13.7 A +10dBm 2.4GHz transmitter with sub-400pW leakage and 43.7% system efficiency

Abstract

Extreme energy constraints inherent in many exciting new wireless sensing applications (such as [1][2][3]) virtually dictate that such systems operate with extremely low duty cycles, harvesting and storing energy over long periods of time before waking up to perform brief measurement and communication tasks. However, such duty cycling only works if the sleep power of the system is less than the average power available from the power source, which may only be as much as a few nA. In this work, we present an RF transmitter designed to operate in an extremely low duty-cycle industrial monitoring system. The primary challenges are achieving high efficiency in the active mode while transmitting as high as +10dBm and simultaneously minimizing the leakage during the sleep mode. We address these in a +10dBm Bluetooth Low Energy (BLE) transmitter test-chip through 1) low voltage design (0.68V) for switching power and short-circuit power reduction, 2) extensive power gating of unused blocks and 3) a negative-VGS biasing technique for PA leakage reduction without affecting its on-performance.Typically, high-VT power switches are used to power gate low-VT active circuits [1]. But, the switch resistance, which is in the direct path of active current, degrades on-performance. Increasing switch size will in turn increase leakage. Negative gate-biasing of the gating switch has been shown to give significant leakage reduction [3]. However, in this work, we study the effect of negative-gate biasing of the low-VT active device itself, thereby eliminating a switch in the direct path of active current and simultaneously reducing leakage. This is especially useful for the PA in our work, which operates at +10dBm and is the largest active power consumer. Fig. 13.7.1 shows the measured drain current (ID,PA) and gate current (IG,PA) of the NMOS PA transistor as a function of the negative gate bias applied. The 65nm CMOS transistor, because of its thin gate oxide, has significant gate leakage. It is exponential with the gate bias. The drain current, which is the sum of sub-threshold current and gate leakage, decreases exponentially with increasing gate bias while the sub-threshold current is dominant, but increases as gate leakage becomes larger. The red curve (in solid) shows the achievable total PA leakage assuming the negative bias is supplied by an ideal −½ charge pump (ILEAK = ID,PA-½IG,PA). The minimum, 430pA, occurs at −200mV and represents a 30× reduction in leakage. Fig. 13.7.1 also shows a histogram of the minimum (ID,PA−½IG,PA) current for 25 measured chips. Simulations indicate that the minimum leakage points range from −150mV to −300mV, which implies that a −½ charge pump generating VNEG =−½VIN is sufficient. Low VDD ensures device reliability even with VNEG biasing (VDD-VNEG < 1.2V). Fig. 13.7.2 shows the complete TX system block diagram. The RF signal path has a 12MHz crystal oscillator feeding a divided 1MHz clock to an integer-N PLL. The PLL provides 2MHz-spaced BLE channels at 2.4GHz. The PLL output is then buffered ont...

Prolonged energy harvesting for ingestible devices

“…Research in ultra-low-power electronics continues to push the boundaries of the average power consumption, and already provides a range of options for circuits that could be adapted for use in GI applications at the nanowatt level. For example, energy harvesters (for sub 10 nW available power 10 – 12 ), ADCs and signal acquisition circuits (under 10 nW 39 , 40 ), far field wireless transmitters (under 1 nW standby 41 ), and mm-scale sensor nodes with sensing, processing (sub nW standby 42 ). Such systems could allow the electrode area to scale to just a millimeter or two on a side, and could enable broad applications for extended power harvesting from alternative cells for long term monitoring of vital signs 4 and other parameters in the GI tract, especially with the introduction of devices that are deployed endoscopically 43 or self-administered 13 and have the capacity to reside in the gastric cavity for prolonged periods of time.…”

A 0.68V 0.68mW 2.4GHz PLL for ultra-low power RF systems

“…This gives a VCO phase noise FOM of −182dB. This phase noise data was shown in the presentation of [7]. Figure 9 shows the transient response of the PLL.…”

“…The resulting leakage power of the PLL is 170pW. The transmitter system in [7] that uses this PLL also provides a negative bias voltage around −0.2V in sleep. When this is used to strongly cutoff the NMOS switches, the total PLL leakage reduces to 27pW.…”

“…The PLL in this work is part of a complete BLE transmitter, optimized for leakage in addition to active performance [7], for use in extremely energy constrained low duty cycle systems. The PLL is designed in 65nm LPCMOS to co-optimize RF and leakage performance.…”

Enhanced Broadband RF Differential Rectifier Integrated with Archimedean Spiral Antenna for Wireless Energy Harvesting Applications

“…Owing to the tremendous achievements in low-power radio transceivers, many low-power wireless sensors that consume only several microwatts have been developed. More recently, researchers have developed a method for designing picowatt radio chips [1]. The power requirement declination has encouraged researchers to implement an independent wireless node that could obtain energy from the surrounding environment via several energy conversion techniques, for instance, solar, wind, vibration, thermal, and RF energy harvesting (RF-EH) [2].…”