Over the past few decades, the development of electronic products has been rapid. Taking personal communication equipment as an example, from the initial luxury to the current widespread popularity, electronic products have undergone earth shaking changes in weight, volume and efficiency. However, no matter how fast electronic products change and how many kinds they have, power supply, the “power” device of electronic products, is often easily ignored by many electronic engineers. Taking max8640y / Z as an example, this paper focuses on how to select the peripheral devices of DC / DC converter and the matters needing attention in circuit design.
Selection of DC / DC converter
Nowadays, electronic devices are developing towards lighter, thinner and smaller. Devices with small packages, such as SOT23, tdfn or SC70, are becoming more and more popular among designers.
For DC / DC converters, the higher the operating frequency, the lower the required external components, such as inductance and capacitance, and the smaller the size. However, the switching loss of MOSFET is also proportional to the switching frequency. Therefore, the choice of switching frequency should be a compromise between physical size and conversion efficiency.
Selection and layout of peripheral devices of converter
Here, it includes the selection of output inductance value, inductance type, freewheeling diode (for asynchronous rectifier converter), and input / output capacitance.
The first is the selection of inductance
For portable devices, the space occupied by inductance is often the largest, which also increases the cost of switching regulator. The smaller the inductance value, the smaller the physical size of the inductance and the faster the transient response, but at the same time, it will also produce a large ripple current, which will reduce the efficiency of the regulator and increase the ripple of the output voltage. In theory, the minimum inductance value should make the circuit work in the critical conduction state, that is, when the load current is maximum, the inductance current just returns to zero in each cycle. In practical applications, the inductance value is usually selected in combination with LIR (ratio of inductance ripple current to the designed maximum load current), input / output voltage, operating frequency and maximum load current. Generally, it is recommended to select LIR value between 20% ~ 50%. The value of inductance can be selected by referring to Formula 1.
Where Vin is the input voltage, Vout is the output voltage, f is the switching frequency of the converter, LIR is the ratio of inductance ripple current to the designed maximum load current, and Iload (max) is the maximum load current.
In addition, in addition to the inductance value, the type of inductance is also a key to optimize the circuit design, which is also the key to the cost of mobile phone manufacturers. Taking the wound inductor and laminated inductor commonly used in mobile phones as an example, the laminated inductor has a greater cost advantage than the looped inductor, and the size is more than 50% smaller than the wound inductor. In addition, on the premise of meeting the requirements of size and inductance value, the inductance with the smallest DC impedance should be selected to reduce the DC loss of the inductance.
In addition, the inductor core material is also an important factor that should be considered in the selection of inductors. The commonly used magnetic core materials are divided into two categories. One is powdered magnetic core, including iron powder core, high flux magnetic powder core and Kool M μ Represented by; The other is ferrite core. The two magnetic core materials have their own advantages and disadvantages. When selecting, designers should comprehensively consider the factors such as cost, performance and size. Compared with the powder core inductance, the ferrite core inductance has lower core loss and higher switching frequency (up to MHz). Therefore, ferrite cores are usually the best choice for such applications. Of course, iron powder magnetic core also has its advantages. For example, the cost of iron powder magnetic core is much lower than that of ferrite; In addition, the inductance of the powder core decreases slowly with the increase of the DC bias current flowing through the inductance, which is equivalent to “soft” saturation, while the inductance value of the ferrite core will decrease rapidly after saturation (see Figure 1). Therefore, more margin needs to be reserved in the design to prevent saturation of the inductance. As shown in Figure 1, the inductance of the powder core decreases relatively slowly with the increase of the load current. Iron powder cores are often used in applications that are not very sensitive to power consumption requirements and low switching frequency (200kHz), but pay attention to cost.
Fig. 1 the inductance of ferrite core will decrease rapidly after saturation
2 selection of output capacitance
Two points should be considered when selecting the output capacitance. One is the capacitance of the output capacitance; The other is the equivalent series resistance (ESR) of the output capacitor. The capacitance shall have a certain capacitance value to ensure the stability of output. The ESR must be low enough to reduce the ripple of the output voltage. Refer to formula 2. The ESR of the output capacitor depends on the acceptable output ripple voltage. The ripple voltage of the buck controller is approximately equal to the ESR of the output capacitor multiplied by the ripple current of the inductor.
The actual selection of capacitor is not only related to the physical size required to meet the low ESR requirements, but also related to the chemical type of capacitor. Therefore, when selecting capacitors, ESR and rated voltage should be considered first, rather than the size of capacitance (this principle is used for tantalum, OS-CON and other types of electrolytic capacitors).
At the same time, the ESR of the capacitor should not be too low. It must be ensured that the ESR is high enough to meet the stability requirements, and the zero point of the ESR should be far lower than the switching frequency. In view of the low ESR characteristics of ceramic capacitors, they are recommended in many applications.
3 selection of input capacitance
The input capacitance can reduce the current spike on the power side and the switching noise of the IC. The impedance of the input capacitor should be as small as possible. Ceramic capacitors are recommended because of their small size and low ESR.
4 rectifier diode
For asynchronous rectifier DC / DC converter, the voltage drop of rectifier diode will produce relatively large power loss. Therefore, in order to improve efficiency, Schottky diode with ultra-low forward voltage drop should be selected.
Figure 2 shows the application circuit of max8640z for SDRAM power supply in mobile phone application. The circuit design can output 1.8V voltage and provide 100mA maximum output current. The capacitance is lb1608t1r0m or lbmf1608t1r0m of Taiyo Yuden, and the input / output capacitance is c1005x5r0j105k of TDK. The total area occupied by all devices is equal to 2mm × 2.1mm (size of SC70) + 1.6mm × 0.8mm (size of inductance) + 2 × 1.0mm × 0.5mm (size of input / output capacitor) = 6.48mm2, saving more than 25% of the area compared with the existing solution.
Figure 2 application circuit of max8640z for SDRAM power supply in mobile phone application
The selection of peripheral components of DC / DC converter is directly related to the electrical performance of the circuit (including voltage output ripple, output load capacity, power consumption), volume, cost and so on. Therefore, we must have a clear understanding of the requirements of power supply at the early stage of design, so as to leave sufficient margin for the design.
Responsible editor: GT