How to quickly select LDO? 5-minute tutorial to get started with LDO parameters and applications (recommended for collection)

Latest update time：2024-10-23

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LDO (Low Dropout Regulator) is a low voltage dropout linear regulator widely used in step-down conversion of various electronic equipment and automotive electronics. It is cost-effective and flexible to use . By explaining the key parameters of LDO, clarifying the test methods , and interpreting the data sheet curves, engineers can quickly understand the product specifications and performance and improve the selection efficiency.

Key performance parameters

Taking the SCT data sheet as an example, a large amount of product information can be obtained on the homepage:

Figure 1 SCT71403Q data sheet home page shows product overview

As shown in the layout, the left side below the title is the product features , which show some important parameters that reflect the product performance (the red serial numbers correspond to the order of the description in this article); the right side is the product description , which shows the product to engineers in all aspects; and the bottom is the common application scenarios and typical application block diagrams .

The following article will guide you to interpret 15 common key performance parameters : 1 input voltage, 2 output voltage, 3 accuracy, 4 output current, 5 quiescent current, 6 shutdown current, 7 line regulation rate, 8 load regulation rate, 9 load transient response, 10 power supply rejection ratio, 11 leakage voltage, 12 protection function, 13 integrated function, 14 thermal index, 15 external device selection.

0 1

Input voltage range (Vin)★

Refers to the voltage range that the LDO can accept.

Determine whether the application requirements are met based on the given minimum and maximum operating voltages. A wide input voltage range can leave a certain amount of redundancy and be applicable to more application scenarios. Some products are marked to support higher transient input voltages , such as SCT71403Q, which can support 45V transient voltage, providing higher protection performance for special needs.

0 2

Output voltage (Vout)

Refers to the stable output voltage of the LDO.

Usually divided into fixed output and adjustable output versions. Fixed version The output voltage is fixed. Compared with the adjustable version, it can save external voltage divider resistors , but the larger number of parts increases the management complexity.

In the adjustable version, users can flexibly configure the output voltage , but external feedback resistors need to be added. Xinzhou's LDO products, such as SCT71405Q and SCT71005Q, are designed with two output versions for customers to flexibly choose.

0 3

Accuracy

Refers to the percentage by which the output voltage deviates from the normal value.

Different output voltage versions require different accuracy. The fixed version corresponds to the output voltage accuracy , and the adjustable version corresponds to the feedback voltage accuracy , which are all reflected in the EC parameter table of the data sheet. Taking SCT71005Q as an example, the specification provides the accuracy corresponding to the two voltage versions, with a normal temperature accuracy of ±1% and a full temperature accuracy of ±2%.

Figure 2 SCT71005Q accuracy display P8

0 4

Output current

Determines the load current that the LDO can drive.

The output current of the chip is affected by many factors, such as input and output voltage, temperature, etc. In order to avoid the actual application not being able to meet the requirements, it is best to leave a 1.5 times margin. Engineers may have concerns about the false marking of some manufacturers, or have doubts about whether the margin can meet the requirements . Xinzhou provides many reference curves related to output current in the data sheet, including the relationship between leakage voltage and output current and actual test waveforms, etc., to dispel engineers' doubts by showing the output current performance in all aspects .

Figure 3 Relationship between output current and output voltage of SCT71403Q

0 5

Quiescent current

Refers to the current consumed by the chip itself when it is unloaded and maintains normal output voltage.

The quiescent current is the current flowing out of the GND pin of the LDO, and its size is often directly linked to the standby power consumption.

How to measure the static current? Xinzhou has two ways to test the demo: for the package with pins outside , such as SOT23-5, ESOP8, etc., the GND pin of the chip will be lifted up during the test, separated from the GND of the circuit board and then welded, and the ammeter will be connected in series between the lifted GND pin and the GND pad of the circuit board. The tested current value can provide the most realistic feedback of the static current of the chip; the other way is to test the input current and output current at the same time , try to avoid the error caused by heating, and read the current value, and then subtract the output current from the input current to get the static current.

In order to avoid engineers from the tedious task of testing the static current under various conditions, Xinzhou provides three sets of curves in the data sheet for engineers to refer to.

Figure 4 Relationship between SCT71403Q output current and quiescent current

0 6

Shutdown current

Refers to the current consumed when the LDO is turned off but the battery is still connected to the system.

The test condition for the shutdown current on the demo is that EN is grounded and the input current at this time is measured with an ammeter.

Figure 5 SCT71403Q shutdown current P8

0 7

Line Regulation

Refers to the current consumed when the LDO is turned off but the battery is still connected to the system.

Line Regulation (in millivolts per volt, mV/V or percentage per volt, %/V) is a measure of the LDO's ability to maintain a normal output voltage when the input voltage changes . The smaller the value, the better the LDO performance. This parameter is usually marked in the EC parameter table, and the calculation formula is:

0 8

Load Regulation

Characterizes the degree to which the output voltage deviates from the ideal value due to changes in load current.

Load Regulation (%/mA) is a measure of the LDO's ability to maintain a normal output voltage when the load changes . The smaller the value, the better the LDO performance. This parameter is usually marked in the EC parameter table, and the calculation formula is:

Figure 6 SCT71403Q line regulation rate, load regulation rate P9

0 9

Load transient response

Refers to the transient response of the output voltage when a step change occurs in the LDO load.

The magnitude and response time of the output voltage change caused by the load current step are mainly affected by factors such as the bandwidth of the LDO system, the output capacitance and the load current. Load transient response can often reflect the loop stability of the LDO .

Figure 7 Measured waveform of SCT71403Q dynamic response P23

Power Supply Rejection Ratio

Refers to the ratio of input voltage ripple to output voltage ripple.

PSRR will fluctuate with some parameters such as frequency, temperature, current, output voltage and voltage difference (in decibel dB). How to intuitively judge whether PSRR meets the requirements? Taking SCT71010Q as an example, in addition to the data annotation in P5 of the EC parameter table, Xinzhou provides PSRR curves under different conditions in P9-P10.

Figure 8 SCT71010Q more intuitive display of PSRR curve

Dropout voltage (Vdropout)

Refers to the minimum difference between VIN and VOUT required for normal voltage regulation.

The voltage difference determines the minimum voltage difference for the chip to work properly. Application scenarios where VIN and VOUT are very close must pay attention to this parameter . Taking SCT71005Q as an example, the EC parameter table clearly marks the leakage voltage under different output voltage versions and different output current conditions. For example, if the adjustable output voltage version is loaded with 500mA, if you want to output 1.8V normally, then the input voltage must be greater than 1.8V+0.143V=1.943V.

Figure 9 SCT71005Q dropout voltage P8

[Explanation] The test condition in the data sheet is "Vin = Vout - 0.1V", which means that when testing the dropout voltage of the 1.8V output voltage, the test is done at Vin = 1.7V. You may wonder: Why should Vin < Vout? Why is it 0.1V?

First of all, testing this parameter requires ensuring that the power tube is in the linear region, which requires that the output voltage cannot reach the normal expected value at this time. Under this condition, the input voltage can only be smaller than the output voltage. The output voltage accuracy of Xinzhou's LDO products is guaranteed to be ±1% at room temperature, that is, the minimum value of the fixed 5V output voltage within the accuracy range is 4.95V, and there is still margin from Vin=Vout-0.1V=4.9V.

Protection function

Protection function

Xinzhou's LDO integrates some protection functions such as over-temperature protection, over-voltage protection, over-current protection, short-circuit protection , etc. to prevent the chip from being damaged or burned under abnormal conditions.

Integrated functions★

Integrated Features

The functions integrated by LDO are closely related to the pin definition. Paying attention to the pin information of the chip can help you understand its functions such as EN (enable), PG (power good), PGDL (power good delay), etc.

Some functions are built-in, such as soft-start, input voltage undervoltage lockout protection (UVLO) , output discharge function (Active output discharge), etc., making the chip more flexible and reliable in practical applications.

How to judge whether these functions meet the selection requirements? Xinzhou's data sheet describes the function explanation, trigger threshold, parameter calculation, etc. of each integrated function, with full details!

Figure 10 SCT71403Q programmable UVLO function

Thermal Index★

Thermal resistance is a heat dissipation index that describes the thermal conductivity characteristics of a material.

For the explanation of terms, see Question 06 of "Answers to Common DC-DC Application Problems (I)"

Compared with DCDC products, LDO thermal indicators have an additional parameter R _q_{JA_EVM} : the thermal resistance of the junction to the environment under still air conditions.

This parameter is based on the actual measured value of the Xinzhou DEMO board, which is closer to the performance of the chip in actual applications. Customers are welcome to apply for Xinzhou DEMO to test chip performance , which will make thermal performance evaluation easier.

Because LDO has a wide variety of package types, the data sheet not only marks the thermal indicators of each package, but also provides the corresponding dissipated power and thermal performance curves at the end . For detailed calculation process and explanation, please contact SCT technical support.

Figure 11 Four heat dissipation index curves of SCT71403Q

[Explanation] The first row shows the curves of two fixed output voltage versions - based on the relationship between the maximum load and input voltage under extreme conditions, each curve includes a comparison of different packages as a reference for the extreme performance of the chip .

The second row is the actual application curve of the TDFN2*2-6 package, which are the relationship between the maximum power dissipation and temperature, and the relationship between the maximum load and input voltage of different fixed output voltage versions, all of which are based on the maximum junction temperature at which the chip can work for a long time.

External Component Selection

External Device Selection

The data sheet recommends the selection range of external input and output capacitors, and recommends the selection of feedforward capacitors and voltage divider resistors for the adjustable output voltage version. In the application information section, P24, a selection table of output capacitors and feedforward capacitors based on a large number of simulations is given (exceeding the recommended range will affect the dynamic response capability of the chip), which allows engineers to flexibly select peripheral devices.

Figure 12 SCT71005Q output capacitor and feedforward capacitor selection range

LDO selection recommendation

Xinzhou's R&D and design team has rich design experience and has currently laid out a matrix of LDO products with low voltage, high voltage and special performance requirements to meet the different needs of customers. Xinzhou's higher voltage products required for 48V electrical architecture will also be sampled soon.

Figure 13 LDO selection table

The reason why Xinzhou's LDO products stand out from the market is that they have lower quiescent current, lower leakage voltage, higher reliability and rich functional integration .

Xinzhou's product manual (click to download) provides some selection recommendations in application scenarios such as battery applications, photovoltaic inverters, automotive electronics , etc., to help engineers better match their needs.