DC/DC is a switching power supply chip. Switching power supply refers to the use of the energy storage characteristics of capacitors and inductors to perform high-frequency switching through controllable switches (MOSFET, etc.), storing the input electrical energy in capacitors (inductors). When the switch is disconnected, the electrical energy is released to the load to provide energy. Its output power or voltage capability is related to the duty cycle (the ratio of the switch on time to the entire switch cycle). Switching power supplies can be used for boosting and bucking. There are two types of DC-DC products we commonly use. One is a charge pump (Charge Pump), and the other is an inductor energy storage DC-DC converter. This article explains the relevant knowledge of these two DC/DC products in detail.

Compared with the core voltage requirements of 2.8V~3.3V in previous years, more and more chips can run smoothly at a low voltage of 1.2V~1.8V recently. In this way, in portable products that mainly use lithium (polymer) batteries or nickel-metal hydride batteries as system working energy, choosing a suitable voltage converter has become a factor that designers need to consider. Due to the reduction of working voltage, the gap between input voltage and output low voltage of low dropout linear regulator (LDO) is getting bigger and bigger. Linear regulator generates great energy loss in the process of voltage conversion, so that its efficiency may even be as low as 50%. More designers are beginning to prefer step-down DC/DC converters.

For the considerations of inductor selection in DC/DC switching converters, usually, size, equivalent resistance and current capacity determine the selection of inductor, and it can also include design factors such as cost, delivery time and technical support; and it is generally believed that under the same conditions, in order to reduce the internal loss of inductor devices, it is best to choose an inductor with a smaller ESR (equivalent series resistance) value. But the actual situation requires more consideration, and this article attempts to give a better compromise.

Inductor Components that can produce inductance are collectively called inductors, often simply referred to as inductors. It works on the principle of electromagnetic induction. Function: Blocking AC and passing DC, blocking high frequency and passing low frequency (filtering), that is to say, when high frequency signal passes through the inductor, it will encounter great resistance and it is difficult to pass, while the resistance presented to low frequency signal passing through it is relatively small, that is, low frequency signal can pass through it more easily. The resistance of the inductor to DC is almost zero. Inductance is the ratio of the magnetic flux of the wire to the current producing this magnetic flux, which is the alternating magnetic flux generated around the inside of the wire when AC current passes through the wire. When DC current passes through the inductor, only fixed magnetic lines of force are presented around it, which do not change with time; but when AC current passes through the coil, magnetic lines of force that change with time will appear around it. According to Faraday's law of electromagnetic induction - magnetoelectricity, the changing magnetic lines of force will generate induced potential at both ends of the coil, which is equivalent to a "new power supply". When a closed loop is formed, this induced potential will generate induced current. According to Lenz's law, the total amount of magnetic lines of force generated by the induced current should try to prevent the change of the magnetic lines of force.

The loss of an inductor comes from its DC resistance (Rdc) and AC resistance (Rac). The DC resistance is determined by the wire diameter and coil length of the coil, while the AC resistance is the eddy current loss caused by the leakage flux of the ferrite core and the GAP part and the interlocking of the copper wire. Generally, when working with DC/DC, the current flowing through the inductor is considered, and the DC current

loss is represented by Rdc×Idc; the AC current loss is represented by Rac×ΔI. When the current flow amplitude ΔI through the inductor is large and Idc is small, even if the DC resistance is small, if the AC resistance is large, its efficiency will decrease; on the contrary, even if the DC resistance is large, if the AC resistance is small, its efficiency may increase.

When the inductor output current (Iout) is small, the average current passing through the inductor is very small. When the DC resistance Rdc is slightly different, the loss of the DC resistance part is small, but the difference in current amplitude (ΔI) will affect the loss power of the AC resistance part. When Iout is large, the average current passing through the inductor is large, and the difference in DC resistance Rdc will lead to a large difference in loss. In comparison, the power loss of AC impedance is not the main factor.

 


Figure 1 is a current consumption waveform of a GSM mobile phone in standby mode, which is the case where Idc is very small and the current flow amplitude ΔI is large. Based on the above conclusion, after replacing the inductor with a smaller AC impedance, the average standby current is reduced from 2.4465mA to 2.1337mA. The average standby current is reduced by 12.8%, which means that the standby time of the mobile phone can be extended by 14.7%. So how is the value of the inductor with a smaller DC impedance reflected? It is suitable for the working mode where Idc is very large and the current flow amplitude ΔI is small. This is exactly the working environment of portable products when users use functions such as calls, multimedia playback, games, GPS navigation, etc. At this time, it can be expected that the inductor with a smaller DC impedance will bring a longer use time. However, since the average current is relatively large, a certain degree of improvement will not make much difference to the actual use time. On the contrary, we believe that the temptation of longer standby time can make designers sacrifice the use time.

The next generation of digital processors for 3G is evolving towards 90nm and 65nm process technology, which will reduce the power supply to nearly 1V. We will see more DC/DC converters in system-level power supply design. The balance between standby time and use time is one of the trade-offs that designers need to face during the design process, and the important impact of standby time is worth our careful selection of inductors.