In PCB circuit design, there will be times when IC needs to be replaced. Here’s to share the skills of IC replacement, so as to help designers to be more perfect in PCB circuit design.
Direct substitution
Direct replacement refers to the direct replacement of the original IC with other IC without any change, which will not affect the main performance and index of the machine.
The replacement principle is: the function, performance index, packaging form, pin usage, pin number and interval of the replacement IC are the same. Among them, the same function of IC means not only the same function, but also the same logic polarity, that is, the polarity of input and output levels, voltage and current amplitude must be the same. Performance index means that the main electrical parameters (or main characteristic curves), maximum dissipated power, maximum working voltage, frequency range, input and output impedance of each signal of the IC should be similar to those of the original IC. For low-power substitutes, the heat sink should be enlarged.
1 same model
The replacement of the same type of IC is generally reliable. When installing the integrated PCB circuit, pay attention not to make a mistake, otherwise, the integrated PCB circuit may be burned when it is powered on.Although the models, functions and characteristics of some in-line power amplifier IC are the same, the direction of pin arrangement is different.For example, the dual channel power amplifier icla4507 can be divided into “positive” and “negative” pins, and the direction of starting pin marking (color dot or pit) is different: there is no IC with Suffix “R”, such as m5115p and m5115rp.
2 model letter prefix is the same, the number is different
As long as the pin functions of each other are identical, the internal PCB circuit and electrical parameters are slightly different, so it can also be replaced directly.For example, icla1363 and la1365 are played in the accompanying sound. The latter adds a voltage stabilizing diode in the fifth pin of IC compared with the former, and the others are exactly the same.
In most cases, the prefix can be replaced by the letters of the manufacturer.But there are also a few special cases. Although the numbers are the same, the functions are completely different.For example, ha1364 is associated sound 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, they can not be replaced.So we must look at the pin function.
Some manufacturers introduce unpackaged IC chips, and then process them into products named after our factory, and some products are improved to improve some parameters. These products are often named by different models or distinguished by model suffixes. For example, AN380 and uPC1380 can be directly replaced, and AN5620, TEA5620, DG5620, etc. can be directly replaced.
Indirect substitution
Non-direct replacement refers to the method that the IC that cannot be directly replaced slightly modifies the peripheral PCB circuit, changes the original pin arrangement or adds or subtracts individual components, etc., so as to make it a replaceable IC.
Replacement principle: the IC used for replacement can have different functions and shapes from the original IC pins, but the functions should be the same and the characteristics should be similar; The replacement should not affect the performance of the original machine.
1. different packages
The IC chip of the same type, but with different package shapes, should be replaced by reshaping the pins of the new device according to the shape and arrangement of the pins of the original device. For example, AFTPCB circuits CA3064 and CA3064E, the former is a circular package with radial pins; The latter is a dual in-line plastic package, and the internal characteristics of the two are exactly the same, so they can be connected according to the pin function. Two-row ICAN7114 and AN7115 have basically the same package form as LA4100 and LA4102, and the difference between pins and 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 AN5620. The other pins of the two are arranged in the same way, so you can connect the pins 9 and 10 to the ground.
2. Circuit functions are the same, but individual pins have different functions.
Replacement can be carried out according to the specific parameters and instructions of each model IC. For example, the AGC and video signal output in TV sets are different in polarity, so long as an inverter is connected at the output end, it can be replaced.
3. Similar plastic pins have different functions.
This kind of substitution needs to change the peripheral PCB circuit and pin arrangement, so it needs certain theoretical knowledge, complete data and rich practical experience and skills.
4. Some empty feet should not be grounded without authorization.
Some pins in the equivalent PCB circuit and the application PCB circuit are not marked. When there are empty pins, they should not be grounded without authorization. These pins are replacement or spare pins, and sometimes they are also used as internal connections.
5. Combination and substitution
Combination and replacement is the method of recombining the undamaged PCB circuit parts in multiple ICs of the same model into a complete IC to replace the IC with poor function. It is very suitable when the original IC can’t be bought. However, it is required that the intact PCB circuit inside the IC used must have interface leads.
The key to non-direct replacement is to find out the basic electrical parameters, the internal equivalent PCB circuit, the functions of each pin, and the data of the connection relationship between the components of the IC. Pay attention to it in practice.
(1) the serial number of the pins of the integrated PCB circuit shall not be wrong;
(2) In order to adapt to the characteristics of the replaced IC, the components of the peripheral PCB circuit connected to it should be changed accordingly;
(3) The power supply voltage should be consistent with the C after replacement. If the power supply voltage in the original PCB circuit is high, try to step down; Low voltage depends on whether the replacement IC can work;
(4) After replacement, the static 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 generate self-excitation, and then it must be decoupled and adjusted. If the gain is different from the original, the resistance value of the feedback resistor can be adjusted;
(5) The input and output impedance of the replaced IC should match the original PCB circuit; Check its driving ability;
(6) When making changes, make full use of the foot holes and leads on the original PCB circuit board, and the external leads should be neat, so as to avoid the front and back cross, so as to check and prevent the self-excitation of PCB circuit, especially the high-frequency self-excitation;
(7) Before electrifying, it is better to connect a DC ammeter in series in the Vcc circuit of the power supply, and observe whether the change of the total current of the integrated PCB circuit is normal or not by decreasing the resistance value of the step-down resistor.
6. Replace IC with discrete components
Sometimes, the damaged part of IC can be replaced by discrete components to restore its function. Before replacement, you should know the internal function principle of the IC, the normal voltage and waveform diagram of each lead, and the working principle of PCB circuit composed of peripheral components. At the same time, we should also consider:
(1) whether the signal can be taken out of the IC and connected to the input 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 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 amplification stage. The signal input method can be used to find out the damaged part. If the audio amplification part is damaged, it can be replaced by discrete components.
