A Floating N-DMOS Switch for RGB LED Control Vesselina Barzinska Automotive and Power Group ON Semiconductor - Sofia Sofia, Bulgaria V.Barzinska@ecad.tu-sofia.bg Emil D. Manolov FETT – Department of Electronics Technical University – Sofia Sofia, Bulgaria edm@tu-sofia.bg Abstract—This paper proposes a new RGB LED control switch with enhanced characteristics in ON and OFF states. To this aim the main requirements and specifications to the circuit are defined. A well-known conventional solution of the typical floating N-DMOS switch control circuit is described and its disadvantages are analyzed. A new circuit with improved performance is proposed and discussed. The both circuits are simulated and the obtained results are compared. The comparison confirms the usefulness of the proposed circuit. Index Terms—RGB LED driver; floating N-DMOS switch; LED Control I. INTRODUCTION Nowadays the industrial and automotive market is characterized with increasing demand of different LED applications. With regard to this, the need of new LED drivers and controllers, with lower power dissipation, more levels of control and accuracy, vastly increases [1], [2]. Many applications, for example RGB LED control, require high voltage switches that have to pass analog signals in wide range – usually from zero to battery supply. For their implementation CMOS processes with option for integration of high voltage transistors are used. They are high voltage tolerant and capable to assure the operation in maximum drain-source, drain-bulk and source-bulk voltage, but the admissible gate- source voltage is usually limited to few volts. This imposes the using of specific circuits for switch control. Fig. 1 presents a simplified circuit of RGB LED control. It uses three diodes connected in series. The structure meets the following requirements: battery supply from 9V up to 24V and maximum current trough diodes in ON state – 40mA. The color of the RGB LEDs is controlled by the current trough the diodes and/or the proportion of time that they are ON/OFF (i.e. by pulse width modulation – PWM). For the presented case the current through the diodes is in range from 0 to 40mA and the PWM is 8 bit with 400Hz frequency. To guarantee minimum calibrations of the emitted colors, a requirement to have 1% matching accuracy between the currents in any pairs of diodes is defined. For very small currents, down to 1mA, this means maximum 10µA difference between the currents through diodes; respectively for the applications where the nominal current is 10mA, the mismatch has to be below 100µA. Regarding switch time parameters, the necessary accuracy of 1% for 8 bit PWM with 400Hz frequency, determines minimum pulse width of 10µs, which tolerates up to 1µs delay for reaction of the switches when turn- on/off the LEDs. The described above requirements define the following specification toward the switches for this type RGB LED control: • The switches have to withstand high voltage, defined by maximum battery voltage (VBAT). • The voltage drop over the switches in ON state has to be enough low in order to guarantee the turn-off of the LED. • The switching time has to be below 1 µs. • The current through the switches in OFF state has to be below 10 µA. • The current through the switches in ON state has to be equal to the current of the source IC1 with acceptable inaccuracy of 1%. Figure 1. RGB LEDs connected in series and the corresponding control switches. The paper presents a new floating N-DMOS switch for RGB LED control, which fulfils the above defined specifications. To this aim a well-known conventional solution of the typical floating N-DMOS switch control circuit is described and its disadvantages have been analyzed. A new circuit with improved performance is proposed and discussed. The both circuits are simulated and the obtained results are compared. The work is supported by National Research Fund of Ministry of Education and Science of Republic Bulgaria through MU-FS-01/2007 and DTK-02/2005 contracts. MIXED DESIGN MIXDES 2010, 17 th International Conference "Mixed Design of Integrated Circuits and Systems", June 24-26, 2010, Wrocaw, Poland