Replace IC skills to make PCB circuit design more perfect
Direct substitution refers to directly replacing the original IC with other ICs without any modification, 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, and impedance parameters etc. are similar to those of the original IC. Substitutes with low power should increase the heat sink.
1. Replacement of the same type of IC
The replacement of the same type of IC is generally reliable. When installing the integrated PCB circuit, pay attention to the direction not to be wrong, otherwise, the integrated PCB circuit may be burned when the power is turned on. Some single in-line package power amplifier IC though has the same model, function, and characteristic, but the direction of the pin arrangement order is different.
2. 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: sound IC LA1363 and LA1365, the latter adds a Zener diode inside the 5th pin of the IC than the former, and the others are exactly the same.
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 there are a few special cases. Although the numbers are the same, the functions are completely different. For example, HA1364 is a sound IC, and uPC1364 is a color decoding IC; the number 4558, the 8-pin is the operational amplifier NJM4558, and the 14-pin is the CD4558 digital PCB circuit; so the two cannot be replaced at all. So it depends on the pin function.
Some manufacturers import unpackaged IC chips and then process them into products named after the factory, and some improved products to improve certain parameters. These products are often named with different models or distinguished by model suffixes. For example, AN1380 and uPC1380 can be directly replaced, and AN5620, TEA5620, DG5620, etc. can be directly replaced.
Indirect substitution refers to a method in which an IC that cannot be directly replaced is a method of slightly modifying the peripheral PCB circuit, changing the original pin arrangement or adding or subtracting individual components, etc., to make it a replaceable IC.
Substitution principle: The IC used in the replacement can have different pin 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. Substitution of different packaged ICs
For IC chips of the same type but with different package appearances, only the pins of the new device need to be reshaped according to the shape and arrangement of the original device pins. 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, they can be connected according to the pin function. Dual-row IC AN7114, 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 and TEA5620 dual in-line 18-pin package, pins 9 and 10 are located on the right side of the integrated PCB circuit, which is equivalent to the heat sink of AN5620. The other pins of the two are arranged in the same way. Connect the pins 9 and 10 to ground to use.
2. Substitution of ICs 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 in the TV, video signal output have the difference between positive and negative polarity, as long as the output terminal is connected with an inverter, it can be replaced.
3. Substitution of ICs of the same type but different pin functions
This kind of substitution needs to change the peripheral PCB circuit and pin arrangement, which requires certain theoretical knowledge, complete information and rich practical experience and skills.
4. 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 there are empty lead pins, they should not be grounded without authorization. These lead pins are alternate or spare pins, sometimes 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 the poorly functioning IC. It is very suitable when the original IC is not available. But it is required that a good PCB circuit inside the IC used must have an interface pin.
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 IC components. 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 peripheral PCB circuit connected to it should be changed accordingly;
(3) The power supply voltage should be consistent with the replaced IC. If the power supply voltage in the original PCB circuit is high, try to step down; 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. At this time, decoupling and adjustment are required. If the gain is different from the original, the resistance of the feedback resistor can be adjusted;
(5) After the replacement, the input, output and impedance of the IC should 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 should be neat and avoid front and back crossing, so as to check and prevent the PCB circuit from self-excitation, especially to prevent high-frequency self-excitation;
(7) It is best to connect a DC current meter in series in the Vcc loop of the power supply before power-on, and observe whether the change of the total current of the integrated PCB circuit is normal from large to small.
6. Replace IC 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 circuit with peripheral components. Also consider:
(1) Whether can take out signal from the IC and connected to the input terminal of the peripheral PCB circuit;
(2) Whether the signal processed by the peripheral PCB circuit can be connected to the next stage 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 instead.