DDC/DC is a switching power supply chip. Switching power supply refers to the use of capacitors and inductors of energy storage characteristics, through the controllable switch (MOSFET, etc.) for high-frequency switching action, the input electrical energy is stored in the capacitor (sense), when the switch is disconnected, the electrical energy is released to the load to provide energy. The output power or voltage capability is related to the duty cycle (the ratio of the on-time of the switch to the cycle of the entire switch). Switching power supply can be used to boost and reduce voltage. There are two kinds of DC-DC products that we commonly use. One is the Charge Pump, and the other is the inductive energy storage DC-DC converter. This article explains the related knowledge of these two DC/DC products in detail.
    Compared with the core voltage requirements of 2.8V to 3.3V in previous years, more and more chips can operate smoothly at low voltages of 1.2V to 1.8V recently. In this way, in portable products that mainly use lithium (polymer) batteries or nickel-metal hydride batteries as the system's working energy, choosing the right voltage converter becomes a factor that designers need to consider. Low voltage differential linear voltage regulator (LDO) due to the reduction of the operating voltage, the drop between the input voltage and the output low voltage is getting larger and larger, and the linear voltage regulator has a lot of energy loss in the voltage conversion process, so that its efficiency may even be as low as 50%. More designers are beginning to favor buck DC/DC converters.
    For inductance selection considerations in DC/DC switching converters, usually size, equivalent resistance and current capacity determine the selection of inductors, but also can include cost, delivery time and technical support and other external design factors; It is generally believed that under the same conditions, in order to reduce the internal loss of the inductor device, the inductor with a smaller ESR (equivalent series impedance) value is the best. But the actual situation needs more consideration, this paper tries to give a better compromise consideration.
    The components that can produce inductance are collectively referred to as inductors, and are often directly referred to as inductors. It works on the principle of electromagnetic induction. Function: blocking AC through DC, blocking high frequency through low frequency (filtering), that is to say, the high frequency signal through the inductor will encounter a lot of resistance, it is difficult to pass, and the low frequency signal through it when the resistance is relatively small, that is, the low frequency signal can be easier to pass it. The inductor coil has almost zero resistance to direct current. Inductance is the ratio of the magnetic flux of the wire to the current that produces the alternating flux around the inside of the wire when an alternating current is passed through the wire. When DC current is passed through the inductor, only a fixed magnetic field line is present around it, which does not change with time. However, when alternating current is passed through the coil, the magnetic field lines around it will change with time. According to Faraday's law of electromagnetic induction - magnetic generation of electricity analysis, the changing magnetic field line at both ends of the coil will produce an induced potential, which is equivalent to a "new power source ". When a closed loop is formed, this induced potential generates an induced current. The total amount of magnetic field lines generated by induced current is known by Lenz's law to try to prevent the change of magnetic field lines.
    The loss of the inductor comes from its direct current impedance (Rdc) and AC impedance (Rac). The DC impedance is determined by the wire diameter and coil length of the coil, and the AC impedance is the eddy current loss caused by the interlocking of the magnetic leakage beam and the copper wire in the ferrite core and GAP. Generally, when working in DC/DC, the current flowing through the inductor is considered and the DC current is lost
    The missing part is represented by Rdc×Idc; The loss of AC current is characterized by Rac×ΔI. When the amplitude of current flow through the inductor ΔI is large and the Idc is small, its efficiency will decrease if the AC impedance is large, even if the DC impedance is small. On the contrary, even if the DC impedance is large, the AC impedance is small, and its efficiency may increase.
    When the inductor output current (Iout) is small, the average current through the inductor is very small, the DC impedance Rdc is slightly different at the same time the DC impedance part of the loss is small, but the current amplitude (ΔI) will affect the AC impedance part of the loss of power. When the Iout is large, the average current through the inductor is large, and the difference in DC impedance Rdc will lead to a large loss difference, in contrast, the power loss of AC impedance is not the main factor.
    Figure 1 shows the current consumption waveform of a GSM mobile phone in standby mode, which is precisely the situation where the Idc is small and the current flow amplitude ΔI is large. Based on the above conclusions, the average standby current is reduced from 2.4465mA to 2.1337mA by replacing the inductor with smaller AC impedance. The average standby current is reduced by 12.8%, which means that the standby time of the phone can be extended by 14.7%. So how is the value of inductors with small DC impedance reflected? It is suitable for large Idc and small current flow amplitude ΔI working mode, which is the user in the use of portable products such as calls, multimedia playback, gaming, GPS navigation and other functions in the working environment, you can expect the DC impedance of the small inductor will bring 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, but we believe that the temptation of longer standby time can make designers sacrifice use time.
    The next generation of digital processors for 3G are evolving towards 90nm and 65nm processes, which will reduce the power supply to close to 1V, and we will see more DC/DC converters in system-level power design. The balance between standby time and use time is one of the tradeoffs that designers need to constantly face in the design process, and the important impact on standby time deserves our careful selection of inductors.