A low power and low noise power supply design

When designing a hardware system, you need to choose the right power supply chip. Whether you are designing consumer digital electronics or wireless sensor devices, you need to weigh the various functional requirements of the product. Only after evaluating and prioritizing indicators such as noise suppression, power consumption, voltage drop, and power supply voltage and current can you choose the power supply IC.

Each signal path requires “clean” power. Power management is the final part of system design. Figure 1 shows an example system of how to power the signal paths.

I am currently designing a wireless product that requires ultra-low power consumption. A 3AH battery needs to be able to work for 5-6 years. This requires the entire communication mechanism to have a power-saving function, and the product itself needs to have ultra-low power consumption capabilities. A wireless product that needs to have ultra-low power consumption needs to be analyzed from several product components:

1) Power supply

2) RF part

3) CPU part

4) Other parts

Here is an analysis of the power supply part based on my work:

Principles for selecting power chips:

1) Choose products from manufacturers with mature technology, good product quality and good cost performance

2) Choose products with high operating frequency, reduce peripheral components and reduce costs.

3) Use a small package, but consider the output current. Generally, small packages have small currents, while large packages have large currents.

4) Choose a manufacturer with good technical support, especially a small company. Be careful when choosing power supply components. Small companies will ignore you!!!

5) Choose products with complete information, preferably in Chinese, samples can be requested, preferably free, with short delivery cycle, and preferably not often discontinued

The above is an analysis from a macro perspective, including design and procurement considerations.

From the perspective of technical requirements,

1. LDO device selection:

There are four factors to consider when selecting LDO: voltage drop, noise, quiescent current, and common-mode rejection ratio.

From the perspective of power saving, the main factor to look at is the quiescent current. Some LDOs have a very small quiescent current, about 1UA. This is the power consumption of the LDO itself when it is working. This parameter is very critical in power saving. The smaller the better, but it cannot be 0. There are two indicators of LDO power consumption: one is the quiescent current, and the other is the SET_OFF current. You must distinguish them! ! There is also the voltage difference, which is easy to understand. A voltage difference of 0 is an ideal LDO.

I am currently using the S-1206 series, made in Japan, and there is no other way to use Japanese products, SOT23. A friend who passed by recommended a Chinese product to me, which is of good quality, and the R1180X series, which seems to be made in Japan. The above are all IQ values ​​below 5ua.

However, when making RF LDO, you need to consider noise suppression, because RF is too sensitive to noise.

The power supply rejection ratio (PSRR) is an AC parameter that reflects the ability of the LDO output to suppress input ripple when the output and input frequencies are the same. Unlike noise, noise usually refers to the mean square value (RMS) of the output voltage noise of the LDO at a certain input voltage within the frequency range of 10Hz to 100kHz. The unit of PSRR is dB, and the formula is as follows: PSRR=20 log(△vin/△vout)

Power Supply Affects Signal Path Performance

Not surprisingly, the power supply affects analog signal integrity, which ultimately affects overall system performance. A simple way to improve signal path performance is to select the right power supply. When selecting a power supply, a key parameter that affects analog signal path performance is the noise or ripple on the power supply line. Noise or ripple on the power supply line can couple into the output of an op amp, increase jitter in a phase-locked loop (PLL) or voltage-controlled oscillator (VCO), or degrade the SNR of an ADC. A low-noise and low-ripple power supply can also improve signal path performance.

The sources of noise or ripple on the power line are diverse. High-speed data and high-frequency signals in the system will generate noise themselves. If the printed lines and connecting lines of the PCB are not designed properly, they can form the effect of transmitting antennas. Digital ICs, such as microcontrollers and field programmable gate arrays (FPGAs) and complex programmable logic devices (CPLDs), have very fast edge jump speeds, and the magnitude of the current varies greatly, which will generate electromagnetic interference radiation into the system. IC silicon chips generate thermal noise internally, which is caused by the random movement and collision of molecules at temperatures above absolute 0 degrees Celsius.

There are three common methods to minimize noise and ripple in the signal path: very careful system PCB layout, proper power supply bypassing, and correct power supply selection. Although the specific design of the PCB depends on the system, in general, the PCB layout needs to consider correct device placement, minimizing the length of signal path connections, and using solid ground.

Bypassing the power rails is a common method that is often recommended in analog IC data sheets to filter out noise. Signal path ICs may have separate analog, digital, and PLL power inputs, each with its own independent bypassing recommended. The PLL power supply and analog power supplies are the most sensitive to noise and ripple. Bypass capacitors, resistor-capacitor (RC) filters, and EMI suppression filters minimize the power supply noise that enters the signal path.

The right power supply selection can reduce the effects of noise and ripple on signal path ICs. When selecting a power supply, designers first make a basic choice between switching converters and linear regulators. Switching converters offer higher frequencies, and higher frequencies mean lower overall system power consumption. Linear regulators offer an easy-to-use solution while reducing noise/ripple on the power rail. Using linear regulators to reduce noise and ripple can improve signal path performance.

There is no doubt that portable wireless products require an LDO with both low operating current consumption and high PSRR. However, there are very few LDO products on the market that can take these two indicators into consideration. I found an S1167 LDO with a working current consumption of 9UA and a PSRR of 70dB. It can be said that it takes these two indicators into consideration, but it is a Japanese product.

If you only consider PSRR and IQ around 45 is not a big deal, AS1361 is a good choice, as PSRR can reach over 90dB.

2. DC-DC power supply selection

For DC-DC, the main considerations are conversion efficiency, ripple, input and output voltage, etc.

When selecting a DC/DC converter, the circuit design should pay attention to output current, high efficiency, miniaturization, and output voltage requirements:

1. If the required output current is small, you can choose the built-in FET type; if the output current needs to be large, choose the external FET type.

2. Regarding efficiency, the following considerations apply: If ripple voltage and noise elimination under heavy load are given priority, PWM control type can be selected; if efficiency under low load is also important, PFM/PWM switching control type can be selected.

3. If miniaturization is required, high-frequency products that can use small coils can be selected.

4. In terms of output voltage, if the output voltage needs to be above a fixed voltage, or if an unfixed output voltage is required, a VDD/VOUT separation type product with variable output can be selected.

Comparison between PFM and PWM working modes of DC-DC:

The three control modes of PWM control, PFM control and PWM/PFM switching control mode have their own advantages and disadvantages: The DC/DC converter increases or decreases the voltage by switching synchronously with the internal frequency, and controls it by changing the number of switching times to obtain an output voltage that is the same as the set voltage.

In PFM control, when the output voltage reaches above the set voltage, the switching will stop, and the DC/DC converter will not perform any operation before it drops to the set voltage. However, if the output voltage drops below the set voltage, the DC/DC converter will start switching again to make the output voltage reach the set voltage. PWM control also switches synchronously with the frequency, but when the boost setting value is reached, it will minimize the current flowing into the coil and adjust the boost to keep it consistent with the set voltage.

Compared with PWM, the output current of PFM is small, but because the DC/DC converter controlled by PFM stops operating when the voltage reaches above the set voltage, the current consumption becomes very small. Therefore, the reduction of current consumption can improve the efficiency at low load. Although PWM is less efficient at low load, it is easier to design a noise filter and eliminate noise because of its small ripple voltage and fixed switching frequency.

If you need to have the advantages of both PFM and PWM, you can choose a PWM/PFM switching control DC/DC converter. This function is controlled by PWM when the load is heavy, and automatically switches to PFM control when the load is light, that is, a product has the advantages of both PWM and PFM. In a system with a standby mode, products using PFM/PWM switching control can achieve higher efficiency.

Advantages of high frequency:

By actually testing the efficiency of PWM and PFM/PWM, it can be found that the efficiency of PWM/PFM switching products is higher at low loads. As for high frequency, by increasing the frequency of the DC/DC converter, large current, miniaturization and high efficiency can be achieved. However, it must be noted that efficiency can only be improved by matching the characteristics of the coil. Because when the DC/DC converter is high-frequency, the switching loss will increase due to the increase in the number of switches, resulting in a decrease in efficiency. Therefore, efficiency is determined by a compromise between the improvement of coil performance and the increase in switching losses. By using high-efficiency products, coils with relatively low inductance values ​​can be used, and small coils can be used. Even if small coils are used, the same efficiency and output current can be obtained.

External device selection:

In addition to the characteristics of the DC/DC converter itself, the selection of external components cannot be ignored. The coil, capacitor and FET in the external components have a great influence on the characteristics of the switching power supply. The so-called characteristics here refer to the output current, output ripple voltage and efficiency.

Coil: If you need to pursue high efficiency, it is best to choose a coil with a small DC resistance and inductance value. However, if a coil with a small inductance value is used for a DC/DC with a low frequency, it will exceed the rated current of the coil, and the coil will produce magnetic saturation, causing efficiency deterioration or damage to the coil. And if the inductance value is too small, it will also cause the ripple voltage to increase. So when selecting a coil, please pay attention to the current flowing to the coil not exceeding the rated current of the coil. When selecting a coil, it is necessary to make a comprehensive decision based on conditions such as output current, DC/DC frequency, coil inductance value, coil rated current and ripple voltage.

Capacitor: The larger the output capacitor, the smaller the ripple voltage. However, a larger capacitor also means a larger capacitor volume, so please choose the most suitable capacity.

Transistor: As an external transistor, compared with bipolar transistors, FET has a faster switching speed, so the switching loss will be smaller and the efficiency will be higher.

Basic principles of DC-DC:

The DC-DC power supply is a relatively new type of power supply. It has the advantages of high efficiency, light weight, voltage step-up and step-down, and high output power. However, since the circuit works in a switching state, the noise is relatively large. Through the figure below, let's briefly talk about the working principle of the step-down switching power supply. As shown in the figure, the circuit consists of a switch K (a triode or field effect tube in the actual circuit), a freewheeling diode D, an energy storage inductor L, and a filter capacitor C. When the switch is closed, the power supply supplies power to the load through the switch K and the inductor L, and stores part of the electrical energy in the inductor L and the capacitor C. Due to the self-inductance of the inductor L, the current increases slowly after the switch is turned on, that is, the output cannot immediately reach the power supply voltage value. After a certain period of time, the switch is disconnected. Due to the self-inductance of the inductor L (it can be more vividly considered that the current in the inductor has an inertial effect), the current in the circuit will remain unchanged, that is, it will continue to flow from left to right. This current flows through the load, returns from the ground wire, flows to the positive pole of the freewheeling diode D, passes through the diode D, and returns to the left end of the inductor L, thus forming a loop. By controlling the time when the switch is closed and opened (i.e. PWM - pulse width modulation), the output voltage can be controlled. If the time of opening and closing is controlled by detecting the output voltage to keep the output voltage unchanged, the purpose of voltage regulation is achieved.

During the switch-on period, the inductor stores energy; during the switch-off period, the inductor releases energy, so the inductor L is called an energy storage inductor. During the switch-off period, the diode D is responsible for providing a current path for the inductor L, so the diode D is called a freewheeling diode.

In actual switching power supplies, switch K is replaced by a triode or field effect tube. When the switch is open, the current is very small; when the switch is closed, the voltage is very small, so the heat generation power U×I will be very small. This is why the switching power supply has high efficiency.

Principle of boost DC/DC converter:

Boost DC/DC converters are mainly used in situations where the output current is relatively small. Using only 1 to 2 batteries can obtain a 3 to 12 V operating voltage, with an operating current ranging from tens to hundreds of mA, and a conversion efficiency of 70% to 80%.

The basic working principle of the boost DC/DC converter is shown in the figure.

VT in the circuit is a switch tube. When the pulse oscillator sets the bistable circuit (i.e., the Q terminal is 1), VT is turned on, and current flows through the inductor VT and stores energy until the voltage drop of the inductor current on RS is equal to the threshold voltage set by the comparator. The bistable circuit is reset, i.e., the Q terminal is 0. At this time, VT is cut off, and the energy stored in the inductor LT is supplied to the load through a diode VD1, while charging C. When the load voltage is about to drop, the capacitor C discharges, and the output terminal can obtain a stable voltage higher than the input terminal. The output voltage is divided by the voltage divider R1 and R2 and input into the error amplifier, and the pulse width is controlled together with the reference voltage to obtain the required voltage, i.e., VR is the reference voltage.

DC-DC circuit PCB design requirements:

When designing a printed circuit board, design engineers will carefully consider the copper wire routing and component placement. If these two points are not fully considered, the efficiency, maximum output current, output ripple and other characteristics of the printed circuit board will be affected. The two main reasons for these effects are the connection of the ground wire (GND, VSS) and the power line (+B, VCC, VDD). If the ground wire and power line are designed reasonably, the circuit will work normally and obtain better performance indicators. Otherwise, interference and performance deterioration will occur. This article introduces some general design principles and ground connection methods for the design of DC/DC converters.

Figure 1: Wiring pattern based on basic design principles. Figure 2: PCB design example for a boost circuit. Figure 3: PCB design example for a buck circuit.

Design principles

The layout of printed lines and the placement of components often affect the performance of the circuit. The following are four principles for grounding line design:

1. Use planar pattern for grounding;

2. Connect the power cord using a flat wiring method;

3. Place components one by one according to the signal current direction in the circuit diagram;

4. The data obtained from the experiment should not be adjusted in any way when applied, and should be reproduced as is even if it is affected by the size of the plate or other factors.

Paying attention to the above principles and key points in design can reduce circuit noise and signal interference. In addition to the above basic principles, the following two points should be kept in mind when designing copper wire routing patterns and component placement: stray capacitance will be generated between wiring; the length of the connection will generate impedance. Paying attention to stray capacitance between wires and shortening the wiring length in the design will help eliminate noise and reduce radiation.