A novel on-chip CMOS current sensor implemented by switched capacitors for a current-mode control DC-DC Buck converter

Yong Zhang *Wengao Lu Juan Guo Yajing Zhang Zhongjian Chen Yacong Zhang Lijiu Ji

Key Laboratory of Microelectronic Devices and Circuits, Institute of Microelectronics, Peking University, 100871, China

Abstract — A novel on-chip CMOS current sensor implemented by switched capacitors for a current –mode buck

converter is presented in this paper. This proposed current sensing circuit does not need another sense MOSFET and a voltage-to-current and current-to-voltage transform circuit. We

use the 0.35um DPTM CMOS process to design and simulate this circuit. Test result shows that the accuracy and the speed of the proposed current sensing circuit are high.

Index Terms — current sensing circuit current mode DC-DC switched capacitors

I. INTRODUCTION

Switched mode power supply is essential part in the

new generation portable systems, for efficiently

converting time varying input battery voltages to fixed

output voltages which supply power to internal circuit

blocks of the systems. Sensing inductor current is one of

fundamental functions needed in the current-mode

converter, for it is used in the pulse-width-modulation

(PWM) module to generate appropriate duty cycle for

output voltage regulation [1].

Current sensing circuit is one of the most important

circuits in a DC-DC converter. Conventional current

sensing methods include series resistor, MOS conducting

resistor, OP-amplifier-based and current-mirror-based

structures . In this paper, we propose a novel current

sensor circuit, it benefits of both MOS conducting resistor

and OP-amplifier-based current sensing structure without

their drawbacks. It does not need another sense MOSFET

compared to OP-amplifier-based current sensing circuit

and it does not need to include a voltage-to-current and

current-to-voltage transform circuit compared to current-

mirror-based current sensing circuit.

II. PROPOSED CURRENT SENSOR CIRCUIT

A. Structure And Operational Principle

Fig.1 shows the structure of the proposed current sensor

for the buck converter, which consists of a sensing

MOSFET transistor MP (we used the power MOSFET as

sense MOSFET), switches MS1, MS2, and MS3, an

amplifier, an input capacitor C1 and a feedback capacitor

C2. The on resistance of switches MS1, MS2 and MS3

can be neglected. When Q is high, the power MOSFET

MP and the switch MS3 are off and switches MS1 and

MS2 are on. The input capacitor C1 is charged by VIN

and the amplifier works as a buffer. Vsense equals to Vref

which is a reference voltage at the positive end of the

amplifier. The electron charged in the C1 is (VIN-

Vref)*C1 while C2 is zero, because C2 is shorted by the

switch MS1.When Q is low, the power MOSFET MP and

the switch MS3 are on and switches MS1 and MS2 are off.

According to the principle of charge conservation, charge

will redistribute in the C1 and C2. The electron charged in

the capacitor C1 is (VLX

-Vref)*C1, and the electron

charged in the capacitor C2 is (Vref-Vsense)*C2.

In a high frequency buck converter, the leak charge

between Q switch from high-to-low or low-to-high can be

neglected. So we get the following equation as

( ) 1 ( ) 1 ( ) 2IN LXV Vref C V Vref C Vsense Vref C− = − + − (1)

Rearranging equation (1), we get

1( )

2IN LX

CVsense Vref V V

C− = − (2)

From the equation (2) we know that the voltage drop of

power MOSFET MP (VIN

-VLX

) is amplified by C1/C2, and

the value of (Vsense-Vref) is proportionally to the load

current of the buck converter.

Fig. 1. Structure of the proposed current sensor

B. Plot Of Key Point And Further Improvement

978-1-4577-1997-4/11/$26.00 ©2011 IEEE

Fig. 2. Plot of VLX ,VIN-VLX during MP on time and plot of Vsense of the proposed current sensor

Fig.2 shows the plot of VLX,VIN-VLX during MP on

time and plot of Vsense of the proposed current sensor.

This current sense circuit amplifies the voltage drop of

the power MOSFET directly. We can get very high

accuracy for the ratio of C1/C2 can be accurate in the on-

chip design. It is worth to mention that the ratio of C1/C2

can be controlled by digital method. As Fig. shows, the

switches S0, S1, S2 close in different current load. So this

current sense module can support variable gain in different

working states and it makes easier to get a stable system in

current mode than conventional current sense circuit

which can only support a fixed gain. This digital

controlled gain improves the response time of the

proposed current sensor further.

Fig. 3. Digital controlled gain implemented by proposed current sensor

III. EXPERIMENTAL RESULSTS

A DC-DC buck converter using the proposed current

sensing circuit has been implemented with 0.35um DPTM

CMOS process .The layout graph of the proposed current

sensor in the buck converter is showing in Fig.4. This

converter is designed as a constant output 1.8V within 18-

mv ripple with 2.5v-5.5v input voltage. The experimental

results of output ripple are shown in Fig.5 in the state of

5V input and 1A current load. We can see that in the

heavy load situation the output ripple is 18.4mv (The

spikes are noise couple during test mode) .Therefore, the

inductor current can be sensed correctly and timely by the

proposed current sensor in the DC-DC converter.

Fig. 4. Buck converter using the proposed current sensor

Fig. 5. Output ripple of buck converter

IV CONCLUSIONS

A novel on-chip current-sensing circuit for a current-

mode control DC-DC buck converter has been described

and experimentally verified in this paper. The proposed on

-chip CMOS current sensor improves both speed and

accuracy of the current sensing. The proposed current

sensor is suitable for the current-mode converter operating

at high switching frequencies in battery-operated portable

electronic devices.

REFERENCES

[1] R.Erickson and D.Maksimovic, “Fundamentals of power electronics,” Norwell,MA:Kluwer,2001.

[2] Forghani-zadeh , H.P. and Rincon-Mora, G.A. “Current-sensing techniques for DC-DC converters ,” Circuits and Systems ,vol.2,pp.577-580,Aug.2002.

[3] Mengmeng Du, Hoi Lee, “An Integrated Speed-and Accuracy –Enhanced CMOS Current Sensor With Dynamically Biased Shunt Feedback for Current-Mode Buck Regulators,” IEEE Trans, Circuits and systems,vol.57,NO.10.Oct.2011.