Vlad Anghel

Variable Off-Time Control Loop for Current-Mode Floating Buck Converters in LED Driving Applications

A versatile controller architecture, used in current-mode floating buck converters for LED driving, is developed. State-of-the-art controllers rely on a fixed switching period and variable duty cycle, focusing on current averaging circuits. Instead, the proposed controller architecture is based on fixed peak current and adaptable off time as the average current control method. The control loop is comprised of an averaging block, transconductance amplifier, and an innovative time modulator. This modulator is intended to provide constant control loop response regardless of input voltage, current storage inductor, and number of LEDs in order to improve converter applicability for LED drivers. Fabricated in a 5 V standard 0.5 μm CMOS technology, the prototype controller is implemented and tested in a current-mode floating buck converter. The converter exhibits sound continuous conduction mode (CCM) operation for input voltages between 11 and 20 V, and a wide inductor range of 100-1000 μH. In all instances, the measured average LED current variation was lower than 10% of the desired value. A maximum conversion efficiency of 91% is obtained when driving 50 mA through four LEDs (with 14 V input voltage and an inductor of 470 μH). A stable CCM converter operation is also proven by simulation for nine LEDs and 45 V input voltage.

Variable off time current-mode floating buck controller - A different approach

Switch-mode power supplies have become the premier choice for LED backlighting applications. This paper introduces a new current-mode floating buck controller architecture, used for driving constant current through a string of LEDs. Unlike state-of-the-art controllers, this architecture is based on fixed peak current and variable OFF time as the current control method. An innovative time modulator is added to provide a constant feedback loop response regardless of external components. The proposed controller architecture is designed, implemented in a 0.5μm CMOS process and its performances are validated by simulations and measurements.

Time modulation — The exponential way

A solution for obtaining an exponential variation of time versus input voltage of a time-modulating circuit is investigated, implemented and its performances are measured. Most time-modulating circuits produce a delay that is linearly dependent on the input voltage. This paper proposes a new method of controlling the time delay output by making it vary exponentially with the input voltage. This solution is especially useful in aplications where the time-delay is constantly modulated by a prior error amplifier.

Finite Element Analysis of Generation and Detection of Lamb Waves Using Piezoelectric Transducers

The paper reports the use of Finite Element (FE) simulation and experiments meant to explore the operation conditions of the PieZoelectric wafer Transducer (PZT). Piezoelectrics is the coupling of structural and electric fields and may be solved using the multi-physics approach. Accordingly, three different multiphysics models were developed to investigate a plane strain problem. The first one includes two PZTs mounted on an aluminium plate and is used to model both the emission and reception signals. The next two ones are developed to separately model the emission and detection processes, in order to decrease the computational effort. The wave displacements are generated by a PZT-like actuator and the output voltage is obtained at a PZT receiver both by a multi-physics approach. The analysis considered the transducer lengths, the effects of the finite pulse width, the pulse dispersion and the detailed interaction between the piezoelectric element and the transmitting medium. The transmitted and received signals for so-called A0 and S0 modes have maxima close to the frequencies predicted in other works. A series of sensitivity curves relating the generation and receiving of Lamb waves were also determined and plotted as a function of the pulse center frequency and of the PZT lengths.

Overcoming leakage current by output precharging in error amplifiers

Initial control loop conditions are essential in switching circuits where fault detection blocks are present, in order to avoid permanent fault triggers. Furthermore, multi-channel switching interference may affect circuit functionality and cause additional fault triggers. Error amplifiers aquire the difference between sensed and reference signals and output a correction voltage. We propose a solution which ensures that the initial correction voltage does not determine a perpetual fault condition. The proposed solution is implemented in a current mode buck control loop, which delivers constant current through a load.

Forcing the limits — Peak voltage extension for floating buck controller

A solution for increasing a current mode floating buck controllers versatility, through the use of a peak voltage extension block, is investigated, implemented and its performances are measured. Most current controller architectures have limitations that impose harsh application-constraints. This paper proposes a new method of reducing those limitations by allowing precise, linear, external control of the peak voltage threshold. The solution is especially useful in applications where each instance of the buck controller needs to be individually adjusted to meet specifications.

Less is more — The improved variable OFF time current-mode floating buck controller

Current-mode floating buck controllers are frequently used as LED drivers. The focus of this paper is to improve a previously introduced variable OFF time current-mode floating buck controller. The initial design is modified in order to operate in continuous conduction mode for a wider range of external components. The improved architecture is simulated and measured in comparison with the initial design.

Suppressing start-up time variation versus load current — Adaptive soft-start in boost LED drivers

This paper proposes an adaptive soft-start technique which offers both dynamic overshoot protection at start-up and maximum current regulation during steady state. The circuit is included in a boost converter IC Controller manufactured in a 50V, 0.5μm CMOS technology and tested via simulations and measurements. A comparative study is performed regarding converters operation with and without the proposed block. Results emphasize the proposed solutions ability to suppress overshoots and reduce start-up time. Moreover, the measured start-up time is quasi-constant for a fixed number of LEDs, regardless of LED current.

Application enhancement for driving large strings of LEDs

A technique for increasing the number of series connected LEDs driven by a current-mode floating buck converter is analyzed, simulated and implemented. The converter uses an external power switch, driven by a previously introduced low-cost variable frequency controller. The increase in input voltage and number of driven LEDs is achieved by the addition of a level shifter at the controllers output which enhances its capability to drive a power MOSFET switch. Measurements performed on the improved converter biasing a maximum of 16 LEDs at up to 75V emphasize a peak efficiency of 94.6%. An efficiency over 87% can be obtained in all cases.

A matter of isolation — A reset controller using Deep N-Well and floating gate technologies

An integrated reset controller (voltage supervisor) is designed and implemented. The system generates a reset signal, active while power supply brownout conditions are detected and for 250 ms after the supply voltage has increased to acceptable levels. The circuit is built in a Deep N-Well (DNW) 5V 0.35μm CMOS process, which provides good isolation between the on-chip devices and the substrate. The controller is based on a comparator, which uses a programmable voltage reference built with floating gate transistors. The voltage supervisors operation is demonstrated by simulations and measurements.