An ultra-small capacitor-less LDO with controlled-resistance technique and MOSFET-only bandgap

In this paper, we propose an ultra-small low dropout regulator (LDO) for NFC tag combining two new techniques. Firstly, a voltage bandgap is designed using only MOSFET instead of BJT in conventional architecture to reduce significantly the chip size. Secondly, to increase the stability of LDO we proposed a controlled circuit to vary output resistance according to output-load current. The latter technique also allows removing the feedback capacitor normally used in a conventional LDO architecture. The proposed LDO has a stable output voltage at 1.8V with input voltage varying from 2.1V to 3.3 V, a maximum current of 10 mA, and only 0.0058 mm2 chip area.

I. Introduction

In wireless power transfer technology, low dropout regulator (LDO) is an important block to supply a stable voltage which is independent on input voltage, temperature and load current. This element will guarantee the operation of digital signal processing block, active sensor inside the tag. Meanwhile, with the highly integrated trend, the number of transistors in the system has been increasing more and more and the CMOS technology is scaled down continuously to meet small area consumption demand. To achieve this goal in a NFC tag, the size of LDO block has to be reduced in maintaining a stable output voltage for other blocks. In bandgap references (BGRs) that supply a reference voltage for the error amplifier in LDO, bipolar junction transistors (BJTs) are widely used in order to compensate for the variation of output voltage due to the variation of temperature [2]–[5]. The main advantage of BGRs implemented BJTs is that they are easy to achieve low temperature coefficient (TC). The positive TC voltage is created by the difference in base-emitter voltage of two BJTs operating at unequal densities, while the negative TC voltage is created by the base-emitter . The combination of these two voltages supplies a voltage reference with low temperature coefficient. However, large area of BJTs leads to large area of layout design. Moreover, the relation of V_BE and temperature is not a linear function over the entire temperature range. Therefore, curvature compensation is necessary to meet the requirement of high precision voltage reference [4], [5]. The curvature compensation demands additional compensation circuits that makes the BGRs more complex and occupy larger area.