Arun Rao

Noise-shaping techniques applied to switched-capacitor voltage regulators

A delta-sigma control loop for a buck-boost dc-dc converter with fractional gains is presented. This technique reduces the tones caused by the traditional pulse-frequency modulation regulation. The prototype regulator was fabricated in a 0.72-/spl mu/m CMOS process and clocked at 1 MHz. It achieved suppression of tones up to 55 dB in the 0-500-kHz range. The input voltage range was 3-5 V. The output voltage ranged from 1.8 to 4 V for load currents up to 150 mA.

A 1.2-A Buck-Boost LED Driver With On-Chip Error Averaged SenseFET-Based Current Sensing Technique

This paper presents circuit techniques to improve the efficiency of high-current LED drivers. An error-averaged, senseFET-based current sensing technique is used to regulate the LED current accurately. Because the proposed scheme eliminates the series current-regulation element present in all conventional LED drivers, it greatly improves efficiency and reduces cost. The converter operates in three different operating modes, namely buck, buck-boost, and boost modes, and achieves high efficiency over the entire Li-Ion battery range (3-5.5 V). Fabricated in 0.5-μm CMOS process, the prototype occupies an active area of 5 mm2. At 1.2-A LED current, the driver achieves an efficiency improvement of over 13% compared to current-regulation-element-based LED drivers. Measured LED current accuracy is better than 2.8% over the entire range of the battery and its standard deviation measured across seven devices is less than 1.6%. The peak efficiencies are 90.7% and 86% at 600-and 1200-mA currents, respectively.

A 1.2A buck-boost LED driver with 13% efficiency improvement using error-averaged SenseFET-based current sensing

High-current LED drivers suffer from a significant efficiency loss due to the presence of a current regulation element (CRE) in series with the LED. In a conventional driver, either a series current source [1] or a sense resistor [2, 3] acts as a CRE to regulate the LED current (ILED). In this paper, we seek to improve the efficiency by eliminating the series CRE. To this end, we employ a highly accurate current sensing scheme to directly regulate ILED and achieve more than 13% efficiency improvement.

A Highly Digital VCO-Based ADC Architecture for Current Sensing Applications

This paper presents a voltage-controlled oscillator (VCO) based current to digital converter for sensor readout applications. Second order noise shaping of the quantization error is achieved by using implicit capacitance of the sensor to realize a passive integrator and a VCO-based quantizer. The non-linearity in voltage to frequency conversion of the VCO is tackled by placing the VCO in a loop consisting of a simple digital IIR filter and a passive integrator. The IIR filter provides large gain within the signal bandwidth and suppresses VCO input swing. As a result, non-linearity of the VCO is not exercised, thus greatly improving the proposed architectures immunity to VCO nonlinearity. The use of a digital filter instead of an analog loop filter eases the design and makes it scaling friendly. Designed for an ambient light sensor application, the proposed circuit achieves 900 pA accuracy over an input current range of 4 μA. Fabricated in a 0.18 μm CMOS process, the readout circuit consumes a total of 77.8 μA current, and occupies an active area of 0.36 mm2.

High Frequency Buck Converter Design Using Time-Based Control Techniques

Time-based control techniques for the design of high switching frequency buck converters are presented. Using time as the processing variable, the proposed controller operates with CMOS-level digital-like signals but without adding any quantization error. A ring oscillator is used as an integrator in place of conventional opamp-RC or G m-C integrators while a delay line is used to perform voltage to time conversion and to sum time signals. A simple flip-flop generates pulse-width modulated signal from the time-based output of the controller. Hence time-based control eliminates the need for wide bandwidth error amplifier, pulse-width modulator (PWM) in analog controllers or high resolution analog-to-digital converter (ADC) and digital PWM in digital controllers. As a result, it can be implemented in small area and with minimal power. Fabricated in a 180 nm CMOS process, the prototype buck converter occupies an active area of 0.24 mm2, of which the controller occupies only 0.0375 mm2. It operates over a wide range of switching frequencies (10-25 MHz) and regulates output to any desired voltage in the range of 0.6 V to 1.5 V with 1.8 V input voltage. With a 500 mA step in the load current, the settling time is less than 3.5 μs and the measured reference tracking bandwidth is about 1 MHz. Better than 94% peak efficiency is achieved while consuming a quiescent current of only 2 μA/MHz.

Buck-boost switched-capacitor DC-DC voltage regulator using delta-sigma control loop

This paper presents a delta-sigma control loop for a buck-boost DC-DC converter with fractional gains. The charge pump used to convert the input voltage acts as a D/A converter in the loop, and its output ripple is frequency shaped by the loop, which also provides the pulse frequency modulation needed for the conversion. Simulation results show that the delta-sigma loop results in spreading the tones in the frequency domain. A suppression of up to 50 dB is observed in the 0-20 kHz range.

A 1.2A 2MHz tri-mode Buck-Boost LED driver with feed-forward duty cycle correction

A flash LED Buck-Boost driver employs a dual duty cycle control and feed-forward duty cycle correction to achieve high efficiency over the entire Li-On battery voltage range of 3.0 to 5.2V. Fabricated in a 0.5µm CMOS process, the converter uses an external 1µH inductor and a 10µF capacitor and operates at 2MHz switching frequency. The measured peak efficiency is 83% and 87% at 1.2A and 0.6A LED currents, respectively. This achieved efficiency represents an improvement of more than 10% over conventional approaches.

A VCO-based current-to-digital converter for sensor applications

A current sensing VCO based ADC architecture is realized using a passive integrator, VCO-based quantizer, and mostly digital circuits. A digital IIR filter is used instead of an analog loop filter to tackle VCO non-linearity in a power efficient and scaling friendly manner. The prototype circuit designed for an ambient light sensor application achieves 900pA accuracy over an input current range of 4μA, consumes a total of 77.8μA current, and occupies an active area of 0.36mm2 in 0.18μm CMOS process.

A 10–25MHz, 600mA buck converter using time-based PID compensator with 2µA/MHz quiescent current, 94% peak efficiency, and 1MHz BW

A time-based PID compensator that combines the advantages of both analog and digital controllers is used to implement a high frequency low quiescent current buck converter. Fabricated in 180nm CMOS process, the proposed buck converter operates over a wide range of switching frequencies (10-25MHz) and achieves better than 94% peak efficiency while consuming a quiescent current of only 2μA/MHz.

A 3.3V 500mA digital Buck-Boost converter with 92% peak efficiency using constant ON/OFF time delta-sigma fractional-N control

A digitally controlled Buck-Boost converter uses a fully synthesized constant ON/OFF time-based fractional-N controller to regulate the output over a 3.3V-to-5.5V input voltage range. The proposed architecture does not use either a high resolution digital pulse width modulator or an analog to digital converter. Fabricated in a 500nm CMOS process, the prototype achieves a peak efficiency of 92% at 500mA load current. The use of a smaller inductor and no external compensation capacitor makes the proposed solution highly cost effective.