"Recently, aiban.com got tps7a4700-evm, an evaluation board based on LDO tps7a4700 launched by Texas Instruments. As the world's top power management chip manufacturer, Ti has a very rich LDO product line, but rarely designs an evaluation board for an LDO, which makes people wonder what the charm of tps7a4700 is? What kind of product will it be? Before answering these questions, we might as well first understand what LDO is? Under what circumstances will LDO be selected?
LDO, i.e. low-voltage differential linear regulator, is usually the most basic structure, which is composed of reference source, comparison amplifier, sampling resistance, adjustment tube, etc. the sampling voltage obtained on the sampling resistance is compared with the reference source through the negative feedback input comparison amplifier. The output current formed by the amplified differential voltage controls the on voltage drop of the adjustment tube, so as to keep the output voltage constant. In short, a stable DC output voltage can be obtained through LDO, which is similar to DC-DC buck. Now, with the complexity of electronic circuit design environment, undervoltage protection unit, reverse protection, overheat shutdown, short circuit protection and other functions will be added to LDO to ensure the normal operation of LDO.
Since LDO and DC-DC have certain commonalities, how should they be selected in actual circuit design? To understand this problem, we must first understand the respective characteristics of LDO and DC-DC.
Advantages and disadvantages of LDO and DC-DC (Figure 1)
From (Figure 1), we can clearly and intuitively distinguish the advantages and disadvantages of LDO and DC-DC, but it is worth mentioning that on the premise of selecting LDO, the following characteristics are the primary factors to be considered in circuit design.
Noise: noise mainly includes internal noise and external noise, which mainly refers to internal noise. LDO internal noise is mainly generated by reference source and error amplifier. This noise mainly includes thermal, flicker and shot noise.
Ripple: that is, the AC component superimposed on the DC output. At present, the power ripple suppression ratio (PSRR) is mostly used as the degree of LDO filtering the circuit. The higher the PSRR, the stronger the LDO suppression of noise or ripple from the power input at various frequencies.
Differential voltage: the difference between the output voltage and the input voltage. The smaller the differential voltage, the higher the voltage conversion efficiency, but at the same time, the differential voltage is related to PSRR. The smaller the differential voltage, the smaller the PSRR.
Quiescent current: that is, the current consumed by the device itself. Especially in mobile devices sensitive to power consumption, LDO with low quiescent current can help reduce the standby power consumption of the whole system, so as to prolong the service time of the device.
The harm of noise and ripple is essentially the same. In digital circuits, ripple or noise will cause instability of output voltage. If the transient voltage is too high, it may cause damage to devices; In analog circuits, ripple and noise will be superimposed on the original AC signal, resulting in the interference of the original signal. For example, additional noise will appear in audio applications.
These four characteristics of LDO can be classified into two categories in terms of manufacturing process:
Bipolar LDO: current driven, high current gain, suitable for high current applications. Low output voltage ripple and noise (compared with CMOS process).
CMOS process LDO: voltage driving element, with the characteristics of low voltage and low current consumption. Low voltage difference and static current (relative to bipolar process).
Of course, in addition to the typical two processes, there is also the LDO of BiCMOS process. Combined with the advantages of the two, it will not be discussed here.
Therefore, in the circuit design, we need to select the corresponding power supply system components according to the actual circuit characteristics. For example:
If there is a boost circuit in the circuit, there is no doubt that it can only be solved with DC-DC.
If it is a mobile electronic device with high standby power consumption requirements (especially a battery powered system), the CMOS process LDO with low differential voltage and low quiescent current has a better effect.
For RF, GPS and other signal sensitive equipment, LDO with low power ripple and low noise is the most appropriate, and LDO with bipolar process is ideal.
LDO with low noise and high ripple rejection ratio -- tps7a4700
After understanding what LDO is and how to choose LDO, it is easier to start with TI's tps7a4700-evm evaluation board.
Front of tps7a4700-evm evaluation board (Figure 2)
Tps7a4700-evm evaluation board is a simple green 2-layer board with no special peripheral circuits and expansion functions. The whole scheme is just a simple power conversion module based on LDO tps7a4700, which supports a wide input voltage of 0 ~ 35V, the output voltage range is 1.4V ~ 20.5v, and the minimum unit is 100mV, It can be adjusted according to the 8 jumpers on the front of the evaluation board (the specific implementation principle still depends on the LDO internal circuit, which will be mentioned later), (Fig. 3) is the additional voltage value of each jump, (Fig. 4) the jump cap without additional output voltage, (Fig. 5) is the output voltage of the additional 100mV jump cap.
TPS7A4700 8 additional optional voltage values (Figure 3)
Output voltage without any jump cap attached (Figure 4)
Voltage of additional 100mV jump cap (Fig. 5)
The back of the tps7a4700 evaluation board (Figure 6) is simpler, with only two power lines, through-hole routing, some labels and silk screen printing. It is worth noting that 16 vias are grounded on the back of the PCB at the position of LDO tps7a4700. The purpose of this is to fully contact the LDO ground wire with the main ground. The function is:
Reduce power supply noise and ensure power supply accuracy
Heat dissipation, taking the whole main ground as the transmission path of the thermal plane
Back of tps7a4700-evm evaluation board (Figure 6)
From the tps7a4700-evm evaluation board, it is not difficult to find that the circuit of LDO is very simple, and the schematic diagram can be deduced reversely according to the PCB board (Fig. 7).
Schematic diagram of tps7a4700-evm evaluation board (Figure 7)
The hardware design based on LDO is very simple. The difficulty in product design lies in how to select the power scheme, DC-DC or LDO, or what characteristics of LDO should be selected after using LDO. Now, let's take a look at the characteristics of this revered LDO tps7a4700.
Tps7a4700 features (Figure 8)
Tps7a4700 features:
Input voltage range: + 3V to + 36V
Output voltage noise: 4.17 µ VRMs (10Hz, 100Hz)
Power ripple suppression: 82db (100Hz); ≥ 55dB (10Hz，10MHz)
Output voltage range: + 1.4V to + 20.5v
Output current: 1A
Voltage drop: 307mv at 1A
Enable pin compatible with CMOS logic level
Built in fixed current limit and thermal shutdown
High heat dissipation package: 5mm x 5mm square flat leadless package (QFN)
Tps7a4700 adopts bipolar technology. As mentioned earlier, bipolar technology is driven by current and has high current gain, which well explains that it has relatively high load capacity 1a, low output noise and high power ripple suppression, which are two important guarantee parameters of bipolar technology, while differential voltage and static current are not good at bipolar transistors, These two parameters of tps7a4700 can only be said to be general. From (Figure 9), we can clearly understand the specific parameters of its four characteristics.
Noise vs output voltage, differential voltage vs output current, quiescent current vs input voltage, PSRR vs output voltage (Fig. 9)
(Figure 9) the output noise and power ripple rejection ratio of tps7a4700 presented to us are not defined by a single value. They are closely related to the frequency. For example, in the range of 10Hz ~ 100Hz, the output voltage noise can reach a minimum of 4.17 µ VRMs, while PSRR can reach a maximum of 80dB when the frequency is 100Hz. These two parameters are particularly important in noise sensitive applications, Circuit design should be fully considered. Next, take the spectrum noise of the switching voltage regulator operating at 500KHz from before input to after output of tps7a4700 as an example. Through the comparison of the two table curves (FIG. 10), it is obvious that the peak pulse in the blue dotted line area (500KHz) is weakened.
Comparison of spectrum noise before and after tps7a4700 (FIG. 10)
Understand its basic characteristics, and then further explore the internal structure of tps7a4700.
Schematic block diagram of tps7a4700 (Figure 11)
As can be seen from (FIG. 11), Texas Instruments tps7a4700 is mainly composed of UVLO, overheat shutdown, fixed current limit, adjusting tube (bipolar transistor), sampling resistance, band gap reference, error amplifier, etc. among them, band gap reference and error amplifier are the core components of the whole LDO, and their anti-noise ability is the key factor affecting the whole voltage conversion accuracy. It is worth affirming that the bandgap reference is used as the reference source of the amplifier:
First, good temperature stability and
Second, strong anti noise ability
In the schematic block diagram of tps7a4700, we can also see that multiple resistors are integrated inside, which is the key factor to change the LDO output voltage value mentioned earlier (Fig. 2). Its principle is to change the sampling voltage and adjust the output different voltage values by changing the sampling resistance value. Of course, if you are not interested in these specific voltage output values, you can also choose its "twin" LDO tps7a4701. The output voltage can be realized by matching the resistance externally as needed, and the output voltage range is from 1.4V to 34V.
So far, we have a comprehensive understanding of LDO from the process, structure and characteristics. As a bipolar LDO, tps7a4700 has the characteristics of typical low output noise and high power rejection ratio. It is very suitable for occasions where power consumption is not too demanding in communication fields such as RF, GPS, on-board entertainment system, precision instruments and instruments.
What can tps7a4700 do?
The essence of components is to use them in actual design. We have already known several key factors of LDO design selection. Here, we will illustrate with examples. For example, we are very familiar with the IP surveillance camera.
IP surveillance camera (Figure 12)
IP surveillance camera transmits video information in digital form through wired or wireless IP network. In case of power failure, it can be powered by backup UPS power supply to ensure the real-time monitoring. The specific scheme can be composed of video codec chip, audio codec, HD camera, peripherals, power supply system, etc.
Principle block diagram of IP surveillance camera (Figure 13)
IP surveillance cameras need to use ad and DA conversion in processing images and audio. As we all know, this is an extremely noise sensitive application. Low noise power supply solution is the most basic requirement for maintaining signal accuracy and integrity. In audio, the frequency range that human ears can hear is about 20Hz to 20kHz. From (Fig. 9), we can see that in this frequency range, tps7a4700 has high power ripple rejection ratio and very low noise output, which is very in line with the requirements.
In addition, everyone is familiar with the camera, the core component of the IP monitor. As a monitoring camera, it needs to deal with image acquisition and processing in different harsh environments. In addition to its own devices requiring high signal-to-noise ratio, it is also necessary to prevent the introduction of new noise superimposed on the collected image signal, affecting the image quality, and the power supply is an important part, The ripple and noise of its output voltage are directly related to the image quality processed by the camera, which requires the power supply system to have the characteristics of high power ripple suppression ratio and low output noise.
The characteristics of IP surveillance camera determine the necessity of selecting low-noise LDO power supply scheme. (Figure 14) illustrates some low-noise LDOS of Texas Instruments, a famous power management scheme manufacturer. Combined with the whole design scheme of IP surveillance camera, including input voltage range and other factors, it is a very good choice to select tps7a4700 as the LDO for power supply conversion.
LDO with low noise and high power ripple rejection ratio (FIG. 14)
Through the comparison (FIG. 14), we can also find that tps7a4700 is far superior to other products in output noise control. So far, we basically understand why an LDO is worth designing an evaluation board specially by Texas Instruments.
In addition to the IP Camera scheme, the LDO tps7a4700 can play a role in high-precision instruments such as temperature measuring instrument, automobile power supply system, RF, etc., which needs more designers to explore and realize. Moreover, in the actual product design, the selection of tps7a4700 LDO can also cooperate with the WEBENCH design tool of Texas Instruments to shorten the product design cycle.
Summary
Through the hands-on experience of LDO tps7a4700 and its evaluation board, we understand its structure, principle and design of related applications. With the increasing requirements of electronic equipment for power supply system, the advantages and status of LDO are becoming more and more obvious. The tps7a4700 LDO of Texas Instruments has made a qualitative breakthrough in low noise. Combined with high power ripple rejection ratio and wide input voltage, it has good advantages in RF, vehicle, medical and other fields. The quotation of tps7a4700 of US $2.10 (purchase reference for 1000 units) is also reasonable. If necessary, you can also click the sample film application on the official website.
reference material
Tps7a4700 Evaluation Board official website
Purchase link
LDO TPS7A4700 DATASHEET
WEBENCH design topic
Original statement: This article is original by Aiban. Reprint is declined“
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