Let's talk about the skills of replacing IC in PCB circuit design

When IC replacement is needed in PCB design, let's share the skills of IC replacement to help designers to be more perfect in circuit design

Direct substitution

Direct replacement refers to directly replacing the original IC with other ICs without any change, which will not affect the main performance and indicators of the machine.

The replacement principle is that the functions, performance indicators, packaging forms, pin uses, pin numbers and intervals of the replacement IC are the same. The same function of IC not only means the same function, but also the same logic polarity, that is, the polarity of output and input levels, voltage and current amplitude must be the same. Performance index refers to that the main electrical parameters (or main characteristic curve), maximum dissipated power, maximum working voltage, frequency range, signal input and output impedance of the IC shall be close to the original IC. The heat sink shall be increased for the substitute with small power.

1 Same model

The replacement of IC of the same model is generally reliable. When installing the integrated PCB circuit, pay attention to the correct direction, otherwise, the integrated PCB circuit is likely to be burned when it is powered on. Some single in-line power amplifier ICs have the same model, function and characteristics, but the direction of pin arrangement is different. For example, the dual channel power amplifier ICLA4507 has "positive" and "negative" pins, and its starting pin markings (color dots or pits) have different directions: there is no suffix and IC with "R" suffix, such as M5115P and M5115RP.

2. The prefix of model letter is the same, but the number is different

As long as the functions of the pins between them are identical and the internal PCB circuits and electrical parameters are slightly different, this replacement can also be directly replaced with each other. For example, ICLA1363 and LA1365 are put in the accompanying sound. The latter adds a regulator diode inside the fifth pin of the IC, which is the same as the former.

Generally, the prefix letter indicates the PCB manufacturer and the category of PCB circuit. The numbers behind the prefix letter are the same, and most of them can be directly replaced. But there are also a few special cases. Although the numbers are the same, the functions are completely different. For example, HA1364 is audio IC, while uPC1364 is color decoding IC; The number is 4558, the 8-pin is the operational amplifier NJM4558, and the 14 pin is the CD4558 digital PCB circuit; Therefore, the two cannot be replaced at all. Therefore, it must depend on the pin function.

Some PCB manufacturers introduce unpacked IC chips, and then process them into products named by their own factory, as well as products improved to improve some parameter indexes. These products are usually named by different models or differentiated by model suffix. For example, AN380 and uPC1380 can be directly replaced, and AN5620, TEA5620, DG5620, etc. can be directly replaced.

Indirect substitution

Indirect replacement refers to the method that IC that cannot be directly replaced slightly modifies peripheral PCB circuits, changes the arrangement of original pins, or adds or subtracts individual components, so as to make it a replaceable IC.

Replacement principle: The IC used for replacement can be different from the original IC pin in function and shape, but the function should be the same and the characteristics should be similar; The replacement shall not affect the performance of the original machine.

1. Different packages

For IC chips of the same type but with different package shapes, the pins of the new device should be shaped according to the shape and arrangement of the original device pins. For example, AFTPCB circuit CA3064 and CA3064E, the former is round package with radial pins; The latter is a dual in-line plastic package, and their internal characteristics are exactly the same. They can be connected according to the pin function. The dual row ICAN7114 and AN7115 are basically the same package as LA4100 and LA4102, and the difference between the pin and the heat sink is exactly 180 degrees.

The aforementioned AN5620 dual in line 16 pin package with heat sink and TEA5620 dual in line 18 pin package have pins 9 and 10 on the right side of the integrated PCB circuit, which are equivalent to the heat sink of the AN5620. The other pins of the two are arranged in the same way. They can be used after connecting pins 9 and 10 to ground.

2. The circuit functions are the same, and individual pins have different 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 outputs in the TV set have positive and negative polarity differences, which can be replaced as long as the inverter is connected to the output end.

3. Similar plastic pins have different functions

This replacement needs to change the peripheral PCB circuit and pin arrangement, so it requires certain theoretical knowledge, complete data and rich practical experience and skills.

4. Some empty feet should not be grounded without authorization

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

5. Combination substitution

Combination replacement is a method to reassemble the undamaged PCB circuit parts of multiple ICs of the same model into a complete IC to replace the IC with poor functions. It is very applicable when the original IC cannot be purchased. However, it is required that PCB circuits with intact internal ICs must have interface pins.

The key of indirect replacement is to find out the basic electrical parameters of the two ICs that are replaced each other, the internal equivalent PCB circuit, the function of each pin, and the data of the connection relationship between the IC components. Pay attention during actual operation.

(1) The numbering sequence of integrated PCB circuit pins must not be connected incorrectly;

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

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

(4) After replacement, measure the static working current of the IC. If the current is far greater than the normal value, it means that the PCB circuit may generate self excitation, and decoupling and adjustment must be carried out. If the gain is different from the original, the feedback resistance can be adjusted;

(5) The input and output impedance of the replaced IC should match the original PCB circuit; Check its driving ability;

(6) Make full use of the foot holes and leads on the original PCB circuit board when making changes. The external leads should be neat and avoid crossing back and forth to check and prevent PCB circuit from self excitation, especially high frequency self excitation;

(7) Before power on, it is better to connect a DC ammeter in series in the power supply Vcc circuit, and observe whether the change of the total current of the integrated PCB circuit is normal from high to low.

6. Replacing IC with Discrete PCB Components

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

(1) Whether the signal can be taken out of the IC and connected to the input end of the peripheral PCB circuit:

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