IC Substitution Techniques in PCB Circuit Design

In the PCB circuit design will encounter the need to replace the IC time, the following to share the skills in the replacement IC, to help engineers in the PCB circuit design can be more perfect.

First, direct replacement

Direct replacement refers to other IC without any changes and directly replace the original IC, replacement does not affect the main performance of the machine and indicators.

The principle of substitution is: the function of the replacement IC, performance indicators, package form, pin usage, pin number and spacing and other aspects are the same. The same IC function not only means the same function, should also pay attention to the same logic polarity, that is, the output input level polarity, voltage, current amplitude must be the same. Performance index refers to the main electrical parameters of the IC (or the main characteristic curve), the maximum power dissipation, the maximum operating voltage, frequency range and the signal input, output impedance and other parameters should be similar to the original IC. Smaller power substitutes to increase the heat sink.

The same type of IC substitution of the same type of IC substitution is generally reliable, the installation of integrated PCB circuit, pay attention to the direction not to be mistaken, otherwise, the integrated PCB circuit is likely to be burned when the power is on. Some single-row plug-in amplifier IC, although the model, function, features the same, but the direction of the pin order is different. For example, dual-channel amplifier ICLA4507, pin “positive”, “reverse” points, the beginning of the foot labeling (color dots or pits) in different directions: there is no suffix and the suffix for the “R ICs without suffix and with suffix “R”, such as M5115P and M5115RP.
The model prefix letters are the same, the number of different IC replacement of this replacement as long as the pin function is identical to each other, its internal PCB circuit and electrical parameters are slightly different, can also be directly replaced each other. Such as: accompanied by the amplifier ICLA1363 and LA1365, the latter than the former in the IC foot 5 internal increase of a regulator diode, the other is exactly the same.

In general, the prefix letter is to indicate the manufacturer and the category of PCB circuit, the prefix letter behind the same number, most can be directly replaced. But also a few exceptions, although the same number, but the function is completely different. For example, HA1364 is a companion IC, and uPC1364 is a color decoding IC; the number is 4558, 8 feet is an operational amplifier NJM4558, 14 feet is CD4558 digital PCB circuit; therefore, the two can not be replaced. So be sure to also look at the pin function.

Some manufacturers introduce unpackaged IC chip, and then processed into the factory named according to the product, as well as in order to improve some of the parameters and indicators to improve the product. These products are commonly used in different models for naming or using the model suffix to distinguish. For example, AN380 and uPC1380 can be directly replaced, AN5620, TEA5620, DG5620, etc. can be directly replaced.

Second, non-direct substitution

Non-direct substitution refers to the IC can not be directly replaced slightly modified peripheral PCB circuit, change the arrangement of the original pin or increase or decrease individual components, so that it can become a replacement IC method.

Replacement principle: the replacement of the IC can be used with the original IC pin function is different, the shape is different, but the function should be the same, the characteristics should be similar; replacement should not affect the performance of the original machine.

1.Different packages IC replacement of the same type of IC chip, but the package shape is different, as long as the replacement of the new device will be the pin according to the shape of the original device and the arrangement of the pin for shaping. For example, AFTPCB circuit CA3064 and CA3064E, the former for the round package, radial pins: the latter for the double in-line plastic package, the two internal characteristics are exactly the same, according to the pin function can be connected. Dual-row ICAN7114, AN7115 and LA4100, LA4102 package form is basically the same, pins and heat sinks are exactly 180 degrees apart. Previously mentioned AN5620 with a heat sink double in-line 16-pin package, TEA5620 double in-line 18-pin package, 9, 10 feet is located in the integrated PCB circuit on the right side, equivalent to the AN5620 heatsink, the two other feet are arranged in the same way, will be 9, 10 feet connected to ground can be used.

2.PCB circuit function is the same, but individual pin function is different lC replacement replacement can be based on the specific parameters of each model IC and instructions. Such as television sets in the AGC, the video signal output has a positive and negative polarity difference, as long as the output is connected to the inverter can be replaced.

3.The same class of plastic but different pin function IC replacement of this replacement needs to change the peripheral PCB circuit and pin arrangement, thus requiring a certain degree of theoretical knowledge, complete information and extensive practical experience and skills.

4.Some empty feet should not be grounded without authorization, the internal equivalent PCB circuit and the application of PCB circuits in the pin is not marked, meet the empty pin, should not be grounded without authorization, these pins for the replacement or spare feet, and sometimes also as an internal connection.

5.The combination of replacement is a combination of replacement of the same type of IC internal undamaged part of the PCB circuit, re-assembled into a complete IC, used in place of a dysfunctional IC method. Can not buy the original IC is very applicable to the case. However, it is required to utilize the intact PCB circuit inside the IC to have the interface pin.

The key to non-direct replacement is to find out the basic electrical parameters of the two ICs replaced each other, the internal equivalent PCB circuit, the function of the pins, the IC Department of the connection between the components of the relationship between the information.

Attention should be paid to the actual operation:

The numbering sequence of the integrated PCB circuit pins, do not connect the wrong one.
In order to adapt to the characteristics of the replacement IC, connected to the peripheral PCB circuit components should be changed accordingly.
The power supply voltage should be replaced with the work of the C in line with the original PCB circuit, if the power supply voltage is high, you should try to step down; voltage is low, depending on whether the replacement IC can work; (4) After the substitution of measurement, we must not connect the wrong pin numbering sequence.
After the replacement to measure the IC’s quiescent operating current, such as current is much higher than normal, then the PCB circuit may produce self-excitation, which must be decoupled, adjusted. If there is a difference between the gain and the original, you can adjust the feedback resistor resistance; (5) After the replacement of the IC, we should measure the quiescent operating current of the IC.
Replacement of the IC’s input and output impedance to match the original PCB circuit; check its driving ability.
In the alteration of the original PCB circuit board to make full use of the foot holes and leads, external leads require neat, avoid crossing back and forth, in order to check and prevent the PCB circuit self-excitation, especially to prevent high-frequency self-excitation.
in the power supply before the Vcc circuit is best connected to a current ammeter, step-down resistor resistance value from large to small to observe the integrated PCB circuit total current changes are normal.

6.With discrete components to replace the IC can sometimes be used discrete components to replace the damaged part of the IC, so that it restores functionality. Replacement should be understood before the internal function of the IC principle, the normal voltage of each pin, waveforms and peripheral components with the working principle of the PCB circuit.

It should also consider:

Whether the signal can be taken out of the IC and connected to the input of the peripheral PCB circuit:
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 when connecting should not affect its main parameters and performance). If the amplifier IC is damaged, from the typical application PCB circuit and the internal PCB circuit, by the accompanying audio in the amplifier, frequency identification, as well as the promotion of frequency amplification stage into the available signal input method to find out the damaged part, if the audio amplifier is mostly damaged, can be used in place of discrete components.