There are 0402,0603,0805 types of table stick resistance packaging commonly used in mobile phone boards, which may be used in some high-density boards.
The main functions of resistors on the circuit include shunt, current limiting, partial voltage, bias, filtering (used in combination with capacitors) and impedance matching, etc. The following is a detailed discussion of the various functions of resistors based on the principles of mobile phones.
Resistance is used as a shunt in parallel circuits. It seems not to have been used in the cell phone board. If anyone has seen a similar usage, please advise.
Resistance plays a role of voltage division in series circuit. On the USB to serial port line we use, the general manufacturer USES two resistors to get 3.3V or 2.8V level of the serial port.
The bias action of resistors is typical in the MIC loop in mobile phone theory.
The filtering action of resistance is generally composed of RC filtering circuit and capacitor, which can be divided into low-pass circuit and high-pass circuit.
Impedance matching of resistors is generally used in high-speed signals, such as the 100R resistance in parallel between TXP and TXN of DIGRF.
The above Outlines some of the USES of resistors. The following highlights the USES of resistors in the schematic at work. Please understand that the exposition may be messy.
0 ohm resistance:
1. There is no function in the circuit, but it is only on the PCB for debugging convenience or compatible design. Commonly used -
2. Can be used for jumper, if a section of the circuit is not used, directly do not stick the resistance (does not affect the appearance) -- commonly used
3. In the case of uncertain matching circuit parameters, 0 ohm should be used instead. In the actual debugging, parameters should be determined and then specific numerical components should be used instead. - not commonly used
4. When you want to measure the current consumption of a certain part of the circuit, you can remove the 0ohm resistor and connect it with the ammeter, so as to measure the current consumption conveniently. -- Measure the charging current on the multimeter
5. When wiring, if it is too difficult to pass, you can also add a 0 ohm resistor -- no
6. Used as inductor or capacitance and (related to external circuit characteristics) inductor in high-frequency signals, mainly to solve EMC problems. Between ground and ground, power and ICPin -- to be learned
7. Single point grounding (refers to protection grounding, working grounding, and DC grounding are separated from each other on the equipment and become independent systems.) -- Not commonly used, as seen on some products, such as electronic equipment powered by POE.
8. Fuse function -- No
9. Single point ground, analog and digital -- not commonly used, same as 7
Expanded description of Action 9:
There are four ways to solve this problem for analog and digital single point grounding:
(1) Connect with magnetic beads;
(2) Connect with capacitance;
(3) Inductance connection;
(4) Connect with 0 ohm resistance.
The equivalent circuit of magnetic beads is equivalent to a band stop-limiter, which can significantly suppress the noise at a certain frequency point. Therefore, it is necessary to estimate the noise point frequency in advance in order to select the appropriate model. For uncertain or unpredictable frequency, magnetic beads are incompatible.
Capacitance is separated by straight intersection, resulting in floating ground.
Large volume, many stray parameters and instability of inductance.
The 0 ohm resistance is equivalent to a very narrow current path, which can effectively limit the loop current and suppress the noise. Resistors have attenuation across all bands (0 ohm resistors also have impedance), which is stronger than magnetic beads.
The actual resistance of a typical 0R resistor is about 50 milliohms +-5% off, so you can roughly estimate the current rating of each package.
0201-1/20 w I = 1 a
0402-1/16 w I = 1.118 A
0603-1/8 w I = 1.58 A
0805-1/4 w I = 2.23 A
Overdesign is recommended for practical use.
In the design of mobile phones, apart from the extensive use of 0R resistors, there are also some resistors, such as connected in series with LED, connected in series with spare battery, and ISET (or other names) PIN pins of some IC, which act as current limiting. Some resistors are connected in series with capacitors, typically the input end of audio power amplifier, which ACTS as a filter, which will be explained in detail later. The two resistors at the feedback end of DCDC are generally acting as partial voltage to adjust the output voltage of DCDC.
Another common use of resistors in schematic design is to pull them up and down. Let's look at the summary of the pull up and down resistors on the web
Pull-up resistance:
1. When TTL circuit drives COMS circuit, if the high level output by TTL circuit is lower than the lowest high level of COMS circuit (generally 3.5V), then the TTL output end needs to be connected with a pull resistor to improve the output high level.
2, OC gate circuit must be added with pull resistance, can be used.
3. In order to increase the driving ability of output pins, pull-up resistors are often used in some microcontroller pins.
4. On THE COMS chip, in order to prevent damage caused by static electricity, the unused pin cannot be suspended. Generally, it is connected with the pulling resistance to reduce the input impedance and provide the discharge path.
5. The pin of the chip is added with pull resistance to improve the output level, so as to improve the noise tolerance of the input signal of the chip and enhance the anti-interference ability.
6. Improve the anti-electromagnetic interference ability of bus. A dangling pin is more susceptible to electromagnetic interference from the outside world.
7. The resistance mismatch in long line transmission is easy to cause the reflected wave interference, and the pull-down resistance is resistance matching, which can effectively suppress the reflected wave interference.
The selection principles of pull-up resistance include:
1. Considering the power saving and current filling capacity of the chip, it should be large enough; High resistance, low current.
2. The consideration of ensuring sufficient driving current should be small enough; Small resistance, large current.
3. For high-speed circuits, too much pull-up resistance may flatten the edge. Taking the above three points into consideration, it is usually chosen between 1K and 10K. The same is true for pull-down resistors.
The selection of pull-up resistance and pull-down resistance should be set in combination with the characteristics of the switching tube and the input characteristics of the subordinate circuit, mainly considering the following factors:
1. Balance of driving power and power consumption. Generally speaking, the smaller the pull-up resistance is, the stronger the driving ability is, but the higher the power consumption is. Therefore, attention should be paid to the balance between the two in design.
2. Driving requirements for subordinate circuits. Similarly, when the output is high, the switch tube is disconnected and the pull up resistance should be appropriately selected to provide sufficient current to the lower circuit.
3. High and low level setting. The threshold level of high and low level of different circuits will be different, and the resistance should be set appropriately to ensure that the correct level can be output. Take the above pull resistance as an example. When the output is low, the partial voltage values of switch tube conduction, pull up resistance and switch tube conduction resistance should be below the zero level threshold.
4. Frequency characteristics. Take the above pull resistance as an example, the RC delay will be formed between the capacitance between the pull resistance and the leakage source stage of the switch tube and the input capacitance between the subordinate circuits. The greater the resistance, the greater the delay will be. The pull-up resistance should be set to take into account the needs of the circuit in this respect.
The pull-down resistance is set in the same principle as the pull-up resistance.
OC gate output high voltage is usually in a high resistance state, and the pull up current is provided by the pull up resistance. Let each input port be no more than 100uA, and let the outlet drive current be about 500uA. The standard operating voltage is 5V, and the high and low level threshold of the input port be 0.8V(lower than this value is low level). 2V(high level threshold).
When the pull resistance is selected:
500Uax8.4K =4.2 that is, the output end can be pulled down below 0.8V if it is larger than 8.4K, which is the minimum resistance value, and it cannot be pulled down if it is smaller. If the outlet drive current is large, the resistance value can be reduced to ensure that it can be lower than 0.8V when pulling down.
When the output is high, the leakage current of the pipe is ignored, and 200uA is required for two input ports
200uAx15K=3V, that is, the pull-up resistance voltage drop is 3V, the output can reach 2V, this resistance value is the maximum resistance value, no more than 2V. And that's choice 10K. COMS doors refer to the 74HC series
Cannot be ignored when the design of pipe leakage current, IO mouth actual current under different level is different, the above is just the principle that a word summarized as: high output power to feed the back of the input port at ordinary times, feed through the output low level don't put the output (or excess current fed to the input port of cascade, higher than the low level threshold values are not reliable).
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