In many circuits will series small resistance values of resistance, do not underestimate them, in fact, a great role.
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1, SPI signal line
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The series resistance on the SPI signal is generally about dozens of ohms, and it generally has the following functions:
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1) Impedance matching. Because the impedance of the signal source is very low, the impedance between the signal line is not matched, and the matching situation can be improved after a resistor is strung on to reduce reflection.
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2) The SPI rate is high, series a resistor, and the online capacitor and load capacitor to form the RC circuit, reduce the signal steepness, avoid overshoot, overshoot sometimes damage chip GPIO, of course, EMI is also good, especially high-speed circuit.
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3) Easy debugging, many of the current chips are BGA, QFN package, series a resistor, debugging with oscilloscope to grasp the waveform is convenient.
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2. LDO input terminal
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When the VIN absolute maximum of the LDO is close to the power supply voltage, at this time you do not want to change the high specification LDO, in order to save costs, then you can string a small resistance resistance, can absorb a part of the voltage and current, when the power side of the larger surge, the resistance will take the lead, the cost is less.
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It is assumed that LDO breakdown, VIN and GND short circuit, because of the presence of series resistance R, will also avoid the short circuit between the power supply SYS_5V and GND.
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3, TVS before and after series resistance
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TVS series resistance generally has two connection methods, Figure A resistor in front of TVS, Figure B resistor after TVS, the two circuit use scenarios are different.
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First ask you a question, resistance and TVS that surge resistance ability is strong, the answer is undoubtedly, of course, TVS.
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1) For Figure A, first consider the surge size, if it is not large, you can choose a suitable power resistor, the resistor in front of the TVS, will absorb a small part of the current, the surge current IPP is small, the corresponding TVS Vc (clamp voltage) will be smaller, better protection of the back-end load.
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2) For Figure B, TVS first absorbs most of the surge current, and part of the residual voltage or residual current will pass through the resistor R2 for secondary voltage and current limiting, which can better protect the back-end load. If the back-end load is much larger than R2, the partial voltage and current limit will be negligible, and R2 will not be useful.
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Several circuit components are connected to each other along a single path, and each node is connected to a maximum of two components, which is called series connection. Circuits connected in series are called series circuits.
The current flowing through each resistor in a series circuit is equal. Because each section of the same branch in the direct current circuit has the same current intensity.
Series resistance can play the role of voltage division and current limiting
In the general circuit we commonly use, the series resistance can increase the total resistance value of the resistance, which can reduce the circuit current, which is equivalent to a part of the voltage. This sliding resistor in middle school physics is a good example.
In the general circuit we commonly use, the series resistance can increase the total resistance value of the resistance, which can reduce the circuit current, which is equivalent to a part of the voltage. This sliding resistor in middle school physics is a good example.