In the new generation portable expense class product integrated more and more functions, the function increased is advantageous to the attraction user, enhances the sales volume. But, to is light-sized along with the user, the longer battery life hope, the system design personnel will also face a bigger challenge. Each kind of additional function needs the extra space, the consumption extra power, to leave battery’s space to be smaller, is higher to power source’s request, i.e., needs the power source to provide a bigger electric current in a smaller space by a higher efficiency.
In the recent two years, the majority hand-hold products use a group voltage dropping switch (buck) and the multi-channel low differential pressure linear voltage regulators (LDO) the plan, some designs only use the linear voltage regulator structure. This kind of plan can provide the good operating performance, because the majority processors use 3.0V or the 3.3V power supply, in the single-node Li battery power supply’s situation, LDO may provide the suitable transfer efficiency. But, along with to a processor power loss’s higher request and the IC technique of production to the smaller submicron technology’s development, microprocessor’s nuclear voltage reduced to 1.8V, 1.5V, 1.3V even 0.9V. Moreover, the typical I/O voltage also drops to 2.5V or 1.8V from 3.3V. The voltage reduced cut the LDO efficiency greatly, it produced the quantity of heat has counter-balanced the advantage which the low voltage nucleus and the I/O mouth brought. In order to maintain the high efficiency, the designers can not but consider that selects the buck switch. Moreover, many system interior have used many processors (e.g. honeycomb telephone and PDA combination), contains a baseband processor and an application processor, these processors need the independent electric power supply. On the handset and PDA uses the camera module favors in uses the LDO power supply, but is related the graphics processor requests the lower supply voltage frequently. As the matter stands, in the multi-purpose designs, must use the multi-channel buck power sources frequently. Now installed three group buck power sources on a PCB board was already not a rare matter.
Figure 1
Had custom-made the power source management chip (PMIC) already to integrate group or the multi-channel buck switches newest promoted, but these products frequently were also insufficient to satisfy the user the request. Increases a new function every time, has the possibility to need other group buck power source or needs to sharpen the buck power source’s driving force, only then these can use separation power source IC to meet the new design need rapidly the manufacturer to be able to maintain its product performance and the market demand synchronization grow. Speaking of the present design level, if wants to increase a buck switch in the system not only to need to increase the cost, moreover must take certain line board area. Three year ago buck switch seal size is 15mm2, may work in 1MHz or the lower turn-on frequency, needs the great size the exterior inductance and the tantalum electric capacity. But the near future will promote TDFN will seal 1MHz the buck switch’s size to drop to 9mm2, and the outside may use the ceramic electric capacity and the light-sized inductance, but its size by far will be bigger than LDO. The advanced submicron BiCMOS composite signal processing craft solves the size question key technologies, it can further reduce power source IC the size, raises its operating frequency, thus enables its to select the light-sized exterior part. Many IC manufacturers already could provide 2MHz even high-frequency power source IC, simultaneously has also used a smaller seal. Shown in Figure 1 is LDO, 1MHz the Buck switch and MAXIM Corporation 4MHz the Buck switch’s comparison schematic drawing. May see: Maxim provides 4MHz buck switch (MAX8560) the size equally is nearly small with LDO! But may use the higher operating frequency, simultaneously permits the use miniature inductance (for example: Taiyo Yuden CB2012 series, 0805 seals). Switch’s fabrication technology progress looked from present 1MHz and 4MHz the buck that 4MHz the buck switch’s efficiency must switch’s efficiency be lower than 1MHz the buck, because the high turn-on frequency will have the big switching loss, but the miniature inductance also has the big magnetic core loss. Shown in Figure 2 is the Buck switch efficiency and the load current relational curve. What is worth rejoicing, 1MHz and the 4MHz Buck switch efficiency difference is not big, moreover, their efficiency is higher than LDO by far (41%).
May see from the above analysis, the system power source design has three kind of selection schemes: First, light-sized; second, high efficiency; third, raises the efficiency light-sized. When design may carries on the balance between the battery life and the system physics size. Because the third kind of plan can, in increases the line board size under the premise to raise power source’s transfer efficiency greatly not obviously, thus becomes the multi-purpose portable expense class products naturally the optimal plan. In addition, considered the buck switch produces the quantity of heat is extremely low, it does not need to consider the radiation question, therefore, has the possibility to substitute for LDO by a smaller size.
