Guide: The power supply system plays an extremely important role in the design of electronic products, especially in communication fields such as radio frequency, GPS, and the output ripple, noise, etc. of power supply system, and noise, etc., which directly affects the sensitivity of these product signals, and determines The quality of the product is very suitable, and the LDO (ie, low pressure difference linear regulator) is very suitable. LDO has low pressure difference, low noise, and power ripple suppression ratio, static current relative to traditional DC-DC. Small and other features. Therefore, in the product circuit design, a suitable power supply system is a very important job.
I have been fortunate to get a TPS7A4700 power supply evaluation board for Texas instruments - TPS7A4700EVM, as shown below
The entire evaluation board is very simple, and a power conversion module is made based on the LDO TPS7A4700.
TPS7A4700 Features:
Input voltage range: + 3V to + 36V
Output voltage noise: 4.17μVRMS (10Hz, 100Hz)
Power ripple suppression: 82dB (100 Hz); ≥ 55dB (10Hz, 10MHz)
Output voltage range: + 1.4V to + 20.5V
Output current: 1A
Pressure drop voltage: 1A is 307mV
Enable pin compatible with CMOS logic level
Built-in fixed current limit and heat
High cooling performance packages: 5mm x 5mm quadruple flat wireless package (QFN)
It can be seen that the TPS7A4700 is an ultra-low noise (4.17UVRMS) without an external resistance or a feedforward capacitor, and the output voltage (maximum can be 20.5V) can be adjusted to provide a linear regulator of the load circuit of 1A.
Look at this TPS7A4700 evaluation board knows that its schematic is also very simple:
There is a more interesting, controversial issue, the input voltage on the schematic of the evaluation board, not 36V indicated by the LDO TPS7A4700, why?
I guess whether it is because the filter tantalum capacitance TAJE476M035RNJ at the TPS7A4700 evaluation board input is only 35V, and the input voltage of 36V is easy to break down the gallbladder capacitance. The patch tantalum capacitor with 47uf and 35V voltage resistance may be considered for the following reasons:
The tantalum capacitor has a low ESR characteristic, which reduces the ripple of output voltage.
Tantalum capacitors with 47uF, 35V above, are not common, mostly in-line capacitors, this may be beautiful
More importantly, according to the actual situation, the LDO TPS7A4700 maximum output voltage is 20.5V, and the pressure drop voltage 1a is 307mA, and excessive input voltage affects the efficiency of LDO, is not recommended.
Of course, these are just the guess, I don't know if some netizens can answer questions.
In the actual product design, TPS7A4700 circuit design can match the Webench design tool for Texas Instruments to speed up your product design. Engineers with design experience should be understood that in actual product design, the packaging of the image unit is an extremely time consuming project, and it is not careful.
However, use Webench, which is an example of voltage conversion output with 5V to 3.3V, open the Webench Home, and enter the design of the rough parameters in the label field of the single power supply.
After clicking the start design, you will jump out a big push plan, enter the TPS7A4700 directly in the search box to find the appropriate design.
The corresponding TPS7A4700 summary design box will appear after the background calculation is completed, which contains subfranes such as optimized adjustment, chart, schematic, material list, and final design results can also be adjusted according to different content in the sub-box.
The most important step to achieve the most simplified design - Finally determine the design plan, in the lower right corner "Your Over Design" sub-box output through the CAD File Export option, output the library files we need, the output library file format includes the current popular EDA design tools. Such as Altium Designer, Orcad, DesignSpark, etc.
With DesignSpark design tools (comparison, only 70M or so size installation files, more importantly, do not need License as an example, will import files from WebEnch output into Schematic Symbols, PCB Symbols, and Components in DesignSpark, PCBOLs, and Components. These components can then be added directly from the library to complete the design, and do not need to be packaged in additional to the package, saving a lot of time.
In the design of the circuit power supply system, if the design is designed with the existing LDO, it is very simple, but the real difficulty is to design a product to select the appropriate LDO (especially in circuitry including the simulated digital mixed signal). As mentioned before, it is necessary to consider all aspects. For example, mobile RF devices must focus on the reference voltage output ripple, noise; low power consumption devices, the LDO's static current is as small as possible; such as battery-powered equipment, using LDO with low pressure differences as much as possible Voltage performance conversion efficiency, these factors need to be considering in practical design.
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