Electronic Design - What are the IC substitution techniques in PCB design?

In the PCB circuit design, when it is necessary to replace the IC chip, I will talk about my skills when replacing the IC chip to help the designer to be more perfect in the PCB circuit design.

1. Direct substitution

Direct substitution refers to directly replacing the original IC with other IC chips without any changes, and the main performance and indicators of the machine will not be affected after the substitution.

The replacement principle is: the function, performance index, package form, pin usage, pin number and interval of the replacement IC are the same. The same function of the IC not only refers to the same function, but also the same logic polarity, that is, the output and input level polarity, voltage, and current amplitude must be the same. Performance indicators refer to the IC’s main electrical parameters (or main characteristic curves), maximum power dissipation, maximum operating voltage, frequency range, and various signal input and output impedance parameters that are similar to those of the original IC. Substitutes with low power should increase the heat sink.

1. Substitution of IC chips of the same type

The replacement of the same type of IC chip is generally reliable. When installing the integrated PCB circuit, be careful not to make the wrong direction, otherwise, the integrated PCB circuit is likely to be burned when the power is turned on. Some single in-line power amplifier ICs have the same model, function, and characteristic, but the direction of the pin arrangement order is different. For example, the dual-channel power amplifier ICLA4507 has "positive" and "reverse" pins, and its starting pin markings (color dots or pits) are in different directions: there is no suffix and the suffix is "R", IC, etc., for example M5115P and M5115RP.

2. Substitution of different types of IC

(1) Substitution of ICs with the same prefix letter and different numbers. As long as the pin functions of this kind of substitution are exactly the same, the internal PCB circuit and electrical parameters are slightly different, and they can also be directly substituted for each other. For example, the audio amplifier ICs LA1363 and LA1365, the latter adds a Zener diode inside IC pin 5. compared with the former, and the others are exactly the same.

(2) Substitution of ICs with different model prefix letters and the same number. In general, the prefix letter indicates the manufacturer and the category of the PCB circuit. The numbers after the prefix letter are the same, and most of them can be directly replaced. But also count, although the number is the same, but the function is completely different. For example, HA1364 is a sound IC, and uPC1364 is a color decoding IC; 4558, 8-pin is an operational amplifier NJM4558, 14-pin is a CD4558 digital PCB circuit; therefore, the two cannot be replaced at all.

(3) Substitution of IC with different model prefix letters and numbers. Some manufacturers import unpackaged IC chips and then process them into products named after the factory. Another example is to improve products in order to improve certain parameters. These products are often named with different models or distinguished by model suffixes. For example, AN380 and uPC1380 can be directly replaced; AN5620, TEA5620, DG5620, etc. can be directly replaced.

2. Indirect substitution

Indirect substitution refers to a method in which an IC that cannot be directly replaced is a method of slightly modifying the PCB circuit, changing the original pin arrangement or adding or reducing individual components, etc., to make it a replaceable IC.

Substitution principle: The IC used in the replacement can have different functions and different appearances from the original IC, but the functions should be the same and the characteristics should be similar; the performance of the original machine should not be affected after the replacement.

1. Replacement of IC chips with the same plastic but different pin functions

This kind of substitution needs to change the PCB circuit and pin arrangement, which requires certain theoretical knowledge, complete information, and rich practical experience and skills.

2. Substitution of IC chips with the same PCB circuit function but different individual pin functions

The replacement can be carried out according to the specific parameters and instructions of each type of IC. For example, the AGC and video signal output in the TV have the difference between positive and negative polarity, as long as the inverter is connected to the output terminal, it can be replaced.

3. Some empty feet should not be grounded without authorization

Some of the lead pins in the internal equivalent PCB circuit and the application PCB circuit are not marked. When encountering empty lead pins, they should not be grounded without authorization. These lead pins are alternate or spare pins, and sometimes they are also used as internal connections.

4. Substitution of different packaged IC chips

For IC chips of the same type, but with different package shapes, only the pins of the new device need to be reshaped according to the shape and arrangement of the pins of the original device. For example, the AFTPCB circuit CA3064 and CA3064E, the former is a circular package with radial pins: the latter is a dual in-line plastic package, the internal characteristics of the two are exactly the same, and they can be connected according to the pin function. Dual-row ICAN7114, AN7115 and LA4100, LA4102 are basically the same in package form, and the lead and heat sink are exactly 180 degrees apart. The aforementioned AN5620 dual in-line 16-pin package with heat sink, TEA5620 dual in-line 18-pin package, 9 and 10 pins are located on the right side of the integrated PCB circuit, which is equivalent to the AN5620 heat sink. The other pins of the two are arranged in the same way. Connect the 9th and 10th pins to the ground to use.

5. Combination substitution

Combination replacement is to reassemble the undamaged PCB circuit parts of multiple ICs of the same model into a complete IC to replace the poorly functioning IC. It is very applicable when the original IC is not available. But it is required that a good PCB circuit inside the IC used must have interface leads.

The key to indirect substitution is to find out the basic electrical parameters of the two ICs that are substituted for each other, the internal equivalent PCB circuit, the function of each pin, and the connection relationship between the components of the IC. Be careful in actual operation.

(1) The numbering sequence of the integrated PCB circuit pins should not be wrongly connected;

(2) In order to adapt to the characteristics of the replaced IC, the components of the PCB circuit connected to it should be changed accordingly;

(3) The power supply voltage should be consistent with the replacement IC. If the power supply voltage in the original PCB circuit is high, try to reduce the voltage; if the voltage is low, it depends on whether the replacement IC can work;

(4) After the replacement, the quiescent working current of the IC should be measured. If the current is much larger than the normal value, it means that the PCB circuit may be self-excited, and decoupling and adjustment must be carried out at this time. If the gain is different from the original, the resistance of the feedback resistor can be adjusted;

(5) After the replacement, the input and output impedance of the IC must match the original PCB circuit; check its drive capability;

(6) Make full use of the pin holes and leads on the original PCB circuit board when making changes, and the external leads must be neat and avoid front and back crossing, so as to check and prevent the self-excitation of the PCB circuit, especially to prevent high-frequency self-excitation;

(7) It is better to connect a DC current meter in series before power-on Vcc loop, and observe whether the change of the total current of the integrated PCB circuit is normal from large to small.

6. Replace IC chips with discrete components

Sometimes discrete components can be used to replace the damaged part of the IC to restore its function. Before replacement, you should understand the internal function principle of the IC, the normal voltage of each pin, the waveform diagram and the working principle of the PCB components. Also consider:

(1) Whether the signal can be taken out from the work C and connected to the input terminal of the PCB circuit:

(2) Whether the signal processed by the PCB circuit can be connected to the next level inside the integrated PCB circuit for reprocessing (the signal matching during connection should not affect its main parameters and performance). If the intermediate amplifier IC is damaged, from the typical application PCB circuit and internal PCB circuit, it is composed of audio intermediate amplifier, frequency discrimination and frequency boosting. The signal input method can be used to find the damaged part. If the audio amplifier part is damaged, discrete components can be used. The device replaces.