IC substitution skills in PCB circuit design
2024-03-14
In PCB circuit design, there will be times when IC needs to be replaced. Let`s share the tips when replacing IC to help designers become more perfect in PCB circuit design. 1. Direct substitution Direct substitution refers to directly replacing the original IC with other IC without any modification. The main performance and indicators of the machine will not be affected after the substitution. The principle of substitution is: the function, performance index, packaging form, pin usage, pin number and spacing of the replacement IC are the same. The same function of the IC not only means 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 should be similar to the original IC. Substitute parts with low power need to enlarge the heat sink. 1. Replacement of the same model IC Replacement of the same type of IC is generally reliable. When installing the integrated PCB circuit, be careful not to get it in the wrong direction. Otherwise, the integrated Printed Circuit Board circuit is likely to be burned when power is turned on. Some single in line power amplifier ICs have the same model, function, and characteristics, but the direction of the pin arrangement is different. For example, the two channel power amplifier ICLA4507 has "positive" and "reverse" pins, and its starting pin markings (color dots or pits) are in different directions: no suffix and "R" suffix, IC, etc., for example M5115P and M5115RP. 2. Substitution of ICs with the same model prefix letters and different numbers This kind of substitution can also be directly substituted with each other as long as the pin functions are exactly the same and the internal PCB circuit and electrical parameters are slightly different. For example: ICLA1363 and LA1365 are placed in the audio. The latter has a Zener diode inside IC pin 5 than the former, but everything else is exactly the same. Generally speaking, the prefix letters indicate the manufacturer and the type of Pcba Circuit Board. The numbers after the prefix letters are the same, and most of them can be directly replaced. But there are a few exceptions where although the numbers are the same, the functions are completely different. For example, HA1364 is a sound IC, while uPC1364 is a color decoding IC; the number is 4558, the 8-pin one is the operational amplifier NJM4558, and the 14-pin one is the CD4558 digital PCB circuit; therefore, the two cannot be replaced at all. So you must also look at the pin function. Some manufacturers introduce unpackaged IC chips and then process them into products named after their own manufacturers, as well as products improved 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, and AN5620, TEA5620, DG5620, etc. can be directly replaced. 2. Indirect substitution Indirect substitution refers to the method of slightly modifying the peripheral PCB circuit of an IC that cannot be directly replaced, changing the original pin arrangement or adding or subtracting individual components, etc., to make it a replaceable IC. Principle of substitution: The IC used for substitution can have different pin functions and shapes from the original IC, but the functions must be the same and the characteristics must be similar; the performance of the original machine should not be affected after substitution. 1. Substitution of different package ICs IC chips of the same type but with different package shapes. When replacing, just reshape the pins of the new device 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. The packaging forms of dual row ICAN7114, AN7115 and LA4100, LA4102 are basically the same, and the pins and heat sinks are exactly 180 degrees different. 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 located 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 the same. Just connect pins 9 and 10 to ground. 2. PCB circuit function is the same but individual pin functions are different IC replacement Replacement can be carried out according to the specific parameters and instructions of each model of IC. For example, the AGC and video signal outputs in TVs have positive and negative polarities, which can be replaced by adding an inverter to the output end. 3. Substitution of ICs with the same plastic but different pin functions This kind of substitution requires changing the peripheral PCB circuit and pin arrangement, so it requires certain theoretical knowledge, complete information and rich practical experience and skills. 4. Some empty feet should not be grounded without permission Some pins in the internal equivalent PCB circuit and application PCB circuit are not marked. When encountering empty pins, you should not ground them without authorization. These pins are replacement or spare pins, and sometimes they are also used as internal connections. 5. Combination substitution Combination replacement is a method of recombining the undamaged PCB circuit parts of multiple ICs of the same model into a complete IC to replace malfunctioning ICs. It is very suitable for situations where the original IC cannot be purchased. However, it is required that the intact PCB circuit inside the IC used must have interface pins. The key to indirect substitution is to find out the basic electrical parameters of the two interchangeable ICs, the internal equivalent PCB circuit, the functions of each pin, and the connection relationship between the IC components. Please pay attention during actual operation. (1) The numbering sequence of the 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 must be changed accordingly; (3) The power supply voltage must be consistent with the replaced 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 replacement, the static operating current of the IC must be measured. If the current is much greater than the normal value, it means that the PCB circuit may be self excited, and decoupling and adjustment must be carried out. If the gain is different from the original, the feedback resistor value can be adjusted; (5) The input and output impedance of the IC after replacement must match the original PCB circuit; check its driving capability; (6) When making changes, make full use of the pin holes and leads on the original PCB Circuit Board. The external leads must be neat and tidy to avoid crossing before and after, so as to check and prevent PCB circuit self excitation, especially to prevent high frequency self excitation; (7) Before powering on, it is best to connect a DC ammeter in series to the power supply Vcc loop, and observe whether the change in the total current of the integrated PCB circuit is normal from large to small with the resistance of the voltage reducing resistor. 6. Replace IC with discrete components Sometimes discrete components can be used to replace damaged parts of the IC to restore functionality. Before replacement, you should understand the internal functional principles of the IC, the normal voltage of each pin, the waveform diagram, and the working principle of the PCB circuit composed of peripheral components. Also consider: (1) Can the signal be taken out from 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 (signal matching during connection should not affect its main parameters and performance). If the mid amplifier IC is damaged, judging from the typical application PCB circuit and internal PCB circuit, it consists of audio mid amplifier, frequency identification and frequency amplification stages. The signal input method can be used to find the damaged part. If the audio amplification part is damaged, discrete components can be used replace.
