current-sensing technique, instead of using the CRE connected in series with each LED. This paper is organized as follows. In Sect. 2, the architecture and control strategy of Fig. 2 a Proposed single-output current-regulated SC converter. b Timing diagram the proposed multiple-output SC converter are described. In Sect. 3, the circuit implementation of the proposed converter is explained. The measurement results are provided in Sect. 4 The conclusions are presented in Sect. 5. 2 Proposed multiple-output SC converter In this section, a cost-effective multiple-output structure, which can generate multiple outputs using only one flying capacitor, is introduced. First, a single-output current-regulated SC converter, which uses the current-sensing technique for the brightness control of the LED, is introduced. Then, a multiple-output SC converter extended from a single-output SC converter is explained. 2.1 Single-output architecture Figure 2 shows the proposed single-output current-regulated SC converter and the timing diagram. The converter uses a current-sensing circuit to regulate the output current without using a series CRE, which may increase the power consumption. As shown in Fig. 2, the proposed circuit has two operations: charging (S a = closed) and regulation (S b = closed). During the charging phase, the flying capacitor (C F )is charged to V IN, and the charging current flows through r a, as shown in Fig. 3. During this phase, the load capacitor (C L ) supplies energy to the load. Figure 3 shows the regulation phase. During this phase, the pumping current

Analog Integr Circ Sig Process (2014) 79:345 354 347 I LED ¼ð1 DÞI pump : ð3þ In the regulation phase, V sense and V ref have the relationship of V sense = V ref due to the negative feedback loop, which consists of the current-sensing circuit and the VCCS circuit. Based on (2) (3), the I LED can be rewritten as follows: I LED ¼ð1 DÞV ref K : ð4þ R sense Therefore, the proposed SC converter can regulate the forward current of the LED without using a series CRE, which incurs power loss and area penalty. 2.2 Multiple-output architecture and control strategy Fig. 3 Steady-state average current model of the SC converter. a Charging phase (S a = closed). b Regulation phase (S b = closed) supplied to both R L and C L is regulated. Therefore, the load device can be supplied with a constant current during both charging and regulation operations. In the average current model in Fig. 3, resistances r a and r b are the equivalent onresistances of switches S a and S b, respectively. The power efficiency of the SC converter is determined by the following formula. V OUT I LED g sc \ 100 %; V IN ðdi chðavgþ þð1 DÞI pumpðavgþ Þ ð1þ where I ch(avg) is the average charging current during the charging phase, I pump(avg) is the average pumping current during the regulation phase, and D is the duty ratio for switch S a. During the regulation phase, the converter uses a current-sensing circuit to regulate the pumping current (I pump ), as shown in Fig. 2. The I pump is sensed as K:1 by the current-sensing circuit, and the sensed current is transformed into a voltage (V sense ), which can be demonstrated by the following equation: V sense ¼ I pump K R sense; ð2þ where K is the sensing ratio and R sense is the resistance used to transform the sensed current into a voltage. In the voltage-controlled current source (VCCS) circuit, the error amplifier compares V sense with the reference voltage (V ref ) to control the current of the pumping cell. The I pump supplied through the pumping cell has the following relationship with the I LED : In this section, the single-output architecture is enhanced for multiple outputs. The architecture and the timing diagram of the proposed multiple-output SC converter are shown in Fig. 4. The converter consists of power transistors (MP 0 - MP 3, MN, MS 1 - MS 3 ), a flying capacitor (C F ), load capacitors (C L1 C L3 ), a current-sensing circuit, VCCS circuits, and a switch controller. This work basically uses single-output SC converters, as shown in Fig. 2, to individually control the RGB LEDs, but the converter is reconfigured so that multiple outputs can be generated using only one flying capacitor and sharing active devices to reduce the system volume. The timing diagram in Fig. 4 demonstrates the operating principle of the proposed SC converter. The converter has three outputs V OUT1 - V OUT3 for RGB LEDs, and all of the outputs share a single flying capacitor and the power transistor MN by the timeinterleaved manner. Here, V OUT1 has the highest voltage level for the body-bias control. As shown in Fig. 4, each output is operated by three non-overlapping phases (S 1 - S 3 ) that have the same duration. During S 1 = high, first, MP 0 and MN are turned on and the charging operation begins. Then LED 2 and LED 3 are supplied with currents through C L2 and C L3, and, at this time, the output currents (I LED2, I LED3 ) ramp down due to the discharging operations of C L2 and C L3. Until D a1 T is finished, C F is charged by V IN, I LED1 ramps down due to the discharging operation of C L1, and LED 1 is supplied with energy from C L1. Here, D a1 is the duty ratio of the charging section, and T is the switching period. During D b1 T, MP 1 and MS 1 are turned on and the regulation operation begins. A pumping cell supplies the pumping current to C L1 and LED 1. At this time, the pumping current is sensed as K:1 by the current-sensing circuit, and VCCS circuit1 controls the pumping current by comparing V sense1 and V ref1. Thus, LED 1 is supplied with a constant current through the

348 Analog Integr Circ Sig Process (2014) 79:345 354 Fig. 4 a Proposed multiple-output SC converter. b Timing diagram

Analog Integr Circ Sig Process (2014) 79:345 354 349 charging and regulation operations. Here, if the dead-time intervals in the switching section are ignored, D a1, D b1, and D c1 will satisfy the following relationship. D a1 þ D b1 ¼ 1 3 D a1 þ D b1 þ D c1 ¼ 1: ð5þ During S 2 = high (or S 3 = high), a similar switching action repeats to regulate I LED2 (or I LED3 ). As mentioned earlier, in the proposed circuit, since all outputs are separated by the time-interleaved control, each output current can individually be regulated by the charging and regulation operations. In each regulation phase, the Fig. 5 Current-sensing circuit of the proposed converter Fig. 6 VCCS circuit

350 Analog Integr Circ Sig Process (2014) 79:345 354 Fig. 7 Body-bias control circuit Fig. 8 a Chip micrograph. b Prototype for testing the proposed SC converter pumping current is controlled by the negative feedback loop, which consists of a current-sensing circuit and a VCCS circuit, as shown in Fig. 2, and the current formula of each output can be extended, as follows, using (2) (4). I LED1 ¼ D b1 V ref 1 K ; R sense1 I LED2 ¼ D b2 V ref 2 K ; ð6þ R sense2 I LED3 ¼ D b3 V ref 3 K ; R sense3 where D b1 - D b3 are duty ratios of MP 1 - MP 3, and V ref1 - V ref3 are reference voltages used to define the output currents. Thus, the proposed SC converter can independently regulate the multiple outputs using only one flying capacitor. 3 Circuit implementations 3.1 Current-sensing circuit The proposed SC converter uses the current-sensing technique to control output currents. Figure 5 shows the currentsensing circuit used in the multiple-output SC converter. In this work, the SC outputs share one current-sensing circuit, in order to reduce the die area. Current sensing is accomplished using the MOS transistor scaling technique, and the performance of the current sensor is determined by a feedback loop consisting of a voltage mirror and an output stage [18]. Therefore, the larger magnitude of the loop gain enables a higher sensing accuracy. The loop gain of the current-sensing circuit is as follows:

Analog Integr Circ Sig Process (2014) 79:345 354 351 Fig. 9 Measured output voltages and sensing voltages of the proposed SC converter at V IN = 2.8 V, F SW = 200 khz. a The output voltages and sensing voltage. b The output voltages (ac) and sensing voltage g m3 g m10 r o6 R sense A sense ¼ A ð1 þ g m3 r o6 Þð1 þ g m10 R sense Þ ; ð7þ where A is the DC gain of an op-amp, g m3 and g m10 are the transconductances of M 3 and M 10, and r o6 is the smallsignal output resistance of M 6. In Fig. 5, when the regulation operation of SC output1 has begun, M a1 is turned on and the voltages of the V A and V B nodes become identical as a result of an op-amp; thus, the source-drain voltages of M 1 and MP 1, as well as their drain current density, become identical. Furthermore, MP 1 and M 1 are scaled so that MP 1 has an aspect ratio much greater than that of M 1. As a result, 1/K times of the current flowing through MP 1 comes to flow through M 1. However, the sensed current becomes (I pump /K) I b due to the bias current (I b ); thus, the sensing accuracy decreases. In [18], the condition I pump /K I b was Fig. 10 a Measured charging and regulation control signals. b Measured charging and regulation operation waveforms of the SC output1. Measurement conditions are V IN = 2.8 V, F SW = 200 khz satisfied in order to obtain a high sensing accuracy. However, this condition requires either making I b very small or I pump /K very large, and thus imposes restrictions on the determination of the sensing ratio in the regulation phase. In Fig. 5, the current-sensing circuit is configured so that I b can be compensated by additionally connecting a transistor (MC). The regulation operations of SC output2 and SC output3 are accomplished using the same mechanism as SC output1. 3.2 VCCS circuit Figure 6 shows a VCCS circuit that controls the pumping current during the regulation phase. The VCCS circuit consists of an error amplifier and a pumping cell. The pumping cell works as a current source that supplies a constant current during the regulation phase, based on the V err signal, and which is implemented by a power transistor (M pump ). The error amplifier is a single-stage construction for the loop stability of the SC converter.

352 Analog Integr Circ Sig Process (2014) 79:345 354 (c) Fig. 11 Measured output currents at different supply voltages. a I LED1. b I LED2. c I LED3 Table 1 Performance of the proposed SC converter Technology 0.13-lm 1P6M CMOS Occupied active area 1.72 9 1.02 mm 2 Switching frequency (F SW ) 200 khz Flying capacitor (C F ) 2.2 lf Output capacitor (C L ) 2.2 lf Number of outputs 3 Input voltage (V IN ) 2.5 3.3 V Peak efficiency 71.8 % Output V OUT1 V OUT2 V OUT3 Fig. 12 Measured efficiency Output voltage (V OUT ) 4.14 V 3.42 V 3.63 V Output current (I LED ) 10.3 ma 18.2 ma 20.1 ma Current variation with V IN 0.89 % 0.72 % 0.63 % Output ripple voltage 51 mv 85 mv 90 mv

Analog Integr Circ Sig Process (2014) 79:345 354 353 3.3 Body-bias control circuit In Fig. 4, biasing the substrates of the PMOS power transistors in a wrong way can lead to operation failure and additional conduction power loss [19]. Therefore, a bodybias control is necessary to fully turn off the PMOS power transistors. Figure 7 shows the body-bias control circuit used in the multiple-output SC converter. V OUT1, which has the highest voltage level in the system, and V IN are leveled down by source followers composed of M 3, M 4, M 10, and M 11. The V 1 and V 2 are compared by the comparator. The decision circuit, which is composed of an inverter chain and a level shifter, determines whether V DDH should be connected to V IN or V OUT1.IfV IN is lower than V OUT1,V a falls to a very low level, M 22 is turned on, and then V DDH is connected to V OUT1. 4 Measurement results The proposed multiple-output SC converter has been implemented in a 0.13-lm 1P6M CMOS process. The proposed converter shares one flying capacitor and active devices in a time-interleaved manner to generate the multiple outputs for the individual brightness control of RGB LEDs. Therefore, both the on-chip die area and the number of off-chip passive components can be significantly reduced. Each output current of the proposed converter is regulated using the current-sensing circuit and the VCCS circuit, unlike conventional converters that use CREs in series with LEDs. A chip micrograph of the proposed converter is shown in Fig. 8; the consumed die area is 1.72 mm 9 1.02 mm. Figure 8 shows a prototype for testing the multiple-output SC converter. Two series of red LEDs that generate the highest forward voltage, a green LED, and a blue LED are connected with load devices to the SC converter. The forward voltages of the RGB LEDs are 1.9 2.4, 2.8 3.7, and 3.0 3.8 V in forward current ranges of 10 20 ma. The SC converter has input voltage ranges of 2.5 3.3 V and operates at a switching frequency of 200 khz. Figure 9 shows the measured output voltage and sensing voltage (V sense ) waveforms of the proposed SC converter. The measurement conditions are V IN = 2.8 V and F SW = 200 khz; the output voltages are 4.1, 3.4, and 3.6 V; and the load conditions are 10, 18, and 20 ma. Figure 10 shows the control signals for the charging and regulation operations. Figure 10 shows the charging and regulation operation waveforms of SC output1. During S 1 = high, the SC output1, which is shown in Fig. 5, performs the charging operation by CK 0 and the regulation operation by CK 1, respectively. During D b1 T, the SC output1 is regulated and V OUT1 ramps up due to the pumping-current supply. During D c1 T, V OUT1 ramps down because the output capacitor supplies the load current. While S 2 (or S 3 ) is high, SC output2 (or SC output3) is charged and regulated by the same switching action as SC output1. Figure 11 shows the output current regulation characteristics of the fabricated converter versus the supply voltage for two load conditions. Over the variation of the input voltage ranges from 2.5 to 3.3 V, the fabricated converter performs very well, and the current variations are less than 0.89, 0.72, and 0.63 %, respectively. The measurement results show that the proposed converter has good current regulation performance during supply voltage variations. Figure 12 shows the measured efficiency of the SC converter. A peak efficiency of 71.8 % is achieved at measurement conditions, which are V IN = 2.5 V and F SW = 200 khz; the output voltages are 4.1, 3.4, and 3.6 V; and the load conditions are 10, 18, and 20 ma. Table 1 summarizes the performance of the proposed converter. There are three outputs for the individual brightness control of the RGB LEDs, and the values of the flying capacitor and the output capacitors are all 2.2 lf. The output voltage ripples are 51, 85, and 90 mv, respectively. 5 Conclusion In this paper, an integrated multiple-output SC converter for controlling the individual brightness of RGB LEDs has been presented. The time-interleaved control is employed to generate the multiple outputs sharing a single flying capacitor and active devices. Thus, the number of external passive components and active areas are significantly reduced. In addition, the proposed converter uses the current-sensing circuit, instead of using resistors or current regulators in series with LEDs, to efficiently regulate the output currents. The measurement results demonstrate the functionality of the scheme of the multiple-output SC converter and its good current-regulation characteristics. Since the proposed structure can significantly reduce the number of external components and the board area when multiple SC converters are required for mobile electronic devices, it can provide a cost-effective solution. Acknowledgments This research was supported in part by the Ministry of Knowledge Economy, Korea, under the University ITRC support program supervised by the National IT Industry Promotion Agency (NIPA-2013-H0301-13-1007), and supported in part by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013R1A1A2011973).

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