A Low EMI DC-DC Buck Converter with a Triangular Spread-Spectrum Mechanism

Abstract

In this paper, a triangular spread-spectrum mechanism is proposed to suppress the electromagnetic interference (EMI) of a DC-DC buck converter. The proposed triangular spread-spectrum mechanism, which is implemented in the chip, can avoid modifying the printed circuit board of switching regulators. In addition, a lower ripple of output voltage of switching regulators and a better system stability can be realized by the inductive DC resistance (DCR) current sensing circuit. The chip is fabricated by using TSMC 0.18-μm 1P6M CMOS technology. The chip area including PADs is 1.2 × 1.15 mm². The input voltage range is 2.7~3.3 V and the output voltage is 1.8 V. The maximum load current is 700 mA. The off-chip inductor and capacitor are 3.3 μH and 10 μF, respectively. The experimental results demonstrate that the maximum spur of the proposed DC-DC buck converter with the triangular spread-spectrum mechanism improves to 14dBm. Moreover, the transient recovery time of step-up and step-down loads are both 5 μs. The measured maximum efficiency is 94% when the load current is 200 mA.

1. Introduction

In recent years, portable electronic devices have become an indispensable necessity in daily life. Power management integrated circuits (PMICs) are widely used in portable electronic devices such as smart phones, tablets, laptops, and digital cameras. These products are constantly being updated to make them more practical and convenient. In general, fixed operating frequency control and variable operating frequency control are used in the switching regulator. Switching regulators with fixed operating frequency include both voltage mode control [1,2] and current control mode [3,4,5]. Fixed operating frequency control mechanisms must utilize an error amplifier and additional compensation components to stabilize the output voltage of the DC-DC buck converter. Another type of control scheme is variable operating frequency control, including hysteresis control [6,7,8] and constant on-time control [9,10,11]. First, the hysteresis control is controlled by the ripple of output voltage of the DC-DC buck converter. When the ripple of output voltage is higher than the upper threshold voltage or lower than the lower threshold voltage, the control circuit will generate a corresponding switching signal to drive power transistors. In addition, to stabilize the system it is necessary to use a capacitor with large equivalent series resistance (ESR) as an output capacitor of the DC-DC buck converter, such as an electrolytic capacitor. The constant on-time control is another control method, as shown in Figure 1. When the output voltage of the DC-DC buck converter is lower than the reference voltage, the power transistors are controlled with a fixed on-time. This method also needs to be controlled by the ripple of output voltage of the DC-DC buck converter, so it is also necessary to use an electrolytic capacitor as an output capacitor of the DC-DC buck converter to stabilize the system.

Compared with electrolytic capacitors, a multilayer ceramic capacitor (MLCC) has the advantages of long used time, low cost, better high frequency response, and small size. However, the use of an MLCC as the output capacitor of a DC-DC buck converter in a variable operating frequency system must be accompanied by additional control mechanisms to maintain system stability. Hence, there are some studies that propose a combination of low ripple of output voltage and system stability [12,13]. In addition, since the variable frequency control system has a gradually increasing switching loss ratio of the power transistor at a light load, the constant on-time control can improve the efficiency of the DC-DC buck converter by lowering the operating frequency.

Switching regulators have many advantages, such as high conversion efficiency and small size. However, there are still some shortcomings that must be carefully considered, the most important of which is the electromagnetic interference (EMI) of the DC-DC buck converter [14,15,16]. Figure 2 shows the EMI of a portable electronic device. EMI is an electromagnetic noise that is not conducive to other systems during the normal operation of a DC-DC buck converter, which is a serious problem for safety regulations. Therefore, in order to eliminate the influence of EMI, it is necessary to modify the layout of the printed circuit board of the switching regulators or to add additional control circuits, which will increase the cost of the product. Random switching frequency is one of the most effective ways to reduce EMI [17]. This method disperses the power of spurs of the converter in the frequency domain by randomly adjusting the switching frequency. However, the disadvantage is that the instantaneous jitter of the switching frequency is too large, which will increase the ripple of the output voltage. To suppress the EMI of the DC-DC buck converter and the ripple of output voltage, the triangular spread-spectrum mechanism is presented in this paper. The EMI of the DC-DC buck converter is introduced in Section 2. The circuit implementation of the proposed triangular spread-spectrum mechanism is described in Section 3. Finally, the experimental results and conclusions are shown in Section 4 and Section 5, respectively.

2. Electromagnetic Interference of the DC-DC Buck Converter

Electromagnetic compatibility (EMC) is a very important issue for switching regulators. If the standard of EMC is not taken into account in the design of the switching regulator, it will take more time to deal with the effects of EMI, such as modifying the layout of the printed circuit board of the switching regulators.

Figure 3 depicts the inductor current distribution of the DC-DC buck converter. There are two dramatically varying current loops (I₁ and I₂) in the DC-DC buck converter. First, when the power transistor M_p is turned on, the current loop I₁ passes through the input voltage V_IN, the power transistor M_p, the inductor L, the output capacitor C_OUT, and the load. Next, in order to maintain the continuity of the inductor current, the power transistor M_n will be turned on. The current loop I₂ flows through the power transistor M_n, the inductor L, the output capacitor C_OUT, and the load at this time. According to the operating principle of the switching regulator, we can find that the current loops I₁ and I₂ are not continuous. This will therefore cause a large current change rate, resulting in the generation of high frequency components. There are two areas (A₁ and A₂) included in the inductor current distribution of the DC-DC buck converter. The area A₁ has a large high frequency component because only the current loop I₁ passes. On the other hand, since both current loops I₁ and I₂ pass through the area A₂, the inductor current forms a smooth and continuous triangular wave. Thus, the high frequency component of the area A₂ is reduced. In summary, the EMI of area A₁ will be higher than the area A₂. As a result, the input terminal of the DC-DC buck converter will generate a large EMI. Consequently, the EMI of the DC-DC buck convertor can be suppressed by the spread-spectrum technique, as shown in Figure 4.

3. Circuit Implementation

The triangular spread-spectrum mechanism proposed in this paper uses a triangular wave to modulate the on-time of the power transistor. Due to the switching frequency being gradually increased or decreased, the waveform of the inductor current is smoother. In addition, reducing the ripple of output voltage can also make the output voltage of the converter more stable. The block diagram of the proposed DC-DC buck converter is shown in Figure 5. To realize the spur reduction, this paper proposes a low EMI DC-DC buck converter with a triangular spread-spectrum mechanism that effectively mitigates the effects of EMI. In addition, the system stability and the ripple of the DC-DC buck converter are improved by the inductive DC resistance (DCR) current sensing circuit.

3.1. Proposed Triangular Spread-Spectrum Generator

The schematic of the proposed triangular spread-spectrum generator and simulation waveform are shown in Figure 6 and Figure 7, respectively. The triangular spread-spectrum generator is composed of a triangular wave generator and an on-time generator. First, the triangular wave generator includes current sources I₁ and I₂, a capacitor C₁, comparators CMP₁ and CMP₂, an SR latch, and transistors M₁-M₄. Assuming that the initial voltage of the capacitor C₁ is 0V, the SR latch will provide a high-level voltage to turn off the transistor M₁. Thus, the current source I₁ will charge the capacitor C₁. When the voltage of the capacitor C₁ is higher than the upper threshold voltage V_H, a low-level voltage will be produced by the SR latch to turn on the transistor M₁. If the current source I₁ is designed to be half of the current source I₂, the capacitor C₁ will be discharged through the transistors M₃ and M₄ and the current source I₂. When the voltage of the capacitor C₁ is lower than the lower threshold voltage V_L, the current source I₁ will charge the capacitor C₁ again. By repeating the above operation, a triangular wave V_TRI can be obtained. In general, the upper threshold voltage V_H and the lower threshold voltage V_L will affect the slope of the triangular wave. Thus, it would appear that a smoother triangular wave has a smaller impact on the output voltage, that is, it has a lower ripple of output voltage and a smooth output voltage. Next, the on-time generator comprises a current source I₃, a comparator CMP₃, a capacitor C₂, and transistors M₅-M₉. The current source I₃ charges the capacitor C₂ via the transistors M₅-M₈. When the voltage of the capacitor C₂ is higher than the triangular wave V_TRI, the comparator CMP₃ will generate a pulse signal to the SR latch to modulate the on-time. The voltage of the capacitor C₂ will be discharged at this time by using the transistor M₉.

3.2. Inductive DCR Current Sensing Circuit

The inductive DCR current sensing circuit is depicted in Figure 8. Due to the reduced ripple of output voltage of the DC-DC buck converter, the output capacitor with low ESR is employed. However, lower ripple of output voltage of the DC-DC buck converter causes system instability. Thus, to prevent sensing incorrect information on the inductor current, the inductive DCR current sensing circuit is adopted to obtain information on the inductor current. It is composed of an inductor with DCR R_DCR and filter circuits R_S and C_S. The operating principle of the inductive DCR current sensing circuit is to sense the inductor current by the inductive DCR R_DCR and obtain the sensing voltage V_SEN through the filter circuits R_S and C_S. It can be expressed as

$$V_{sen}\left(s\right)=\frac{i_L\left(R_{dcr}+sL\right)}{1+s{R}_s{C}_s}=(i_L{R}_{dcr})[\frac{1+s\left(\frac{L}{R_{dcr}}\right)}{1+s{R}_s{C}_s}]$$

(1)

When the time constants in Equation (1) are equal, it can be determined as

$$R_s{C}_s=\frac{L}{R_{dcr}}$$

(2)

By substituting Equation (2) into Equation (1), V_SEN can be simplified as

$$V_{sen}\left(s\right)=i_L{R}_{dcr}$$

(3)

The output voltage of the inductive DCR current sensing circuit V_SUM can be calculated as

$$V_{sum}\left(s\right)=V_{out}+V_{sen}\left(s\right)$$

(4)

It can be found from Equation (4) that the output voltage of the inductive DCR current sensing circuit V_SUM is composed of the output voltage of the DC-DC buck converter V_OUT and the sensing voltage V_SEN. Thus, even if the MLCC is used as the output capacitor C_OUT, the output voltage of the inductive DCR current sensing circuit V_SUM can acquire the correct information on the inductor current.

4. Experimental Results

In order to verify the proposed DC-DC buck converter with the triangular spread-spectrum mechanism, the chip is implemented using TSMC 1P6M 0.18-μm CMOS technology. The chip microphotograph is shown in Figure 9, and consists of the power transistors, gate driver and proposed triangular spread-spectrum mechanism.

The chip area including PADs is 1.2 × 1.15 mm². The off-chip inductor and capacitor are adopted as 3.3 μH and 10 μF, respectively. The operating frequency ranges from 0.7 MHz to 1.3 MHz. The nominal input voltage and output voltage are 3.3 V and 1.8 V, respectively. The maximum load current is 700 mA. The experimental results are shown in Figure 10, Figure 11, Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17, Figure 18 and Figure 19, including the steady-state response, ripple of output voltage, transient recovery time, spectrum waveform and conversion efficiency.

Figure 10 and Figure 11 show the measured results of the light load (I_LOAD = 200 mA) and the heavy load (I_LOAD = 700 mA) steady-state response of the proposed DC-DC buck converter, respectively. The measured results include the output voltage V_OUT of the DC-DC buck converter, the connection of the power transistors V_LX and the inductor current I_L. It can be found that the output voltages of the proposed DC-DC buck converter are both 1.8 V from the measured results.

The light load (I_LOAD = 200 mA) and heavy load (I_LOAD = 700 mA) output ripples of the proposed DC-DC buck converter, as shown in Figure 12 and Figure 13, are both less than 3 mV.

The measured transient response of the proposed DC-DC buck converter is shown in Figure 14. Figure 15 and Figure 16 depict the transient recovery times for the step-up and step-down loads of the proposed DC-DC buck converter, respectively. According to the measured results, the transient recovery time of the proposed DC-DC buck converter is 5µs for both step-up (from I_LOAD = 200 mA to I_LOAD = 700 mA) and step-down (from I_LOAD = 700 mA to I_LOAD = 200 mA) loads.

The measured spectrum waveforms of the Fast Fourier Transform (FFT) without and with the proposed triangular spread-spectrum mechanism are shown in Figure 17 and Figure 18, respectively. The measured maximum spurs of the DC-DC buck converter without and with the proposed triangular spread-spectrum mechanism are –46 dBm and –60 dBm, respectively. The experimental results show that the maximum spur of the proposed DC-DC buck converter with the triangular spread-spectrum mechanism improved by 14 dBm. The measured efficiency of the proposed DC-DC buck converter is illustrated in Figure 19. The maximum conversion efficiency is 94% when the load current is 200 mA. The specification of the proposed DC-DC buck converter with the triangular spread-spectrum mechanism is listed in

Table 1. Specification of the proposed DC-DC buck converter.

Parameter	This Work
Technology (nm)	180
Control scheme	Constant on-time
Switching frequency (MHz)	0.7~1.3
Input voltage (V)	2.9~3.3
Output voltage (V)	1.8
Inductor (μH)/Capacitor (μF)	3.3/10
Peak efficiency (%)	94
Load current (mA)	50~700
Transient recovery time for step-up load (μs)	5
Transient recovery time for step-down load (μs)	5
Chip area (mm2)	1.38

.

5. Conclusions

This paper presents the possibility of a low EMI DC-DC buck converter with a triangular spread-spectrum mechanism to reduce the EMI. The proposed triangular spread-spectrum mechanism is implemented in the chip, so there is no need to modify the PCB in order to mitigate the effect of EMI. On the other hand, reducing the ripple of output voltage of the DC-DC buck converter and maintaining system stability can be realized by the inductive DCR current sensing circuit. The chip is implemented by using the TSMC 0.18-μm 1P6M CMOS process. The chip area including PADs is 1.2 × 1.15 mm². The input voltage range is 2.7–3.3 V and output voltage is 1.8 V. The maximum load current is 700 mA. The off-chip inductor and capacitor are 3.3 μH and 10 μF, respectively. The measured results show that the maximum spur of the proposed DC-DC buck converter with a triangular spread-spectrum mechanism is suppressed to 14 dBm. Moreover, the transient recovery time of step-up and step-down loads are both 5 μs. The measured efficiency is 94% when the load current is 200 mA.