IC Substitution Skills in PCB Circuit Design

1、 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 functions of IC not only mean the same functions; It should also be noted that the logic polarity is the same, that is, the output and input level polarity, voltage and current amplitude must be the same. For example, if ICTA7607 and TA7611 are displayed in the image, the former is a reverse high-level AGC, and the latter is a forward high-level AGC, so they cannot be directly replaced. In addition, ICs with different output polarity, AFT voltage and synchronous pulse with different output polarity cannot be directly replaced, even if they are products of the same company or manufacturer, they should be distinguished. 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. Replacement of IC of the same model

The replacement of IC of the same model is generally reliable. When installing the integrated circuit, pay attention to the correct direction, otherwise, the integrated circuit may 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, dual channel power amplifier IC LA4507 has "positive" and "negative" pins, and its starting pin markings (color dots or pits) have different directions: there is no suffix and IC with suffix "R", such as M5115P and M5115RP

2. Replacement of ICs of different models

Substitution of IC with the same model prefix and different numbers. As long as the functions of the pins between them are identical and their internal circuits and electrical parameters are slightly different, they can also be directly replaced with each other. For example, IC LA1363 and LA1365 are amplified in the accompanying sound. The latter adds a voltage regulator diode inside the ⑤ pin of IC, which is the same as the former.

Substitution of IC with different model prefix letters and the same number. Generally, the prefix letter indicates the manufacturer and circuit category. The numbers behind the prefix letter are the same, and most of them can be directly replaced. But the number is the same, but the function is completely different. For example, HA1364 is audio IC, while uPC1364 is color decoding IC; 4558, 8-pin is the operational amplifier NJM4558, 14 pin is the CD4558 digital circuit; Therefore, the two cannot be replaced at all.

Model prefix letter and number are different IC substitution. Some manufacturers introduce unpacked PCB and then process it into products named by our factory. Another example is to improve the product in order to improve some parameter indicators. These products are usually named by different models or differentiated by model suffix. For example, AN380 and uPC1380 can be directly replaced; AN5620, TEA5620, DG5620, etc. can be directly replaced.

2、 Indirect substitution

Indirect replacement refers to the method that IC that cannot be directly replaced slightly modifies the peripheral circuit, changes the arrangement of original pins or adds or subtracts individual PCB components 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 performance of the original machine should not be affected after replacement.

1. Different packages

IC replacement

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, AFT circuit CA3064 and CA3064E, the former is circular package with radial pins, and the latter is dual in-line plastic package with identical internal characteristics, which can be connected according to the pin function. The package form of dual row IC AN7114 and AN7115 is basically the same as that of 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 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 by connecting pins 9 and 10 to ground.

2. Replacement 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 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. Substitution of Ic with the same plastic but different pin functions

This replacement requires changing the peripheral 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 circuit and application 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. PCB combination replacement

Combination replacement is a method to reassemble undamaged 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 the circuit in good condition inside the IC used must have an interface pin.

The key of indirect replacement is to find out the basic electrical parameters, internal equivalent circuits, functions of each pin and the connection relationship between IC components of the two ICs that are replaced each other. Attention shall be paid during actual operation:

The numbering sequence of integrated circuit pins must not be connected incorrectly;

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

The power supply voltage shall be consistent with the replaced voltage. If the power supply voltage in the original circuit is high, try to reduce the voltage;

The voltage is low. It depends on whether the replacement IC can work.

After replacement, measure the static working current of the IC. If the current is far greater than the normal value, it means that the 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:

The input and output impedance of the replaced IC should match the original circuit;

Check its driving ability.

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

Before power on, 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 circuit is normal from high to low.

6. Discrete component replacement IC

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 instrument C, the normal voltage of each lead pin, the waveform diagram and the working principle of the circuit with the peripheral components. Also consider:

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

(2) Whether the signal processed by the peripheral circuit can be connected to the next level inside the integrated 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 circuit and internal circuit, it is composed of audio intermediate amplifier, frequency discrimination and boost frequency amplification stage. The damaged part can be found by the method of information signal input. If the audio amplification part is damaged, discrete PCB components can be used to replace it.