PCB Technical - IC Substitution Skills in PCB Circuit Design

When there is a need to replace IC in PCB circuit design, let’s share some tips when replacing IC to help designers be more perfect in PCB circuit design.

1. Direct substitution

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 impedance parameters that are similar to those of the original IC. Substitutes with low power should increase the heat sink.

1. Substitution of the same type of IC

The replacement of the same type of IC is generally reliable. When installing the integrated PCB circuit, be careful not to make a mistake in the direction, otherwise, the integrated PCB circuit may 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, dual-channel power amplifier ICLA4507, the pins are divided into "positive" and "reverse", and the initial 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 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: ICLA1363 and LA1365 are put in the sound, the latter adds a Zener diode inside IC pin 5 than the former, and the others are exactly the same.

In general, the prefix letter indicates the PCB 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 also 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 is 4558, the 8-pin is an operational amplifier NJM4558, and the 14-pin is a CD4558 digital PCB circuit; therefore, the two cannot be replaced at all. So it must depend on the pin function.

Some manufacturers introduce unpackaged IC chips and 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, AN380 and uPC1380 can be directly replaced, and 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 peripheral PCB circuit, changing the original pin arrangement or adding or removing 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. Substitution of different packaged ICs

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 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 9th and 10th pins to the ground to use.

2. Substitution of lC 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. Substitution of ICs with the same plastics 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, and sometimes they are also used as internal connections.

5. Combination substitution

Combination substitution 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 peripheral 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 best to connect a DC current meter in series in the power supply Vcc loop 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 the signal can be taken out from the industrial C 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 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 replace.