# Small series resistance, big effect. Classification and summary of common series resistance

Hello everyone, I am Lisa Wang.

In many circuits, small-value resistors are connected in series. Don't underestimate them, they actually play a big role.

1. SPI signal line

The series resistance on the SPI signal, usually about tens of ohms, generally has the following functions:

1) Impedance matching. Because the impedance of the signal source is very low, it does not match the impedance of the signal line. After adding a resistor in series, the matching situation can be improved to reduce reflection.

2) The speed of SPI is high. A resistor is connected in series to form an RC circuit with the line capacitance and load capacitance, which reduces the steepness of the signal and avoids overshoot. Overshoot sometimes damages the chip GPIO. Of course, it is also good for EMI, especially for high-speed circuits. .

3) It is easy to debug. Many of the current chips are BGA and QFN packages, and a resistor is connected in series. It is convenient to use an oscilloscope to capture the waveform during debugging.

2. LDO input

When the VIN absolute maximum of the LDO is close to the power supply voltage, you do not want to change to a higher specification LDO at this time. In order to save costs, a small resistance resistor can be connected in series, which can absorb a part of the voltage and current. When the surge occurs, the resistor will take the lead, and the cost will be less.

Assuming that the LDO is broken down, VIN and GND are short-circuited, because the existence of the series resistance R will also avoid the short-circuit between the power supply SYS_5V and GND.

3. Series resistance before and after TVS

There are generally two connection methods for TVS series resistors. The resistor in Figure A is before the TVS, and the resistor in Figure B is after the TVS. The usage scenarios of the two circuits are different.

Let me ask you a question first. Resistor and TVS have strong anti-surge ability. The answer is undoubtedly TVS.

1) For Figure A, the size of the surge should be considered first. If it is not large, a resistor with suitable power can be selected. The resistor is in front of the TVS and will absorb a small part of the current. After the surge current IPP is small, the corresponding TVS The Vc (clamping voltage) will also become smaller, and the protection of the back-end load is better.

2) For Figure B, the TVS first absorbs most of the surge current, and some residual voltage or residual current will pass through the resistor R2 for secondary voltage division and current limiting, which can better protect the back-end load. If the back-end load is much larger than R2, the voltage divider and current limit will be negligible, and R2 actually has no effect.