Enhanced active feedback technique with dynamic compensation
for low-dropout voltage regulator
Chia-Min Chen•Chung-Chih Hung
Received: 16 June 2012 / Revised: 28 December 2012 / Accepted: 18 January 2013 / Published online: 31 January 2013 Ó Springer Science+Business Media New York 2013
Abstract This paper presents a novel frequency com-pensation technique for a low-dropout (LDO) voltage regulator. Enhanced active feedback frequency compen-sation is employed to improve the frequency response. The proposed LDO is capable of providing high stability for current loads up to 150 mA with or without loading capacitors. The proposed LDO voltage regulator provides a loop bandwidth of 7.8 MHz under light loads and 6.5 MHz under heavy loads. The maximum undershoot and over-shoot are 59 and 90 mV, respectively, for changes in load current within a 200-ns edge time, while the compensation capacitors only require a total value of 7 pF. This enables easy integration of the compensation capacitors within the LDO chip. The proposed LDO regulator was designed using TSMC 0.35-lm CMOS technology. With an active area of 0.14 mm2(including feedback resistors), the qui-escent current is only 40 lA. The input voltage ranges from 1.73 to 5 V for a loading current of 150 mA and an output voltage of 1.5 V. The main advantage of this approach is the stability of the LDO circuit when external load capacitors are connected, or even without load capacitors.
Keywords Low dropout (LDO) voltage regulator Frequency compensation Loop stability Transient response Output ripple noise Power management
1 Introduction
Portable devices require an efficient power management system to extend battery life. These devices usually employ multiple on-chip voltage levels, which has led to the widespread adoption of low dropout (LDO) voltage regu-lators for electronic devices, such as MP3 players, cellular phones, and digital cameras [1,2]. LDO voltage regulators result in a small voltage drop across the power transistor and provide a well regulated low-noise voltage. These are particularly suitable for applications such as RF IC and audio IC, which require low noise [3,4]. Another advan-tage of linear regulators is the low standby current, due to the absence of switching. In portable devices, a crucial design consideration is reducing quiescent current to maximize the lifespan of the battery [5,6,7].
For different applications, LDO regulators are able to support load current capacity ranging from several hundred mA to several A. In the design of SoC solutions, a higher output voltage (e.g., 3.3 V) is usually used for I/O ports, and a lower voltage (e.g., 1.5 V) is used for digital cores, as shown in Fig. 1[8,9,10,11,12].
Recently, an increasing number of designs have focused on low supply voltage and low power consumption. A lower supply voltage is required to decrease power con-sumption and extend battery life in portable electronic devices. Conventional LDO error amplifiers must be designed to provide high gain for precision output voltage and good regulation. A cascode architecture precludes the implementation of the amplifier under low supply voltage; therefore, amplifiers with multiple stages are also usually required [13]. However, multiple amplification stages increases the number of poles, which reduces the phase margin, resulting in stability problems. Moreover, because the transconductance and resistance of the output stage of
C.-M. Chen C.-C. Hung (&)
Analog Integrated Circuit Laboratory, Department of Electrical Engineering, National Chiao Tung University, Hsinchu, Taiwan e-mail: cchung@mail.nctu.edu.tw
C.-M. Chen
e-mail: jeremy.cm97g@g2.nctu.edu.tw DOI 10.1007/s10470-013-0034-x
the LDO regulator vary according to load conditions, the phase margin must be designed high enough to maintain stability. Thus, this paper proposes a new compensation topology capable of overcoming these drawbacks. The proposed circuit provides zeros to maintain appropriate phase margins under various load conditions. This enables us to maintain the stability using the proposed compensa-tion technique, regardless of whether the LDO regulator is capacitorless or has Cout connected to the output, where [8,9,10,11,12] are only suitable for the SoC applications without off-chip capacitor, not suitable for the applications with large load capacitance. This paper discusses not only the capacitorless case but also the condition with external load capacitor. All circuit analyses and measurements have covered the both conditions. The proposed LDO regulator with enhanced active feedback technique is capable of providing load current up to 150 mA.
The proposed LDO regulator with a new compensation topology maintains high stability even without external load capacitors and features fast transient response,
enabling it to reduce undershoot and overshoot of output voltage. Section II provides a brief review on the archi-tecture typically used in linear regulators as well as a dis-cussion on stability. Operating principles, compensation methods, and details regarding the implementation of the circuits are discussed in Section III. Measurement results are provided in Section IV, followed by the conclusion in Section V.
2 Conventional topology of linear regulators
2.1 Basic architecture
An LDO regulator usually comprises a pass element (power transistor), an error amplifier, a driver, a voltage reference, and a resistive feedback network, as shown in Fig.2. The topology of an LDO is a closed loop. Series-shunt negative feedback is used to dynamically control the pass element, usually a power PMOS transistor. Because the power PMOS must provide a large transconductance and large load current, it is a large element on the order of ten thousand lm. Unfortunately, the gate capacitance of the power PMOS contributes a parasitic pole at low fre-quencies, thereby degrading the stability of the system. Because the external load capacitor is also very large, it contributes a dominant pole. Three low-frequency poles at the output of the pass element, the driver, and the error amplifier make the structure intrinsically unstable. None-theless, with the series connection of the load capacitor and its equivalent series resistance (ESR), it generates a zero, which provides the required stability, as shown by the solid line in Fig.3. If the loop gain is too high (the dashed line in Fig.3), and Po1is located before the unity-gain frequency,
Fig. 2 Topology of conventional LDO regulator
an even larger load capacitance is required to maintain stability. This compensation method usually results in a narrow bandwidth [14,15].
The LDO regulator operates as a closed loop system, which requires compensation to ensure stability under all operating conditions. The fundamental requirements for stability are: (1) the zero must be located below the unity gain frequency of the loop; and (2) all high-frequency poles must be at least three times higher than the unity gain frequency [16]. However, the ESR values differ according to the material used in the load capacitors making it very difficult to accurately control the value of the ESR.
2.2 Existing techniques
One popular compensation method is the Miller compen-sation technique, as presented in Fig.4 [17]. This tech-nique involves connecting a capacitor between the output and input of the second stage gmII. The poles of the two-stage amplifier are split, one toward low frequency and the other toward high frequency. In this manner, the dominant pole is located at the output node of the first stage. The dominant low-frequency pole, Pn1, is located at node n1 and is given by Pn1 1 ro1ðA2CmÞ ¼ 1 ro1ðgmIIro2CmÞ ð1Þ
where A2refers to the gain of the second stage. At high frequencies, the compensation capacitor Cm shunts the output to node n1, converting the input transistor of the second stage into a diode connected device. This moves the output pole toward higher frequencies
Pn2 gmII Cout
ð2Þ
A right-half plane zero also results from the topology described by (ZRHP= gmII/Cm). This zero is the result of capacitor Cm providing a noninverting feedforward path, which degrades the phase margin. Variations in load cause very wide variations in gmII, and thus in the high frequency pole (Pn2). Consequently, the Miller capacitor must be very large to keep the poles separated, which results in a very large chip area. Other compensation techniques providing amplification greater than that from the Miller effect are capable of improving the performance and reducing the area significantly [18,19,20].
3 Proposed LDO regulator
3.1 Architecture
Figure5 shows the block diagram of the proposed LDO regulator. The architecture of the proposed error amplifier is basically a two stage gain amplifier driving the power transistor. Thus, this LDO regulator can be viewed as a three stage amplifier driving a capacitive load with a par-allel resistive load. The first stage is designed as a high gain error amplifier using a folded cascode topology to enhance the dynamic range of the input and the output impedance for higher gain. The first stage provides a gain of approx-imately 60 dB. An enhanced active feedback (EAF) block comprises gmk1, gmk2, Ck, Rk. This block utilizes the com-pensation capacitor Ckby multiplying the gain factors of two stages (gmII, gmk1) to form a low-frequency pole (dominant pole for Cout= 0). Compared with traditional Miller compensation, EAF compensation appears to have one more gain stage factor, enabling the compensated capacitance Ck to have a much smaller value. This block also generates a zero Z1to cancel the non-dominant pole. The second stage, which must be able to turn the power transistor (power PMOS MPO) on and turn off quickly, provides high signal swing and 25 dB gain. The bias current of the second stage is proportional to the slew rate, while the charge time of the power transistor gate is inversely
Fig. 3 Frequency response of conventional LDO regulator
proportional to the parasitic gate capacitance; therefore, the bias circuit must be carefully designed to enhance overall efficiency. Capacitor Cmconstitutes a fast path capable of reducing response time. Due to the small value of capacitor Cm(3 pF), the combination of feed-forward path (FFP) gmf and the fast path does not produce a right-half plane (RHP) zero inside the unity-gain frequency to degrade phase margin. In addition, capacitor Cm pushes the pole of the second stage output to a lower frequency. The feedforward stage (gmf) accelerates the transient discharge time of the output terminal. We have also designed an embedded RC block (Ct1, Rt1) to add another internal zero compensation to reduce phase shift at high frequencies.
In Fig.5, P1(P01) is the dominant pole, contributed by the output of the first amplifier stage. P2(P02) represents the second pole, contributed by the output of the second amplifier stage. P3(P03) is the third pole, contributed by node n3of the enhanced active feedback. Z1(Z10) is the first
LHP zero, generated by the enhanced active feedback circuit. Z2(Z20) represents the second LHP zero, generated by compensation capacitor Ct1and compensation resistor Rt1. The pole and zero symbols marked ‘‘0’’ represent the poles and zeros generated under heavy loads; otherwise, they refer to light load conditions.
The negative gain of the output stage (power transistor) depends heavily on the load. The gain of the power tran-sistor (gmpRop) is inversely proportional to
ffiffiffiffiffiffiffi Iout p
. The dis-tribution of gain among the stages is an important consideration. To prevent front stages from forwarding noise, DC gain is distributed such that AV1[ AV2[ AVP. A resistive feedback network was added at the output of the three stage amplifier to form a negative feedback system. The feedback resistors must be large enough to reduce quiescent current for low power design. This architecture requires a high loop gain to provide high-precision LDO output voltage.
Fig. 5 Block diagram of the proposed LDO regulator without external load capacitor
3.2 Analysis of stability without external load capacitor
The circuitry of the proposed LDO regulator is detailed in Fig.6. Transistors M1–M8form the first stage of the error amplifier while transistors M9–M12form the second stage. The dominant pole is contributed by the output of the first stage. Capacitor Ck, the transconductance of the second stage gmII, and the gain stage gmk1 introduce the
pole-splitting effect, which pushes the pole of the first stage output toward very low frequencies, resulting in a new dominant pole for the system. Meanwhile, the pole pro-duced by the output node of the negative gain stages gmk1 and gmk2is pushed toward a higher frequency, resulting in the third pole of the feedback loop. The feedforward stage (gmf) reduces the output impedance at the output node of the LDO regulator. Compensation capacitor Cm splits the poles at the input and output nodes of the power transistor
Fig. 7 Simulated loop gain and phase responses of the proposed LDO regulator under different loads without external load capacitor (Cout= 0,
stage. Thus, the pole at the LDO regulator output node is pushed toward frequencies higher than the unity gain fre-quency, by the transconductance of the power transistor and compensation capacitor Cm. The pole at the power transistor MPO gate node is then pushed toward lower frequencies, forming the second pole of the feedback loop. Compensation resistor Rk, compensation capacitor Ck, negative gain stages gmk1, gmk2, and gmIIintroduce a first LHP zero, Z1. The first LHP zero cancels the third pole.
Resistor Rkand capacitor Ckallow independent adjustment over the placement of the first zero. Resistor Rt1 and capacitor Ct1 can also introduce a second LHP zero, Z2. The second LHP zero is presented to improve the phase margin of the LDO regulator. Resistance Rt1 is the small signal equivalent resistance of the transistor Mt1. Capacitor Ct1also allows independent adjustment over the placement of the second zero. All of the major zero and pole rela-tionships are indicated in Fig.5.
Fig. 8 Simulated loop gain and phase responses of the proposed LDO regulator under different loads using external load capacitor (Cout= 10 lF,
Figures7 and 8 are the simulated loop gain and phase responses of the proposed LDO regulator under different loads with different Cout conditions. The frequency response of the proposed LDO regulator without external load capacitor is divided into two states: light load and heavy load, as shown in Fig.9. The open loop gain transfer function of the proposed structure is given by
HðcaplessÞð Þ s ADC 1þzs 1 1þs z2 1þ s P3dB 1þ s p2 1þs p3 ð3Þ ADC ¼ gmIgmIIgmpRo1Ro2Rop Rf 2 Rf 1þRf 2 ð4Þ P3dB¼ P1¼ P01¼ 1 Ckgmk1gmIIRokRo1Ro2 ð5Þ P2¼ P02 ¼ gmIIgmk1 Cmgmpgmk2Rop Cmgmk1gmfRopþ Ct1gmIIgmk1Rt1 ð6Þ P3 ¼ P03 ¼ gmk2Ck Co1Cokþ CkCo1þ CkCok ð7Þ Z1 ¼ Z01¼ gmk2ðgmIIþ gmk1Þ Ck½ðgmk2 gmII gmk1Þ gmk2ðgmII þ gmk1ÞRk ð8Þ Z2 ¼ Z02¼ 1 Ct1Rt1 ð9Þ UGF¼ UGF0¼gmIgmpRop Ckgmk1Rok Rf 2 Rf 1þ Rf 2 ð10Þ
where gmI, gmII, gmp, Co1, Co2, Cop, Ro1, Ro2, and Ropare the transconductance, output capacitance and output resistance of the three stages, respectively. Because Co2 and Copare smaller than the other parameters in the derivation process and are thus neglected, these two symbols do not appear in the above equations.
The elements gmk1, gmk2, Ck, and Rkrepresent the trans-conductance, compensation capacitance, and compensation
Fig. 9 Frequency response of the proposed LDO regulator with Cout= 0
Fig. 10 Block diagram of the proposed LDO regulator with external load capacitor
Fig. 11 Frequency response of the proposed LDO regulator with Cout= 0
resistance of the enhanced active feedback block, respec-tively. The small signal elements Ct1 and Rt1 are the equivalent capacitance and equivalent resistance of the embedded RC block. gmfand Cmare the transconductance of the feedforward stage and the fast path capacitance, respectively.
The unity gain bandwidth location changes with load current. When there is no load capacitor, the loop band-width is larger. This large bandband-width improves signal transient response at Cout = 0 resulting in fast line transient
and load transient states. The loop bandwidth under light load is larger than under heavy load, which degrades sta-bility under light loads. The proposed LDO voltage regu-lator provides a loop bandwidth of 7.8 MHz under light loads and 6.5 MHz under heavy loads. Three poles and two LHP zeros were designed in the loop, as shown in Fig.9. Pole-zero cancellation was achieved by Z1 and P3. Z2 reduces phase shift and maintains an adequate phase mar-gin. Even when heavily loaded, the system still provides three poles and two zeros, which ensure that stability is maintained.
3.3 Analysis of stability using external load capacitor
The other situation involves load capacitors at the output terminal of the LDO regulator. Figure 10shows the block diagram of the proposed LDO regulator with external load capacitor, which also illustrates all of the major zero and pole relationships. Under light loads, the dominant pole (P1) is formed by the large load capacitor and the second pole (P2) is contributed by the output of the first stage, while the dominant pole (P01) is contributed by the output of the first stage and the second pole (P02) is formed by the large load capacitor under heavy loads. The third pole (P3(P03)) is contributed by the output of the second stage, the first LHP zero (Z1(Z10)) is formed by the load capacitor and the ESR, and the second LHP zero (Z2(Z20)) is formed by the EAF circuit. The pole and zero symbols marked ‘‘0’’ represent the poles and zeros generated under heavy loads; otherwise they refer to light load conditions. The transfer function can be derived, and three poles and two LHP zeros are shown as follows.
Fig. 12 Microphotograph of the proposed LDO
Fig. 13 Measured load regulation of the proposed LDO
ADC ¼ gmIgmIIgmpRo1Ro2Rop Rf 2 Rf 1þRf 2 ð11Þ P1¼ P02¼ 1 CoutResrþ Rop ð12Þ P2¼ P01¼ 1 Ckgmk1gmIIRokRo1Ro2 ð13Þ P3¼ P03¼ gmIIgmk1Resrþ Rop Cmgmpgmk2Resrþ Rop Cmgmk1gmf Resrþ Rop þ Ct1gmIIgmk1Rt1 2 6 4 3 7 5Rop ð14Þ Z1¼ Z10 ¼ 1 CoutResr ð15Þ Z2 ¼ Z02¼ gmk2ðgmIIþ gmk1Þ Ck½ðgmk2 gmII gmk1Þ gmk2ðgmII þ gmk1ÞRk ð16Þ UGF¼gmIgmIIgmpRo1Ro2Rop Cout Resrþ Rop Rf 2 Rf 1þ Rf 2 ð17Þ UGF0¼gmIgmpRop Ckgmk1Rok Rf 2 Rf 1þ Rf 2 ð18Þ When load capacitors are added, the loop bandwidth is reduced. When the load current is small, the system gen-erates two low frequency poles and one zero located at a frequency below the unity-gain frequency. When the load current increases continuously, the dominant pole P1shifts to higher frequency pole P02, and pole P03 show up below the unity gain frequency. The proposed EAF compensation network creates one new dominant pole P01 at lower
Fig. 14 Dropout voltage performance at 150 mA load current
Fig. 15 Measured line transient response and line regulation with
frequencies and one new zero Z20 to cancel out pole P03, as shown in Fig.11. When Cout = 10 lF, the proposed LDO voltage regulator provides a loop bandwidth of 71.4 kHz under light loads and 139 kHz under heavy loads, respec-tively. Nevertheless, the circuit must still be rendered carefully to guarantee adequate phase margin to avoid oscillation and over-ringing instability during supply volt-age startup or transient state change.
4 Measurement results
The LDO regulator was implemented using TSMC 0.35-lm standard CMOS technology. Figure12 shows the die microphotograph of the proposed LDO regulator, with an effective die area of 0.14 mm2, including feedback resis-tors. The output voltage of the regulator is shown in Fig.13
as the load current is varied from 0 to 150 mA. The output voltage of the regulator is shown in Fig.14as the supply voltage is varied from 1 to 5 V. The DC load and line regulation performance of the LDO regulator are 0.187 mV/mA and 2.5 mV/V, respectively. The high loop
gain architecture of the LDO limits error in the output voltage. The negative feedback of the LDO regulator adjusts the source-gate voltage of the power PMOS with the change of load current. A continuous increase in load current decreases the gate voltage of power PMOS until it reaches zero, at which point the LDO regulator is unable to regulate. In this design, dependent on the aspect ratio and the maximum source-gate voltage of the power PMOS, the maximum driving capacity of the LDO regulator is larger than 150 mA.
To characterize the line transient response, a supply voltage pulsating between 1.8 and 3.8 V was applied to the input, and the effect at the output was measured. The results are shown in Fig.15. Figure16 demonstrates that the proposed LDO regulator largely reduces input ripple noise from 200 to 5 mV at 1 MHz, which corresponds to a 40-fold reduction.
Another important characteristic is the load transient response. When the load current is rapidly pulsed from 0 to 150 mA and from 150 mA to 0 under capacitor-less con-ditions, the maximum variations in transient output are 59 and 90 mV, respectively, as shown in Fig.17. When the
Fig. 18 Measured load transient response with Cout= 1 lF
regulator output was connected to a 1 lF load capacitor, the maximum variations in transient output were 37 and 30 mV for a transient load current step between 0 and 150 mA, as shown in Fig.18. When the regulator output was connected to a 10 lF load capacitor, the maximum output recovery times were 17 and 40 ls for a transient load current step between 0 and 150 mA, as shown in Fig.19. The rise/fall time of the 0 to 150 mA load current pulse was 200 ns.
A summary of the proposed LDO regulator performance is shown in Table1. The input voltage ranges from 1.73 to 5 V. The LDO regulator is capable of operating down to 1.73 V with a preset output voltage of 1.5 V. The quiescent current is only 40 lA. The dropout voltage for the worst case load current (150 mA) is 230 mV. The power supply rejection ratio (PSRR) is less than -71 dB for frequencies up to 100 Hz. The equivalent output noise at 100 Hz is approximately 3.24 lV ffiffiffiffiffiffiffipHZ. Table 2presents the speci-fications of the proposed LDO regulator and a comparison
with other published designs. The proposed regulator achieved performance in maximum load current (150 mA), quiescent current (40 lA), line regulation (2.5 mV/V), and Figure of Merit (FOM = 0.01 ps) superior to that afforded by other designs. A lower FOM implies better transient response.
5 Conclusion
This paper proposes a novel frequency compensation net-work design for a low dropout voltage regulator and measurement data are presented. Stability in the LDO
Fig. 19 Measured load transient response with Cout= 10 lF
Table 1 Summary of the LDO Performance Supply voltage 1.73–5 V Output voltage 1.5 V Max. output current 150 mA Quiescent current 40 lA Dropout voltage 230 mV
AVout-transient 59 mV, COP= 100 pF @ DIL= 150 mA
Line regulation 2.5 mV/V Load regulation 0.187 mV/mA
PSRR -71 dB @ 100 Hz -69.5 dB @ 1 kHz -63 dB @10 kHz Output noise 3.24 lV ffiffiffiffiffiffiffipHZ@ 100 Hz COUT 100 pF (COP)/1 lF/10 lF Active area 0.14 mm2
Table 2 Comparison with previously published LDO regulators
Parameter [8] [9] [10] This work
Technology 90-nm CMOS 0.35-^m CMOS 0.35-fim CMOS 0.35-nm CMOS
Pass element PMOS PMOS PMOS PMOS
ILoad(mA) 100 100 50 150 Quiescent current (IQLuA) 6,000 170 65 40 Dropout coltage (mV) 300 200 200 230 COP(pF) 600 pF 100 pFa 100 pFa 100 pFa Response time TR (ns)b 0.54 0.06 0.18 0.039 Load regulation (mV/mA) 1.8 0.02 -0.56 0.187 Line regulation (mV/mA) N.A. 3.3 -23 2.5 FOM (ps)b 32.4 0.102 0.234 0.01
a The estimated maxmimum output capacitance C
OP= 100 pF from
parasitics and test equipments
b Adopted from [8], T
R= COUT9 VOUT/ILOAD(Max.) and FOM =
regulator using enhanced active feedback techniques and embedded RC blocks can be achieved with or without loading capacitors. The LDO regulator adopts both a feedforward path and a fast path to achieve fast load transient responses and small overshoots and undershoots. The design provides large loop gain to improve line and load regulation. Compared with previously published LDO regulators, the proposed approach provides low quiescent current and high speed transient response as shown in Table2. The FOM of this regulator compares favorably to other published works.
Acknowledgments The authors would like to thank the National Chip Implementation Center for supporting the chip fabrication. This work was sponsored by the National Science Council.
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Chia-Min Chen received the M.S. degree in electrical engi-neering from National Chiao Tung University, Hsinchu, Taiwan, in 2006. He is currently working toward the Ph.D. degree at the Department of Electrical Engineering, National Chiao Tung University, Hsin-chu, Taiwan. His research interests include mixed-signal integrated circuit design, power management integrated circuits, and energy-efficient integrated circuits and systems.
Chung-Chih Hungreceived the B.S. degree in electrical engi-neering from National Taiwan University, Taipei, Taiwan, in 1989, and the M.S. and Ph.D. degrees in electrical engineering from The Ohio State University, Columbus, Ohio, in 1993 and 1997, respectively. From 1997 to 2003, he worked for several IC design companies in San Jose, CA and San Diego, CA, where he held analog circuit design manager and director positions. Since 2003, he has been with National Chiao Tung University, Hsinchu, Taiwan, where he is currently a Full Professor in the Department of Electrical Engineering. His research interests include the design of analog and mixed-signal integrated circuits for communication and high-speed applications.
