As a common electronic component, resistors play a crucial role in circuits.

1. Series voltage divider

The feedback resistor of DC-DC is connected in series to perform calculations on the error amplifier inside the chip after voltage division, controlling the output voltage to achieve stable output. When laying out the feedback resistor, it should be placed close to the chip (try to make the blue line as short as possible to ensure the minimum parasitic parameters), and the voltage sampling line (the blue line in the second layout) should avoid the power loop (high dv/dt and high di/dt loops to prevent interference).

When using LDO under high pressure difference, a resistor can be connected in series to reduce the heat generation of LDO. This application requires attention to the rated power of the resistor and the value of the resistor.

After voltage division, collect data from the microcontroller ADC (the original voltage range is greater than the microcontroller ADC's collection range), for example: collect the voltage of a 3.7V lithium battery (maximum 4.2V). If necessary, a voltage follower can be added to reduce the output impedance of the voltage divider circuit and achieve stronger driving capability.

NTC resistance (negative temperature coefficient resistance, temperature rise resistance value decreases) is used to measure temperature. It is necessary to use a voltage divider to transfer the divided value to the microcontroller for collection. The microcontroller then uses the collected voltage to deduce the resistance value of the NTC resistance, and then calculates the measured temperature based on the resistance value of the NTC resistance.

2. Parallel shunt

When the resistance power is insufficient, parallel power sharing is used, such as sharing power with LDO as mentioned above. It is also possible to change the resistance value in parallel, for example, if you do not have a 500 Ω resistor on hand, you can use two 1k Ω resistors in parallel. Alternatively, as shown in the figure below, the size of the feedback resistor can be dynamically controlled by controlling the conduction or cutoff of MOSFETs through the parallel connection of resistors, in order to control the output voltage.

3. Current sampling

In some usage scenarios, it is necessary to monitor the current situation of the circuit, such as the motor control board (detecting whether the motor is locked, and if the locked current increases), or the feedback of the constant current source and some constant current drive circuits also require sampling resistors. When the current flows through the sampling resistor, a voltage drop will be generated on the resistor, and then the sampled voltage or the current flowing through the circuit will be processed or calculated through a microcontroller or some processing chip, operational amplifier or comparator, etc.

4. RC filtering

It is possible to attenuate some interference signals and external interference noise through RC filtering circuits. Both the power port and signal port can be used, such as the T-CARD power port shown in the figure below (pay attention to the size of the resistance, too large will affect the load voltage).

5. Impedance matching

For example, some USB or clock lines require series resistance impedance matching to ensure signal integrity.

6. Up-down resistance

In some applications where open drain OD gates/open collector OC gates output, a pull-out resistor needs to be added. For example, in some comparators where the output is open drain, a pull-out resistor needs to be added.

The output of IIC is also an open drain output, and in this case, a pull-out resistor needs to be added.

Some signal lines also add pull-up or pull-down resistors to increase the anti-interference ability of the signal. Although most microcontrollers have built-in pull-up or pull-down resistors, the internal pull-up and pull-down resistors of the chip are often large, belonging to weak pull-up and weak pull-down (50k or larger). It is recommended to add an external pull-up or pull-down resistor (4.7k Ω or 10k Ω), for example, adding pull-up resistors to the serial port in the following figure can resist interference (for the serial port, the low level is the starting bit).

7. ESD protection

Adding a resistor to the IO of the chip is beneficial for ESD, as the resistor absorbs some of the energy and can better protect the IO of the chip.

8. Current limiting

As shown in the figure below, the base resistance of the transistor and the series resistance of the buzzer are both used for current limiting.

9. Capacitor discharge

For example, in the example of resistance capacitance voltage reduction, the parallel resistor on the capacitor is used to provide a discharge circuit for the voltage reduction capacitor when the load is disconnected. Generally, safety regulations require that the voltage decay to a certain extent within a certain period of time after unplugging the power supply depends on this leakage resistor.

?10. RC absorption circuit

Eliminating ringing and attenuating high-frequency noise is usually added to GND at switch nodes in switching power supplies.