Selection and application of power chips for embedded systems

Abstract: This paper introduces the principles and analyzes the characteristics of four types of power chips that can be used in embedded systems: ordinary linear regulators, low-dropout linear regulators, capacitive DC-DC converters (i.e. charge pumps), and inductive DC-DC converters. The principles for selecting power chips are proposed, and finally a power design example is given.

Power supply is an indispensable and important component of embedded systems. The quality of power supply design directly determines the success or failure of system design. The reasons for power supply design problems are, on the one hand, the lack of hardware design experience of designers; on the other hand, there are many varieties of integrated voltage regulator chips and non-standard manual instructions (especially DC-DC converters). In the power supply design process, in addition to the basic requirements of voltage and current, there are also certain constraints on performance indicators such as efficiency, noise, ripple, volume, and anti-interference. In addition, for the power supply of portable embedded systems powered by batteries, power management must also be considered.

1 Power Supply Technology Overview

According to the working state of the regulating tube, DC regulated power supplies can be divided into two categories: one is the linear regulated power supply; the other is the switching regulated power supply [1]. The regulating tube working in the linear state is called a linear regulator; the regulating tube working in the switching state is called a switching regulator. Linear regulated power supplies can be subdivided into two types, one is the ordinary linear regulator; the other is the low dropout linear regulator (LDO). Switching power supply regulators can also be subdivided into two types, one is the capacitive DC-DC converter, which is commonly known as the charge pump; the other is the inductive DC-DC converter, which is commonly known as the DC-DC converter.

1.1 Linear Regulator

Under the premise of ensuring output stability, the voltage value of the input voltage higher than the preset output voltage is called the input/output voltage difference. This parameter is not only related to the adjustment tube used in the voltage regulator, but also to the working state of the tube. The adjustment tube used in ordinary linear regulators is generally a bipolar transistor, which works in a linear state, and the input-output voltage difference is generally 1 to 3 V; while the tube used in low-dropout linear regulators is generally a field-effect tube, with an on-resistance of tens to hundreds of mΩ, so the input-output voltage drop is below 1 V, and a relatively small one can reach below 0.1 V, such as LP3999 and LP3985 of American Semiconductor, with a minimum voltage difference of 0.06 V.

According to the above analysis of power dissipation and efficiency, in order to improve efficiency, the input/output voltage difference and quiescent current must be as small as possible. If the load is not considered, the input/output voltage difference is the key factor in determining efficiency. The working efficiency of LDO is generally between 60% and 75%, and the efficiency will be better with a small quiescent current. In the case of ignoring the quiescent current of LDO, Vout/Vin can be used to estimate the efficiency.

The schematic diagram of a common linear regulator is shown in Figure 1. The sampling voltage is added to the non-inverting input of the comparator U1 and compared with the reference voltage Uref added to the inverting input. The difference between the two is amplified by the amplifier U1 to control the voltage drop of the series adjustment tube, thereby stabilizing the output voltage. When the output voltage Uo decreases, the difference between the reference voltage and the sampling voltage increases, the drive current output by the comparison amplifier increases, and the voltage drop of the series adjustment tube decreases, thereby increasing the output voltage; if the output voltage Uo exceeds the required set value, the front drive current output by the comparison amplifier decreases, thereby reducing the output voltage.

In Figure 1, according to the KVL law, UO=Ui-Vce, Vce is the voltage drop from the collector to the emitter of the tube. For ordinary linear regulators, this voltage drop is generally 1 to 3 V. The input/output voltage difference of LM7805 is generally above 2 V. Of course, this voltage difference varies with the operating temperature and output current, and is not a fixed value. When selecting an ordinary linear regulator, the minimum input/output voltage difference must be met, otherwise the voltage regulator chip cannot work properly. For example, the input voltage range of LM7805 is 5 to 18 V, and the expected output voltage is 5 V. The input voltage must be 2 V higher than the expected output voltage of 5 V, that is, the input voltage must be above 7 V to ensure the normal operation of the chip. This is something that needs special attention during design.

The characteristics of ordinary linear regulators are as follows:

① The power consumption of the adjustment tube is large and the power efficiency is low, generally only about 45%.

② It is large in size and requires a large board space.

③ In situations where heat generation is severe and requirements are high, a radiator needs to be installed.

④ The static current is relatively large, generally at the mA level.

⑤ An external low-frequency filter capacitor with larger capacity is required, which increases the size of the power supply.

Ordinary linear regulators are low in price, have large quiescent current, low efficiency, and a large minimum input/output voltage difference. They can only be used for voltage reduction and are not strictly required for power efficiency and volume, such as chargers, experimental instruments, etc.