Factors to consider when selecting a switching DC/DC converter

As shown in the figure, the working principle of the three converters is to store energy first, and then release the energy in a controlled manner to obtain the required output voltage. For a certain job, the best switching DC/DC converter can meet the overall needs of the system with the lowest installation cost. This can be measured by a set of parameters that describe the performance of switching DC/DC converters, including: high efficiency, small installation size, small quiescent current, small operating voltage, low noise, high functional integration, sufficient output voltage regulation capability, and low installation cost.

Figure Block diagram of three typical DC/DC converters

(1) Working efficiency
① Inductive DC/DC converter: The conversion efficiency of the battery-powered inductive DC/DC converter is 80% to 85%, and its loss mainly comes from the external diode and the modulator switch.

② Charge pump without voltage regulation: It is a basic charge pump (such as TC7660H). It has a very high power conversion efficiency (generally over 90%) because the loss of the charge pump mainly comes from the ESR of the capacitor and the on-resistance (RDS-ON) of the internal switch tube, both of which can be made very low.

③ Charge pump with voltage regulation: It is a linear regulator with a low voltage drop added after the output of the basic charge pump. Although voltage regulation is provided, its efficiency is reduced due to the power consumption of the back-end regulator. To achieve the highest efficiency, the output voltage of the charge pump should be as close as possible to the voltage regulated by the back-end regulator.

The best choice in the design application is: a charge pump without voltage regulation (in applications that do not require strict output regulation) or a charge pump with voltage regulation (if the voltage difference across the back-end regulator is small enough).

(2) Installation size
① Inductive DC/DC converter: Although many new inductive DC/DC converters are available in SOT packages, they usually still require external inductors with larger physical dimensions. In addition, the circuit layout of the inductive DC/DC converter itself also requires a larger board space (additional decoupling, special ground wire processing, shielding, etc.).

② Charge pump without voltage regulation: The charge pump does not use an inductor, but requires an external capacitor. New charge pump devices use SOP packages and operate at higher frequencies, so small capacitors (1μF) that take up less space can be used. The space occupied by the charge pump IC chip and the external capacitor combined is not as large as the inductor in the inductive DC/DC converter. It is also easy to obtain a positive and negative combination of output voltages using a charge pump. For example, the TCM680 device can support an output voltage of +2 UIN with only external capacitors. To obtain the same output voltage using an inductive DC/DC converter, two independent converters are required. If one converter is used, a transformer with a complex topology must be used.

③ Charge pump with voltage regulation: Adding a discrete back-end voltage regulator takes up more space, but many such regulators have SOT packages, which relatively reduces the space occupied. New charge pump devices with voltage regulation, such as TCM850, integrate the charge pump, back-end voltage regulator and shutdown control in a single 8-pin 50lC package.

The best choice in the design application is: no voltage regulation or charge pump with voltage regulation.

(3) Quiescent current
① Inductive DC/DC converter: Frequency modulation (PFM) Inductive DC/DC converter is a switching DC/DC converter with the lowest quiescent current. Voltage regulation through frequency modulation can minimize the supply current under low load current.

② Charge pump without voltage regulation: The quiescent current of the charge pump is proportional to the operating frequency. Most new charge pumps operate at frequencies above 150kHz, so that capacitors of 1μF or even smaller can be used. To overcome the problem of high quiescent current, some charge pumps have a shutdown input pin to shut down the charge pump when idle for a long time, thereby reducing the supply current to near zero.

③ Charge pump with voltage regulation: The back-end voltage regulator increases the quiescent current, so the charge pump with voltage regulation is worse than the basic charge pump in terms of quiescent current.

The best choice in the design application is: inductive DC/DC converter, especially frequency modulation (PFM) switch type.

(4) Minimum operating voltage
① Inductive DC/DC converter: Battery-powered dedicated inductive DC/DC converters (such as TC16) can start working at a voltage as low as 1V or even lower, so they are very suitable for single-cell battery-powered electronic devices.

② Charge pump without voltage regulation/charge pump with voltage regulation: The minimum operating voltage of most charge pumps is 1.5V or higher, so they are suitable for applications with at least two batteries.

The best choice in the design application is: inductive DC/DC converter.

(5) Noise generated
① Inductive DC/DC converter: Inductive DC/DC converter is the source of power supply noise and switching radiation noise (EMI). Wideband PFM Inductive DC/DC converter will generate noise in a wide frequency band. The operating frequency of the inductive DC/DC converter can be increased so that the noise it generates falls outside the frequency band of the system.

② Charge pump without voltage regulation/charge pump with voltage regulation: The charge pump does not use an inductor, so its EMI effect can be ignored. The pump input noise can be eliminated by a small capacitor.

The best choice in the design application is: charge pump without voltage regulation or with voltage regulation.

(6) Integration
① Inductive DC/DC converter: Chips that integrate switching regulators and other functions (such as voltage detectors and line regulators) have been developed. For example, the TC16 chip integrates a PFM boost converter, LD0 and voltage detector in an SO-8 package. Compared with discrete implementations, such devices provide excellent electrical performance and take up less space.

② Charge pump without voltage regulation: Basic charge pumps, such as TC7660, have no additional functions integrated and occupy a small space.

③ Charge pump with voltage regulation: Charge pump chips with voltage regulation that integrate more functions have become a development trend. It is obvious that the functional integration of the next generation of charge pumps with regulation will be comparable to that of inductive DC/DC converter integrated chips.

The best choice in the design application is: inductive DC/DC converter.

(7) Output regulation
① Inductive DC/DC converter: Inductive DC/DC converter has good output regulation capability. Some inductive DC/DC converters also have external compensation pins that allow "fine adjustment" of the transient response characteristics of the output according to the application.

② Charge pump without voltage regulation: The output of this type of device has no voltage regulation. They simply convert the input voltage into a negative or multiple output voltage. Therefore, the output voltage will drop as the load current increases. Although this is not a problem for some applications (such as LCD bias), it is not suitable for applications that require a stable output voltage.

③ Charge pump with voltage regulation: It provides voltage regulation (stabilization) through a back-end linear voltage regulator (on-chip or external). In some cases, it is necessary to increase the number of switching stages for the charge pump to provide sufficient net space for the back-end regulator. In this case, external capacitors need to be added, which will have a negative impact on size, cost and efficiency. However, the back-end linear regulator can make the output voltage stability of the regulated charge pump the same as that of the inductive DC/DC converter.

The best choice in the design application is: charge pump with voltage regulation.

(8) Installation cost
① Inductive DC/DC converter: In recent years, the cost of using inductive DC/DC converters has gradually decreased, and the demand for external components has become less. However, the inductive DC/DC converter requires at least one external inductor, capacitor and Schottky diode. The total cost of the diode, inductor, and relatively expensive switching conversion chip is higher than that of the charge pump.

② Charge pump without voltage regulation: Charge pump without voltage regulation is cheaper than inductive DC/DC converter and only requires external capacitors (no inductors), saving board space, inductor costs, and shielding costs in some cases.

③ Charge pump with voltage regulation: The cost of a charge pump with voltage regulation is roughly equivalent to the cost of the inductive switching DC/DC converter itself. In some cases, an external back-end voltage regulator can be used to reduce costs, but it will increase the required installation space and reduce work efficiency.

The best choice in the design application is: the best choice in situations where strict voltage regulation is not required is a charge pump without voltage regulation; if there is a requirement for output voltage regulation, the cost of choosing a charge pump with voltage regulation and an inductive DC/DC converter is roughly equivalent.