ISSCC 2016 / SESSION 5 / ANALOG TECHNIQUES / 5.7
5.7

A 39.25MHz 278dB-FOM 19µW LDO-Free
Stacked-Amplifier Crystal Oscillator (SAXO)
Operating at I/O Voltage

Shunta Iguchi, Takayasu Sakurai, Makoto Takamiya

Miller effect is added. The area-efficient LPF with M3 and CLPF achieves a cutoff
frequency of 12Hz. The active resistor of 27MΩ with a size of 1800μm2 isachieved with a multiplication of RBIAS (= 80kΩ) by (W/L)M1 / (W/L)M3. 500pF isachieved with a multiplication of CLPF (= 50pF) by the gain (≈ 10) of M4. Figure
5.7.4 shows measured phase noise without and with the LPF in the proposed
SAXO. By adding the LPF, the phase noise at 1kHz offset is reduced from-122dBc/Hz to -139dBc/Hz, which indicates 17dB noise reduction.

University of Tokyo, Tokyo, JapanThe target of this work is to develop a low-power 39.25MHz crystal oscillator(XO) with low phase noise for RF applications and to achieve the highest FOM in
XO’s. To reduce the supply current at a constant negative resistance (RN) in theXO, a stacked-amplifier crystal oscillator (SAXO) operating at I/O voltage isproposed, eliminating the need for a Low-Dropout Regulator (LDO). By stacking
4 amplifiers, the supply current is reduced by 91%. The developed 3.3V,
39.25MHz SAXO with 4-stacked amplifiers in 65nm CMOS consumes 19μW andhas a phase noise of -139dBc/Hz at 1kHz offset, thereby achieving a state-of-theart FOM of 278dB.
Figure 5.7.1 (a) shows a conventional XO. The XO is often designed as an I/O celland operates at I/O voltage (VDD(I/O)) (e.g. 3.3V [1]), because the XO starts its
operation first within an RF SoC. In contrast, the peak-to-peak oscillation
amplitude (VXO) of the XO should be less than a certain voltage (e.g. 0.8V)
determined by a drive level (e.g. 10μW) specified in a datasheet of quartz
crystals. The operation beyond the specified drive level will lead to an inaccurate
output frequency. Therefore, a large voltage difference between VDD(I/O) and VXO(e.g. 3.3V - 0.8V = 2.5V) exists. As shown in Fig. 5.7.1(a), in the conventional
XO, the voltage difference is dropped across an LDO and a current source.
Therefore, in this paper, we focus on the wasted LDO voltage drop and propose
a new circuit topology effectively utilizing the voltage drop with the stacked
amplifiers.
Figure 5.7.1(b) shows a proposed SAXO. Compared with Fig. 5.7.1(a), the CMOS
inverter amplifier in Fig. 5.7.1(a) is replaced with N-stacked CMOS inverter
amplifiers, and the LDO in Fig. 5.7.1(a) is removed. To enable the stacking, each
amplifier includes two coupling capacitors (CC1, CC2) and two decoupling
capacitors (CD1, CD2) as shown in Fig. 5.7.1(b). The coupling capacitors in thelowest stage are removed to provide a common mode level in Out. RN of the XOis determined by gm(TOTAL) as shown in Fig. 5.7.1. To achieve the same RN as theconventional XO in Fig. 5.7.1(a), gm of each amplifier in the SAXO equals togm0/N. When the transistor size of each amplifier in Figs. 5.7.1(a) and (b) are thesame and ideal MOSFET’s in the saturation region are assumed, the gateoverdrive (VGS - VTH) of the SAXO is 1/N of that of the conventional XO, and thesupply current (IDD) and the power consumption of the SAXO is 1/N2 of that ofthe conventional XO. In our implementation, N is 4 at VDD(I/O) = 3.3V considering
the PVT variations, and 1/N2 equals to 1/16.
Figure 5.7.2 shows SPICE-simulated IDD dependence of RN with various N. RN isproportional to N, because gm(TOTAL) is proportional to N. At the target RN of 50Ω,IDD of the proposed SAXO (N = 4) is reduced to 9% (= 1/11) of that of theconventional XO (N = 1).
Figure 5.7.3 (a) shows a detailed circuit schematic of the proposed LDO-free
SAXO in 65nm CMOS. A start-up circuit for a constant-gm bias generator is notshown in Fig. 5.7.3(a) for simplicity. Off-chip components are a 39.25MHz
quartz crystal (DSX221G) and two capacitors (C1, C2) corresponding to the load
capacitance of 8pF. Both 1.2V core transistors and 3.3V I/O transistors are usedfor the 3.3V operation, because the XO design using only I/O transistorsincreases the power consumption. On top of the stacked amplifiers, a current
source (CS) (M4) for a constant-gm biasing is added to achieve robust operation
to PVT variations, because the stacked amplifiers in the SAXO are sensitive to
PVT variations due to the reduced gate overdrive. Figures 5.7.3(b) and (c) show
SPICE-simulated VDD(I/O) dependence of RN variations without and with the CS for
5 process corners at 25°C. Without the CS, the RN variation is -24% to +6%. Incontrast, with the CS, the RN variation is reduced to -3% to +4% because theconstant-gm bias generator is a supply-voltage-insensitive current reference.

A drawback of the LPF for the phase noise reduction is slow start-up time
(tSTART-UP) of the XO, because the RC time constant of the LPF with a cutoff
frequency of 12Hz is 14ms, which is longer than a typical tSTART-UP of an XO(< 4ms). To reduce tSTART-UP, the LPF is disabled during the start-up. Figure 5.7.5
shows measured start-up waveforms enabling and disabling the LPF during thestart-up in the proposed SAXO. As shown in Fig. 5.7.3(a), Amp_en is an enable
signal for both the constant-gm bias generator and the stacked-amplifiers, andLPF_en is an enable signal for the LPF. In Fig. 5.7.5(a), both Amp_en andLPF_en are simultaneously turned on and tSTART-UP is 62ms. In contrast, in Fig.
5.7.5(b), tSTART-UP is 3.9ms, because the LPF is disabled during the start-up andthe LPF is enabled after the start-up. The delayed turn-on of LPF_en does notaffect the waveform of Out. Thus, by disabling the LPF during the start-up,tSTART-UP is reduced from 62ms to 3.9ms, i. e. by 94%.
Figure 5.7.6 shows a comparison with previously published XO’s [2-5]. By
stacking 4 amplifiers, the proposed 3.3V, 39.25MHz, 19μW LDO-free SAXO in
65nm CMOS consumes the lowest supply current of 5.8μA including that of
1.5μA in the constant-gm bias generator. The phase noise at 1kHz offset is-139dBc/Hz, thereby achieving the highest FOM of 278dB. The measured
frequency variation due to temperature variation is within ±7.7ppm from -30°C
to 80°C at VDD(I/O) = 3.3V. The measured frequency variation due to VDD(I/O)
variation is within 0.07ppm/V from 2.97V to 3.63V at 25°C. The measured
supply-current variation due to temperature variation is within ±17% from -30°C
to 80°C at VDD(I/O) = 3.3V. The measured supply-current variation due to VDD(I/O)
variation is within 7.1%/V from 2.97V to 3.63V at 25°C.
Figure 5.7.7 shows a die micrograph of the proposed SAXO fabricated in 65nm
CMOS. The core area is 88200μm2 with a size of 420μm by 210μm.
Acknowledgment:
The authors thank Yoshiaki Yoshihara, Yusuke Niki, and Ryuichi Fujimoto ofToshiba Corporation for technical discussions and Ren Shidachi of theUniversity of Tokyo for measurement support. This work is partly supported bySTARC. Shunta Iguchi is supported by JSPS through the Program for Leading
Graduate Schools (MERIT).
References:[1] Data sheet of TC35667, Toshiba, [Online].
[2] J. Lin, “A Low-Phase-Noise 0.004ppm/step DCXO with Guaranteed
Monotonicity in 90nm CMOS,” ISSCC Dig. Tech. Papers, pp. 418-419, Feb.
2005.
[3] Y. Chang, J. Leete, Z. Zhou, M. Vadipour, Y. Chang, and H. Darabi, “ADifferential Digitally Controlled Crystal Oscillator with a 14-bit Tuning Resolutionand Sine Wave Outputs for Cellular Applications,” IEEE J. Solid-State Circuits,
vol. 47, no. 2, pp. 421-434, Feb. 2012.
[4] S. Iguchi, A. Saito, Y. Zheng, K. Watanabe, T. Sakurai, and M. Takamiya,“93% Power Reduction by Automatic Self Power Gating (ASPG) and Multistage
Inverter for Negative Resistance (MINR) in 0.7V, 9.2μW, 39MHz CrystalOscillator,” IEEE Symp. VLSI Circuits, pp. C142-C143, June 2013.
[5] D. Griffith, J. Murdock, P. Røine, and T. Murphy, “A 37μW Dual-Mode Crystal
Oscillator for Single-Crystal Radios,” ISSCC Dig. Tech. Papers, pp. 104-105,Feb. 2015.
[6] D. Griffith, F. Dülger, G. Feygin, A. Mohieldin, and P. Vallur, “A 65nm CMOSDCXO System for Generating 38.4MHz and a Real Time Clock from a Single
Crystal in 0.09mm2,” IEEE Radio Frequency Integrated Circuits Symp., pp. 321324, May 2010.

A drawback of the CS for the constant-gm biasing is the degradation of the phasenoise of the XO, because the flicker noise of transistors in the constant-gm bias
generator shown in Fig. 5.7.3 (a) modulates the power supply voltage of thestacked-amplifiers. To suppress the effect of the flicker noise, a lowpass filter(LPF) using an active resistor (M3) and an amplified capacitor (CLPF) using the100• 2016 IEEE International Solid-State Circuits Conference

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Figure 5.7.1: (a) Conventional XO. (b) Proposed stacked-amplifier XO (SAXO).

Figure 5.7.2: Simulated IDD dependence of negative resistance (RN) withvarious N.

Figure 5.7.3: (a) Circuit schematic of the SAXO. (b) & (c) Simulated VDD(I/O)
dependence of RN variations without and with current source (CS).

Figure 5.7.4: Measured phase noise without and with the LPF in the SAXO.

Figure 5.7.5: Measured start-up waveforms in the SAXO. (a) Enabling LPF.
(b) Disabling LPF during start-up.

Figure 5.7.6: Comparison with previously published XO’s [2-5].

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Figure 5.7.7: Die micrograph of the SAXO fabricated in 65nm CMOS.

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