> IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS –II EXPRESS BRIEFS, PAPER ID NUMBER: 1 A CMOS Fast Settling Time Low-Noise LowDropout Regulator with Current Limiter Praveen Nadimpalli, Dharma Kadam, Bertan Bakkaloglu, Member, IEEE, Wonseok Oh, Member, IEEE Abstract—This paper presents a low-dropout regulator (LDO) for portable applications with current limiter. To minimize effect of bandgap noise, novel low pass filter associate with bandgap reference circuit, which provides highly filtered reference voltage and fast settling time, is implemented. Dynamically operating current limiter using decent current comparator limits output current. Threshold current of current limiter is set to 230mA of output load current. The proposed LDO with low pass filter and current limiter has been implemented in a 0.6um n-well CMOS process. The proposed LDO dissipates 65uA quiescent current at 150mA full load condition and its output noise, PSR, and load regulation are 407.8nV/√Hz at 100Hz, 51.2dB at 10kHz, and 43mV/150mA respectively. The maximum transient output voltage variation is within 3.2% of the output voltage with load step changes of 150 mA and 1µF output capacitor. Index Terms—low-dropout regulators, reference voltage, noise, power supply rejection L I. INTRODUCTION ow dropout voltage regulator (LDO) is widely demanded in portable electronics market. These LDOs require providing strict characteristics such as low noise, high PSR, and fast settling time. The baseband and analog sections of RF transceivers, especially transmitter power control DACs and receive channel ADCs have finite PSR; therefore, at lower supply rails, they require power supplies which support high PSR -. In addition of analog sections, noise and crosscoupling on the power supply line for RF circuitry starts playing a dominant role in an RF transceiver noise budget. LDOs should provide low noise power line at low frequencies to RF circuit -. Conventional LDOs utilize highly filtered voltage references at their inputs and bypass capacitors at their outputs in order to minimize output noise of voltage reference which is dominant noise source of LDO. This required large Manuscript received June 24, 2008. W. Oh is with the RF Micro Devices, Inc. Chandler, AZ 85226 USA (corresponding author to provide phone: 480-612-5900; fax: 480-763-2111; email: woh@ rfmd.com). P. Nadimpalli is with the RF Micro Devices, Inc. Chandler, AZ 85226 USA (e-mail: PNadimpalli@ rfmd.com). D. Kadam is with the RF Micro Devices, Inc. Chandler, AZ 85226 USA (email: dkadam@ rfmd.com). B. Bakkaloglu, is with Arizona State University, Tempe, AZ 85287 USA. (email: [email protected]). capacitor, especially the capacitor at their inputs is required gigantic values more than 1µF, which can not be integrated and it causes to increase board area. It results in high product cost as well as slow turn-on time -. In power management IC, in order to protect the IC from an y damage when an overload condition is happened or the output of the regulator is shorted, current limiting circuits are required in LDO design -. The current limiter should provide fast response and less variation over process and temperature variation. In this paper, we propose bandgap voltage reference with novel low pass filter, which provides highly filtered voltage reference and fast turn-on time. To protect ICs from overflowing current, current limiting circuit is implemented. The remainder of the paper is organized as follows. Bandgap reference with low pass filter is introduced in Section II. Section III presents the system and circuit level description of the proposed LDO design with current limiter. Section IV presents the silicon evaluation results and conclusion is addressed Section V. II. BANDGAP REFERENCE WITH LOW PASS FILTER A. Noise Sources of LDOs Fig. 1 shows a block diagram of a typical LDO regulator, which consists of an error amplifier, a pass device, a high resistance feedback network and an output capacitor Co. The PMOS regulation FET at the output is configured in commonsource configuration. Due to its high transconductance value and large geometry device, input referred noise of the regulation FET, Sn,p(f), can be ignored. The input referred noise power spectral density (PSD) of the error amplifier is Fig. 1. Block diagram of a typical LDO regulator depicting major noise contributors. Copyright © 2008 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending an email to [email protected] > IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS –II EXPRESS BRIEFS, PAPER ID NUMBER: denoted by Sn,e(f), and total output noise of the voltage reference is defined by Sn,ref(f). Total output noise spectral density of the LDO system Sn,o(f) is represented by 2 2 ⎛ R⎞ ⎛R ⎞ (1) Sn,o( f ) = Sn,ref ( f ) +Sn,e( f ) ⎜1+ 1 ⎟ +Sn,R2⎜ 1 ⎟ +Sn,R1 ⎝ R2 ⎠ ⎝ R2 ⎠ From (1), most critical noise contributor of LDO is reference noise Sn,ref(f) and its flicker (1/f) noise is most critical noise source of LDO at low frequencies. Assumed that target spot noise of LDO at 100Hz is 500nV/√Hz at 100Hz and the desired output voltage of LDO is 2.8V and reference input voltage is 1.226V. So, LDO needs around 2.284 gain stage which is implemented by resistor networks (R1 and R2). Then, LDO itself provides 100nV/√Hz at 100Hz. So, noise of reference circuit should not exceed 175nV/√Hz at same frequency. It means that reference circuit requires also low noise profile at low frequencies. To suppress noise of LDOs, two popular techniques can be applied in standard CMOS process. The first scheme is to use low pass filter following by reference circuit. Using low pass filter, however, bandgap reference noise can be suppressed by sufficient filtering in the expense of increasing setting time and board area due to large filter capacitor. Another method is to use circuit technique such as chopper stabilization. This technique provides excellent noise characteristics, but also generates unwanted characteristic such as ripple voltage due to chopper switches. In order to suppress ripple voltage, low pass filter is also required -. ( 2 ) B. Bandgap Reference with Low Pass Filter Bandgap reference circuit provides very stable voltage reference in regard to both temperature and power supply variations. In LDO design, voltage reference circuit is one of most critical block since it is most dominant contributor of LDO characteristics such as output voltage accuracy and noise profile in regard to both temperature and process variation. Fig 2 shows the block diagram of proposed reference circuit with low pass filter. It consists of bandgap voltage reference, buffer, comparator, and low pass filter composed with MOS transistors M2~M3 and Cfilter. The main key operation of proposed reference circuit depends on comparator which enables low pass filter. The comparator compared bandgap voltage VBG and delayed bandgap signal VBG_d. M1 sets the gate voltage of M3 by Vb so that M3 operates in subthreshold region and generates high resistance. When VBG is higher than VBG_d, Vc is zero. Then, Fig. 2. Block diagram of proposed reference circuit with low pass filter. Fig 3. Block Diagram of Low Dropout Regulator with Current Limiter. M2 operates in triode region and generate small resistance. The corner frequency of low pass filter is located at higher frequencies and it provides fast settling time since total resistance of M2 and M3 is almost same as that of M2. Then after a few µsec delay, VBG_d is higher than VBG. Then Vc is goes to high voltage. M2 operate in sub threshold region. M2 generate very high resistance in order to set the corner frequency of low pass filter at lower frequencies without reference voltage variation since both resistance of M2 and M3 are high value. So, highly filtered reference voltage with fast setting time is achieved. III. LOW DROPOUT REGULATOR WITH CURRENT LIMITER The proposed low-noise LDO (LN-LDO) is composed of a voltage reference, an error amplifier, regulation FET (MP), and feedback network. In order to protect overflow current, current limiter is implemented. Fig. 3 shows the block diagram of the proposed LDO with current limiter. The following sections describe the building blocks. A. Low Dropout Regulator The proposed LN-LDO consists of error amplifier, regulation FET and feedback network resistor. Fig 4 shows the schematic of LN-LDO. To achieve fast transient response, current boost amplifier is utilized to reduce impedance of VPG node and provide large output swing and large slewing current which is a function of transient response time. To assure stability of LN-LDO, external compensation technique is adopted by using output capacitor associated with ESR to introduce zero near unity gain frequency. Fig 4. Schematic of LN-LDO > IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS –II EXPRESS BRIEFS, PAPER ID NUMBER: 3 Fig 6. Schematic of proposed current limiter Sn _1/ f ( f ) = Fig 5. Stability response of proposed LN-LDO. The loop gain LG(s) of LN-LDO is represented by: LG ( s ) = V fb ( f ) Vref ( f ) = A(0) (1 + s ω z1 ) (1 + s ω ) ⎡⎣1 + s ω p1 ⎤ p2 ⎦ (2) where 1 ωp = 2π Co ( rdsmp + Resr ) ω z1 = 1 2π Co Resr 1 , ω p2 A(0) = Ng m1 g mp ( ro 6 ro8 ) rdsmp GBW ≈ Ng m1 g mp ( ro 6 ro8 ) 2π Co γ gm + R2 R1 + R2 R2 R1 + R2 K WLCox f ⎛ µn ⎞ KM 3 LM 1 ⎜⎜ ⎟⎟ 2 ⎝ µ p ⎠ WM 1LM 3 ⎛ µn ⎞ KM 6 LM 1 ⎜⎜ ⎟⎟ 2 ⎝ µ p ⎠ WM 1LM 6 (4) Form (4), large geometry input devices (M1 and M2) are utilized to achieve low noise at the output of LDO. In addition, M3~M6 are also dominant noise source. Therefore, these devices should be large geometry to reduce noise at low frequencies. 1 = 2π C p ( ro 6 ro8 ) From (2), external compensation scheme using ESR resistor associated with output capacitor adds a zero (ωz1) close to ωp2. Therefore, the lineup for the bandwidths are ωp1<ωp2, GBW<ωz1. The locations of ωp2 and ωz1 and GBW are quite close each other. Therefore, the stability of the system can be achieved although phase margin is less than 60deg. The minimum phase margin of LN-LDO is around 35deg at full load condition. Fig 5 shows the simulation results of stability response. Assuming that reference noise is completely filtered out, dominant noise source of LDO is an error amplifier which is CMOS device. Input refereed noise of single CMOS transistor is well known as following. S n ( f ) = 4kT N + Cox f KM 1 3 + Cox f (WL )M 1 Cox f 2 (3) where γ is noise factor, K is a process dependent constant of device. W, L, and Cox represent the transistor’s width, length and oxide capacitance, respectively. From (3), dominant noise source at low frequencies is second part, which is flicker (1/f) noise. The dominant flicker noise contributor of proposed LDO is M1~M6 and input referred 1/f noise from these devices represents as B. Current Limiter In power management ICs, LDO requires current limiter to protect overflow current. As seen Fig 3, the proposed current limiter senses the current (Isense) which is ratio of output current. This sensed current is compared to reference current (Iref), then, generates control signal (VCL). When Isense is smaller than Iref, VCL goes to high and Mpull-up device is off. When Isense is larger than Iref, VCL goes to low and Mpull-up turns on. So, VPG signal goes back to high level to prevent over current flow. Fig 6 shows the schematic of proposed current limiter. As seen Fig 3, sensing device MPS is much smaller than pass device MP. These two devices share the gate-source voltage and operate as current mirror. So, sensing current (Isense) is L times smaller than load current (Iref). However, drain-source voltage mismatch causes the variation of sensing current over load current. To minimize the difference between Iout and Isense, the current sensing part utilize regulated current mirror which is composed with M3~M6 devices and two resistors in Fig 6. This sensing current is copied to one input (M13) of current comparator and compared with reference current of other input (M12). Then, the output of comparator generates control signal (VCL), which controls the pull-up device (Mpull-up) in Fig. 3. This current comparator gives faster response with low current consumption than conventional voltage reference. IV. EXPERIMENTAL RESULTS The proposed LN-LDO IC is designed and fabricated in a 0.6µm digital CMOS process with three layers of metal. The LDO is designed to source a nominal output current of > IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS –II EXPRESS BRIEFS, PAPER ID NUMBER: 150mA. With the help of highly filtered bandgap reference, the output noise of the proposed LDO is quite small value which is able to utilize supply line for RF application. Fast settling time reference voltage results in fast turn-on time of LDO. To protect LDO from overflow current, current limiter is involved. Fig. 7 shows the turn-on settling time response of proposed LN-LDO. The achieved turn-on settling time is 45µsec. This settling time is around 220 times faster than that of RC filter which utilized off-chip2.2µF filter capacitor. Output noise and PSR is measured at the full load condition. The output noise spectral density is measured to be 407.8nV/√Hz at 100Hz. Fig. 8 shows the measured output noise spectrum. Fig. 9 shows the transient response to 150mA load current step. The proposed LN-LDO achieves 43mV/150mA load regulation. Transient response time over full load step is 12µsec for rising load current and 2µsec for falling load current. The LDO achieves 52.1dB PSR at 10kHz offset. Fig. 10 shows the PSR response of the LN-LDO at 150mA loading condition. Fig. 11 shows current limiter operation. Threshold current of current limiter is 230mA. Fig. 12 shows the micrograph of designed LN-LDO. Table 1 shows the summary of the measured performance of proposed LN-LDO. Fig. 9 Measured load regulation of proposed LN-LDO with 150mA load current step. Fig. 10. Measured power supply rejection (PSR) of proposed LN-LDO with 150mA load current. Fig. 7 Turn-on settling time response of proposed LN-LDO with proposed low pass filter and conventional RC filter. Fig. 11. Current limiter operation of proposed LN-LDO. Fig. 8.Measured LDO output noise comparison with proposed low pass filter and conventional RC filter at 150mA load current. Fig. 12. Micrograph of the LN-LDO and associated circuits. 4 > IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS –II EXPRESS BRIEFS, PAPER ID NUMBER: Table 1. Summary of the measured performance. Process 0.6µm CMOS Chip Area 228 µm X 151 µm Vin 3.1~4.5V Vout 2.8V ∆Vout 87mV Co 2.2µF ESR 0.01 ohms Full Load Transient Response Time 12µsec Turn-on Settling Time 45µsec Output Noise 407.8nV/√Hz @100Hz Max. Output Current Imax 150mA Quiescent Current IQ 67µA Current Efficiency 99.96% Current Limit 230mA Power Supply Rejection >[email protected] V. CONCLUSIONS A low noise linear low dropout regulator with a fast settling response voltage reference and current limiter is presented. The proposed LN-LDO achieve low noise, fast settling time under the help of highly filtered voltage reference adopting novel integrated low pass filter. Current limiter protects LDO from overflow current. REFERENCES          G.A. Rincon-Mora, and P. E. Allen, “A Low-Voltage, Low Quiescent Current, Low Drop-Out Regulator,” IEEE J. Solid State Circuits, vol. 33, no.1, pp.36-44, Jan.1998. H. Lee, P. K. T Mok, K. N. Leung, "Design of Low-Power Analog Driver Based on Slew-Rate Enhancement Circuit for CMOS LowDropout Regulators," IEEE Trans. on Circuits and Systems II, vol. 52, no 9. pp. 563 – 567, Sept. 2005. K. N. Leung., P. K. T. 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