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Regulador de voltaje -LDO- completamente integrado de baja caida de tension, alta velocidad y bajo consumo de area.

An area efficient high speed, fully on-chip low dropout -LDO- voltage regulator

1. Introduction

Power management system has an important role on the performance of any mixed-signal circuit (Rincon, 2009). One of its tasks is to extend the battery life and therefore the operating time of the device, varying between different supply voltages depending on the activity of the chip. System on chip (SoC) is a common solution to integrate many functions that can switch simultaneously with the clock. Switching of numerous devices demands both high power and fast response times. Therefore, integrated circuits need regulators in power management in order to avoid substantial voltage and current variations across time and over a wide range of operating conditions (Lam & Ki, 2008; Rincon, 2009).

Regulators can be classified in linear and switching. Switching regulators has a mixed signal nature combining analog and digital functions (Chia, et al., 2012). On another hand, linear regulators linearly modulate a conductance of a series pass switch connected between the input and the output of the circuit, being faster and less noisy than switching counterpart (Rincon, 2009). Low-dropout regulators (LDO) are a type of linear regulators very common in many power management systems, because they can produce a ripple-free output voltage with short transient responses, even considering fast load changes such as high speed memories.

A LDO regulator can be seen as a negative feedback amplifier composed by two or more single stages, in which each one is biased in the saturation region typically, in order to develop a high speed and gain. However, to achieve a low-dropout voltage, the aspect ratio of the power transistor --the last stage--could be greater than 40000 for a load current of 100mA (Lovaraju, et al., 2013; Ming, et al., 2012), which implies a large area consumption as well as a large parasitic capacitance (Gutierrez, 2008). In addition, parameters such as transconductance and output impedance of the power transistor vary more than 500% between full-load and light-load conditions, producing that the open-loop gain, phase margin and GBW of the regulator change dramatically; this could affect the stability of the circuit for any load changes (Man, et al., 2008). To overcome these problems, a common alternative is to use a capacitance at the output of the LDO to ensure stability and reduce voltage spikes. However, due to its high value (several picofarads or almost nanofarad), this capacitance needs to be an external component, reducing the bandwidth of the regulator.

For those reasons, in this paper the design of low area and high speed LDO regulator is presented. The area improvement is achieved due to the use of a power transistor biased in the triode region. Also, dynamic and adaptive bias schemes, as well as a high slew-rate differential pair, are used in order to reduce the settling time of the circuit for any load and input voltage variation. In addition, the stability of the regulator is ensured by using indirect compensation avoiding the need of external components.

2.Voltage regulator topology

Figure 1 shows the topology of the designed regulator. It consists of an error amplifier, a power transistor, and dynamic and adaptive biasing networks.

2.1 Power transistor

The basic equation for the drain current of a MOSFET in saturation is:

The dropout voltage of the regulator is the drainto-source voltage [V.sub.DS] of the power transistor [M.sub.Power], and its minimum value corresponds to the overdrive voltage [V.sub.OV] of equation 1. Therefore, in order to provide a high output current with a low dropout voltage--which means a low overdrive--a very high aspect ratio W/L is needed. This implies large area consumption as well as parasitic capacitances.

On the other hand, the relationship between the drain current and VGS and VDS in the triode region is: (2)

[I.sub.DS] = [K.sub.P] W/L(([V.sub.GS] - [V.sub.TH])[V.sub.DS] - 1/2 [V.sup.2.sub.DS]) (2)

So, for a given [V.sub.DS]--dropout voltage-- there is not necessary to increase the aspect ratio W/L because there is another degree of freedom corresponding to [V.sub.GS]. Hence, it is possible to minimize the area of the power transistor by applying a high [V.sub.GS].

2.2 Error amplifier

The error amplifier is a one-stage current mirror OTA, as figure 2 shows. We propose the use of a source-cross-coupled differential pair as the input stage composed by [M.sub.1, 2, 3, 4] and [M.sub.13, 14, 15, 16] (Baker, 2010). Its main advantage is the high slew-rate that it can achieve using a very low quiescent current needed to drive the input capacitance of the power transistor. In many common amplifiers the slew-rate is limited by the bias current of both the input differential pair and output branch; then, for driving high capacitive loads it is necessary to use an output-stage as a class AB one (Rijns & Wallinga, 1990; Yavari, 2010). However, in a source-cross-coupled amplifier the tail current depends on the amplitude of the input signal: if [] is much higher than []--the slewing condition--the current through M2 and M3 does not copy the current from M15 and rises without any limit; then, the current of [M.sub.21] and [M.sub.20] increases too, providing a high slewrate at the output node. When the transient state finishes, the gate voltage of [M.sub.1, 13] and [M.sub.2,14] tends to be equal, and the current flowing through the pair is set by the current mirror composed by [M.sub.17, 18, 19] i.e. by the source [I.sub.REF]. This reference can be set as low as the needed to guaranty a maximum steady-state error and noise level, thus minimizing the power consumption.

Finally, capacitors [C.sub.C1] and [C.sub.C2] are used to compensate the regulator. The circuit uses indirect miller compensation due to the high variation in both gain and bandwidth with load. Transistors M20,21 and M22,23 --compound transistors--forms the low-impedance node and current buffer which are needed to eliminate the right-plane zero; therefore, phase and gain margin are improved.

3. Dynamic and adaptive biasing

The fact of using a power transistor biased in triode region to deliver the output current of the regulator implies that the circuit could have a lower bandwidth and therefore a higher settling time. A triode-MOSFET-transistor develops almost five times less transconductance than a saturated device; so, recovery time as well as open-loop gain can be compromised.

Figure 3 shows a dynamic bias scheme--[M.sub.(26...29)] (Ho, et al., 2011) and [M.sub.31,33]--, as well as an adaptive one --[M.sub.24, 25] and [M.sub.30,3 2]--. When the regulator is fully loaded (100mA) the power transistor is in triode region, so the bandwidth of the circuit is reduced. But, in the presence of a fast load change i. e. from 100mA to 100[mu]A in 100ns, transistors [M.sub.31] and [M.sub.33] add some extra current to the bias branches of the error amplifier in order to increase even more its slew-rate and discharge the gate capacitance of [M.sub.p] quickly. This extra current is controlled by the voltage of the resistor [R.sub.HP], which is the output of a passive first-order high-pass filter. We propose to use two logic inverters --formed by [M.sub.(2...29)] in order to regenerate the control signal of MP in a shorter time than the given by the bandwidth of the regulator i.e. in a few nanoseconds; then the high-pass filter transmits this signal to the gate of [M.sub.31] to increase the bias current. Once the output voltage has settled down, the output of the passive filter will be zero, and no extra current is added to the error amplifier. It is important to highlight that there will flow current only through [M.sub.31] during the discharge transient response of the circuit, thus the current consumption in steady state is minimized.

When the output current increases from 100[mu]A to 100mA the adaptive bias network add more bias current to the error amplifier, with the same purpose of increase even more its slew-rate. This adaptive network forms another negative-feedback that helps to increase the bandwidth of the loop and to reduce the settling time of the regulator. It is important to emphasize that the copy ratio between [M.sub.p] and [M.sub.25] has to be very low i.e. 10000/1, in order to reduce the quiescent power consumption and enhance the gain of the error amplifier.

4. Simulation results

Simulations were performed using a Silterra 180nm standard CMOS technology, with a power supply of 2 V and a reference and output regulated voltage of 1.8 V. The overall current consumption was only 2.3 [mu]A for no load condition. The power transistors dimensions were 1.5 mm/180 nm which is much lower than many LDO reported (Lovaraju et al., 2013; Ming et al., 2012). In order to validate the specifications, transient simulations are the best option to show more approximate performance. Line and load transient simulation should be made varying power supply and load current respectively. The rise and fall times of the variables are set in values according other papers and for high speed response. Finally, DC simulation is made to validate the operating point according transient results.

Line and load transient response were obtained to validate the performance of the proposed regulator where rise and fall times used were 100 ns is all simulations. Figure 4 shows the line response for a power supply change from 2 to 2.5 V. It can be seen the output regulated voltage reaches steady state very fast with a maximum time in the positive edge of the input (V_DD) of 300 ns probing the stability of the circuit respect power supply variations. The under and overshoot of output voltage do not exceed 100 mV in any case.

Figure 6 shows the load response for a current load variation from 100 [mu]A to 100 mA. It probes the correct operation of dynamic and adaptive bias; despite having low quiescent current, because of the high slew-rate, the output voltage reach the steady state in less than 300 ns considering 1% settling time for positive and negative edge of the load current. Also, circuit stability is ensured. for different load capacitance from 0 to 100 pF avoiding the use of external large capacitors. Overshoot voltage does not reach 150 mV while undershoot is more critical reaching 400 mV.

Figure 6 shows line DC response of the regulator varying power supply from 2 to 3 V where the change in the output regulated voltage is around 5 mV; it means a line regulation of 5 mV/V. On another hand, load DC response is shown in Figure 7, where the variations of the load from 100 [mu]A to 100 mA results in a load regulation of 0.06 mV/mA.

Finally, Table 1 summarizes the specifications of the proposed LDO. Some circuits found in the literature are included. The main idea of this work was to keep in a low value the settling time while reducing power consumption. Table 1 shows that the settling time is in the order of other works accomplishing the first target. In addition, the current consumption is the lowest. This result is obtained by reducing the complex circuitry used in others works in order to improve the performance. By keeping the simplicity of the circuit, less elements can be used and thus reducing the current consumption. Finally, the use of dynamic circuits, as the dynamic biasing proposed, static current consumption is widely decreased improving the figure of merit (FOM) of the circuit.

5. Conclusions

A low-dropout voltage regulator having a low area and power consumption, and a fast transient response is presented. The circuit uses dynamic and adaptive bias for low power operation allowing the power transistor operates in triode region reducing the area. Dynamic bias produces a current increment only during transient operation while adaptive bias increases polarization during and after transient operation for high load current. In both cases, the bias of error amplifier is higher during transient operation allowing high slew-rate. Simulation results using 180 nm standard CMOS technology shows that regulated output can recover within 300 ns for both load and line transient response with a quiescent current of 2.3 ^A. Finally, the stability of the regulator is ensured within a range of load capacitance from 0 to 100 pF avoiding the use of external capacitors.

6. Acknowledgement

The authors would like to thank Universidad Industrial de Santander and INAOE for the support provided by sharing Hspice license and providing process models.

7. References

Baker, J. (2010). CMOS Circuit Design, Layout and Simulation. New Jersey: John Wiley & sons, Inc.

Chia, C. H., Lei, P. S., & Chang, R. C. H. (2012). A high-speed converter with light-load improvement circuit and transient detector. ISCAS 2012-2012 IEEE International Symposium on Circuits and Systems, Seoul, Korea, p. 456-459.

Gutierrez, L. C. (2008). Diseno de un regulador LDO integrable en tecnologia CMOS. Universidad Industrial de Santander. Bucaramanga, Colombia.

Ho, M., And, & Ka, N. (2011). Dynamic Bias Current Boosting Technique for Ultralow -Power Low-Dropout Regulator in Biomedical Applications. In Circuits and Systems II: Express Briefs, IEEE Transactions on, p. 174-178.

Koay, K. C., Chong, S. S., And, & Chan, P. K. (2013). A FVF based output capacitorless LDO regulator with wide load capacitance range. In Circuits and Systems (ISCAS), 2013 IEEE International Symposium on Beijing , China p. 1488-1491.

Lam, Y. H., & Ki, W. H. (2008). A 0.9V 0.35[mu]m adaptively biased CMOS LDO regulator with fast transient response. Digest of Technical Papers IEEE International Solid-State Circuits Conference, Lewiston, USA, 51, 442-444.

Lovaraju, C., Maity, A., & Patra, A. (2013). A Capacitor-less Low Drop-out (LDO) Regulator with Improved Transient Response for System-on Chip Applications. In VLSI Design and 2013 12th International Conference on Embedded Systems (VLSID), Pune, India, p. 130-135.

Man, T. Y., Leung, K. N., Leung, C. Y., Mok, P. K. T., & Chan, M. (2008). Development of Single Transistor-Control LDO Based on Flipped Voltage Follower for SoC. Circuits and Systems I: Regular Papers, IEEE Transactions on, 55 (5), 1392-1401.

Ming, X., Li, Q., Zhou, Z., And, & Zhang, B. (2012). An Ultrafast Adaptively Biased Capacitor less LDO With Dynamic Charging Control. In Circuits and Systems II: Express Briefs, IEEE Transactions on p. 40-44.

Rijns, J. J. F., & Wallinga, H. (1990). A CMOS class-AB transconductance amplifier for switched-capacitor applications. IEEE International Symposium on Circuits and Systems, New Orleans, USA, p. 1-4.

Rincon, G. (2009). Analog IC Design with Low Dropout Regulators. Atlanta: McGraw-Hill.

Yavari, M. (2010). Single-stage class AB operational amplifier for SC circuits. Electronics Letters 46 (14), 977. -LDO- voltage regulator

Andres F. Amaya *, Hector I. Gomez *, Guillermo Espinosa ** ([section])

* Electrical Engineering School, Universidad Industrial de Santander, Colombia ([section]) Instituto Nacional de Astrofisica, Optica y Electronica, Puebla Mexico * * ([section]) ** (Recibido: mayo 4 de 2014 - Aceptado: abril 16 de 2015)

Caption: Figure 1. Proposed LDO regulator.

Caption: Figure 2. Circuit diagram of the Error Amplifier.

Caption: Figure 3. Circuit diagram of the proposed LDO regulator.

Caption: Figure 4. Line transient response.

Caption: Figure 5. Load transient response.

Caption: Figure 6. Line regulation.

Caption: Figure 7. Load regulation.
Table 1. Comparison of different regulators.

Specifications        (Ming     (Lovaraju    (Koay,        This work
                      et al,    et al,       Chong, And,
                      2012)     2013)        & Chan,

[V]                   2.5-4     1.4-1.8      1.2           2-3.5
Dropout Voltage [V]   150       200          200           200
[M]                   7         40           13.2          2.3
[[mu]A]               100       100          50            100
[pF]                  0-100     100          10-100000     0-100
Settling Time [ns]    150       240          925           300
Line Regulation       1         -            217           5
Load Regulation       0.08      -            217           0.06
[mV/ mA]
PSR [dB]@10KHz        -         51           -             52
FOM[ps](Ming          10        96           244           6.9
et al., 2012)
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Author:Amaya, Andres F.; Gomez, Hector I.; Espinosa, Guillermo
Publication:Ingenieria y Competividad
Date:Jun 1, 2015
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