A Loop-Improved Capacitor-Less Low-Dropout Regulator for SoC Power Management Application

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CM
V1
m1Vin C1 R1 -gm4V1
C4 R4 gm2V1
C2 R2
-gm3V2 Cpar R3
Figure 2. Small-signal model of the DFCFC LDO structure
Fig. 2 illustrates the small signal model of this LDO. gmi, Ri and Ci represent the transconductance, output resistance,
Equation (1) can equal to
G(S) ≈
Adc¨¨©§1
+
S Z1
¸¸¹·¨¨©§1 +
S Z2
¸¸¹·
¨¨©§1 +
S P1
¸¸¹·«¬ª1+ S
(CM + Cpar)gm4C2 gm2 gm3CM
II. PROPOSED LDO TOPOLOGY
The structure of the proposed LDO is shown in Fig. 1, including a high gain error amplifier as the first stage, a large swing range buffer as the second stage, a PMOS pass device, a resistance feedback network, and DFC block between the first and second stage.
Considering relatively poor stability of cap-less LDO, our study used Miller compensation technology to set a low frequency dominant pole internally, and configured two other non-dominant poles slightly higher than unity gain frequency(UGF) by DFC block. LDO loop could have only one pole within UGF, and have a proper phase margin, so as to assure system stability and optimize its dynamic performance. The circuit designed by HHNEC 0.25um CMOS technology, was applied to the target SoC chip, to solve the problem of its different regions power supply.
Wuhan, China seion@
Abstract—Stability is the major obstacle for capacitor-less lowdropout regulator (LDO). By using Miller compensation, a low frequency dominant pole is internally generated, and two other non-dominant poles, which frequency are higher than unity gain frequency (UGF), can be configured by Damping-Factor-Control (DFC) block. With opposed zero cancellation, single pole system is formed before UGF and satisfied phase margin is achieved, hence both the system stability and optimized transient performance are ensured. Based on DFCFC, a 1.8V 100mA capacitor-less LDO was designed by using HHNEC 0.25ȝm standard CMOS process in this paper. Simulation results showed that the improved regulator could provide a full load transient response of 2ȝs settling time and both overshoots and undershoots less than 70mV. Furthermore, 50mV dropout voltage, 40ȝA quiescent current, and smaller compensation capacitors cater to low power and low cost SoC application.
Keywords-LDO; Capacitor-less; frequency compensation; DFC
I. INTRODUCTION
Low-dropout voltage regulator (LDO) has been characterized by its low noise, high accuracy and fast-response performance and thereby is widely utilized in many electronic products. With System-on-Chip (SoC) technology developing, LDO design has become more challenging due to the requirement of removing output capacitor. Traditional LDO relies mainly on its output capacitor and equivalent series resistance (ESR) which together form a left-half-plane (LHP) zero[1, 2] to ensure stability. Thus both of them increases the utilization complexity of LDO because application engineer must choose the output capacitor with proper ESR. Additionally, the capacitor takes more PCB area and raises hardware cost. Especially, more than one LDO is adopted to fulfill the requirements of different modules in SoC, achieving optimization of local performance and total efficiency. As the development of SoC’s scale and complexity, chip leads and peripheral components have to be reduced, so as to cut down the cost of manufacture and application. Therefore, it becomes a tendency to utilize capacitor-less LDO[3-6] in SoC.
G(S) ≈
gm1gm2 gm3R1R2 R3¨¨©§1 − S
CM gm2 gm3R2
−
S2
CM C2 g g m2 m3
¸¸¹·
.
(1)
S 3R1R2R3CM C2Cpar + S 2 R1R2 R3C2 (CM + C par )gm4 + SR1R2R3CM1gm3gm2 + (1 + gm4R1)
Recently, some studies on the design of cap-less LDO stability have been taken and made great achievements, such as low turn-on resistance pass device technology[7], Pole-Control frequency compensation(PCFC)[2], dynamic zero compensation[6], multi-zero-ploe pairs compensation[3], active
This work was supported by the National High Technology Research and Development Program (863) of China (2006AA01Z226)
capacitor compensation[5], Damping-Factor-Control (DFC) compensation[6], etc. However, dynamic performance and light load stability still need to be improved.