Selection Criteria for Power Management ICs (PMICs)

Through a certain circuit layout, power management turns various power inputs into output voltages that fit the needs of the system. The power supply has a direct impact on system performance, and the power management chip is the fundamental component that influences power supply performance (PMIC). It is vital to comprehend the power management chip's selection criteria (PMIC).

Ⅰ. Understand the power usage environment

To choose the right power management components, you must first comprehend the application's environmental conditions, such as the system's input and output characteristics, and so on. These are some of the factors to consider:

Is it an alternating current (AC) or a direct current (DC) power source?

Is it USB or battery  powered while using direct power?

Is the input voltage higher or lower than the output voltage that you want?

What is the load current that is required?

Is the load noise-sensitive, does it require a constant current (like in LED applications), or does it require a variable current?

Is there enough room for installation? More power in a smaller package.

Different applications have unique needs that necessitate specialized power conversion systems and power management ICs. Low-dropout linear regulators (LDOs),  DC /DC switching converters, and other ubiquitous ICs are among them. A buck converter (Buck), a boost converter (Boost), and a boost/buck converter are three types of DC/DC switching converters (Buck-boost).

The input-output voltage difference is the first consideration when building a circuit (VIN - VOUT). Second, based on the application's specific needs, such as efficiency, thermal limitations, noise, complexity, and cost, choose the best power management chip (IC).

Ⅱ. There is an option when VOUT is less than VIN

The needed output current and VIN/VOUT ratio are crucial parameters to consider when choosing LDO or Buck when VOUT is smaller than VIN.

(1) Select LDO for low VIN/VOUT applications

The LDO Linear Regulator (Low Dropout Linera Regulator) regulates the output voltage by controlling the conduction of the pass element in a linear fashion. It provides a precise and noise-free output voltage and can respond quickly to load changes at the output end. As a result, it's perfect for low-noise, low-current, and low-VIN/VOUT applications.

Consider the input and output voltage ranges, the LDO's current level, and the package's heat dissipation capability when selecting an LDO. The minimal voltage of  VIN - VOUT  within the configurable range is referred to as the LDO voltage difference.

In micropower applications, the LDO quiescent current IQ must be low enough to avoid needless battery drain; such applications require specific, low quiescent current IQ low dropout Linear Regulators (LDOs).

However, linear regulation means that the power dissipated by the linear regulator's pass element is equal to the voltage difference between the input and output times the average load current. Excessive extra power losses are caused by both high VIN/VOUT ratios and high load currents. Larger package sizes are required for low dropout linear regulators (LDOs) with increased power consumption, which increases cost, PCB space, and thermal energy consumption.

Figure. 1 LDO Basic Circuit Diagram

When the LDO power dissipation exceeds ~0.8W, it is wiser to switch to a Buck buck converter as an alternative.

(2) Buck converter is selected when VIN/VOUT is high

Buck regulators are switching buck converters that may give high efficiency and flexibility output when the VIN/VOUT ratio and load current are both high. Buck converters (Bucks) are also known as buck regulators (buck regulators) and step-down switching regulators due to their wide range of applications (DC-DC step-down switching regulators). The three terms all refer to the same item.

An internal high-side  MOSFET  and a low-side  MOSFET serve as synchronous rectifiers in most buck converters, and an internal duty-cycle control circuit alternately turns them on and off (ON/OFF) to regulate the average output voltage. An external LC filter can be used to filter the noise created by switching.

Figure. 2 Buck Converter Basic Circuit Diagram

The power consumption is low because the two MOSFETs are switched on and off alternately. The duty cycle can be controlled to provide an output with a higher VIN/VOUT ratio. The internal MOSFET's on-resistance RDS(ON) determines the buck converter's current handling capability, while the MOSFET's voltage rating determines the maximum input voltage. The amount of the ripple voltage at the output is determined by the switching frequency and the external LC filter components; the filter components used in a buck converter with a higher switching frequency can be smaller, but the power consumption caused by switching will increase.

At light loads, a buck converter with pulse skip mode (PSM) reduces its switching frequency, hence enhancing efficiency. For applications that require a low-power standby state, this capability is critical.

Some particular buck topologies, such as ACOT, offer a very quick loop response and are ideal for power applications requiring a very fast load transient response, such as DDR, Core SoC,  FPGA, and SIC.

Ⅲ. Select boost converter when VOUT is higher than VIN

In cases where VOUT is greater than VIN, boost regulators are utilized to boost the input voltage to a higher output value. The inductor is charged through the internal MOSFET, and when the MOSFET is turned off, the inductor is discharged to the load through the rectifier. The inductor voltage is reversed when the inductor is charged and discharged, raising the output voltage above VIN.

A boost converter's usual circuit contains an inductor, a power MOSFET, a rectifier diode, a control IC, and input and output capacitors. A typical retrofit setup includes two MOSFETs, one of which substitutes the rectifier diode and switches on when the power switch is turned off.

MOSFETs have a reduced voltage drop, which minimizes power dissipation while improving the regulator's efficiency.

The boost ratio VOUT/VIN is determined by the MOSFET switch's ON/OFF duty cycle, and the duty cycle is also controlled by the feedback loop to maintain a constant output voltage. The output capacitor acts as a buffer, reducing output voltage ripple. The maximum load current is determined by the MOSFET current absolute maximum rating and the boost ratio, whereas the maximum output voltage is determined by the MOSFET voltage absolute maximum rating. To accomplish the effect of synchronous rectification, certain boost converters will merge the rectifier with the MOSFET.

Figure. 3 Boost Converter Basic Circuit Schematic

Ⅳ. Choosing a Buck-Boost Converter When the Input Voltage Is Uncertain

In applications where the input voltage varies, either lower or higher than the output voltage, a boost-buck regulator is utilized. When VIN is more than VOUT, the four internal MOSFET switches automatically set up as a buck converter, and when VIN is less than VOUT, they transition to boost operation. This makes the buck-boost converter suitable for battery  -powered applications, particularly when the battery voltage is lower than the regulated output voltage, which helps to increase battery life. Higher efficiency are attained since the four-switch buck-boost converter operates in a fully synchronous mode. Buck mode has a higher output current capability than boost mode because boost mode requires more switching current under the same load conditions.

The maximum input and output voltage range is determined by the MOSFET's voltage absolute maximum ratings. A buck-boost converter with only a single switch and rectifier can be utilized in situations where the output voltage does not need to be referred to ground, such as LED drivers. The output voltage is usually related to VIN.

Figure. 4 Buck-Boost Converter with Four Internal Switches

The four converter topologies described above are used by the majority of power management components. External MOSFET mode can be considered for some particular applications, such as those requiring a very large switching current (e.g. >10A). A specialized power supply monitoring IC can be used to monitor power supply overvoltage or under voltage conditions.