PCB News - PCB circuit design for IC substitution skills

In PCB circuit design will meet the need to replace IC, the following to share the skills of replacing IC, to help designers in printed circuit board design can be more perfect.

First, direct substitution

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

Its substitution principle is: substitution IC function, performance index, packaging form, pin use, pin serial number and interval are the same. The same function of IC not only means the same function, but also the same logical polarity, that is, the polarity of output input level, voltage and current amplitude must be the same. Performance index refers to IC's main electrical parameters (or main characteristic curve), dissipated power, working voltage, frequency range and signal input, output impedance and other parameters should be close to the original IC. Small power substitute parts to increase the heat sink.

1, the replacement of the same model IC

The substitution of the same TYPE of IC is generally reliable. When installing the INTEGRATED PCB circuit, pay attention to the direction is not wrong, otherwise, the integrated PCB circuit is likely to be burned when energized. Some single-row straight plug type power amplifier IC, although model, function, characteristic is same, but the direction of pin arrangement order is different somewhat. For example, double channel power amplifier ICLA4507, pin has "positive", "negative" points, its starting foot mark (color or pit) direction is different: no suffix and suffix is "R", IC, such as M5115P and M5115RP.

2, model prefix letters are the same, different numbers IC substitution

As long as the pin function between each other is exactly the same, its internal PCB circuit and electrical parameters are slightly different, can also be directly replaced with each other. For example: sound in the ICLA1363 and LA1365, the latter than the former in the IC fifth foot added a regulator diode, the other completely the same.

Generally, prefix letters indicate the manufacturer and PCB circuit type. The numbers after the prefix letters are the same, and most of them can be directly replaced. But there are a few exceptions, although the number is the same, but the function is completely different. For example, HA1364 is a sound IC, and uPC1364 is a color decoding IC; The number is 4558, 8-pin is the operational amplifier NJM4558, 14-pin is CD4558 digital PCB circuit; So the two are not interchangeable at all. So definitely look at the pin function.

Some manufacturers introduce unpackaged IC chips, and then processed into products named by the factory, and there are products improved in order to improve some parameters. These products are often named by different models or distinguished by a model suffix. For example, AN380 and uPC1380 can be directly substituted, and AN5620, TEA5620, and DG5620 can be directly substituted.

Indirect substitution

Indirect substitution refers to the IC that can not be directly replaced slightly modify the peripheral PCB circuit, change the original pin arrangement or increase or decrease individual components, so that it becomes a substitutable IC.

Substitution principle: the IC used for substitution can be different from the original IC pin function, shape is different, but the function should be the same, characteristics should be similar; Replacement should not affect the original machine performance.

1. Substitution of IC in different packages

The same type of IC chip, but the package shape is different, as long as the replacement of the new device according to the shape a

nd arrangement of the original device pin shaping. For example, AFTPCB circuit CA3064 and CA3064E, the former is circular package, radial pin; the latter is double in-line plastic package, the internal characteristics of the two are exactly the same, according to the pin function can be connected. Double column ICAN7114, AN7115 and LA4100, LA4102 package form is basically the same, the pin and heat sink are exactly 180 degrees different. The AN5620 with heat sink dual-in-line 16-pin package and TEA5620 with dual-in-line 18-pin package mentioned above, pin 9 and 10 are located on the right side of the integrated PCB circuit, equivalent to the HEAT sink of AN5620. The other pins of the two are arranged in the same way. Pin 9 and 10 can be used by connecting the ground.

2, PCB circuit function is the same but individual pin function is different lC replacement

The substitution can be carried out according to the specific parameters and instructions of each IC model. Such as TV AGC, video signal output has positive and negative polarity difference, as long as the output end is connected with inverter can be replaced.

3, the same type of plastic but different pin function IC replacement

This kind of substitution needs to change the peripheral PCB circuit and pin arrangement, so it needs certain theoretical knowledge, complete data and rich practical experience and skills.

4, some empty feet should not be grounded without authorization

Some pins in the internal equivalent PCB circuit and the application PCB circuit are not marked, and should not be arbitrarily grounded when the empty pins are encountered. These pins are replacement or spare pins, and sometimes also used as internal connections.

5. Combinatorial substitution

Combinational substitution is to recombine the undamaged PCB circuit parts of multiple ICS of the same model into a complete IC to replace the dysfunctional IC. Can not buy the original IC is very suitable. But requires the use of IC internal intact PCB circuit must have interface pin.

The key of indirect substitution is to find out the basic electrical parameters of the two kinds of IC, the internal equivalent PCB circuit, the function of each pin, and the information of the connection between the IC components. Pay attention to the actual operation.

(1) Serial number of integrated PCB circuit pins, do not connect wrong;

(2) In order to adapt to the characteristics of the IC after the substitution, the components of the peripheral PCB circuit connected with it should be changed accordingly;

(3) The power supply voltage should be consistent with the work C after the replacement. If the power supply voltage in the original PCB circuit is high, try to reduce the voltage; Low voltage, to see whether the substitution IC can work;

(4) After the replacement, the static working current of IC should be measured. If the current is much higher than the normal value, it indicates that the PCB circuit may generate self-excitation. At this time, decoupling and adjustment should be carried out. If the gain is different from the original, the resistance value of the feedback resistor can be adjusted;

(5) The input and output impedance of IC after replacement should match the original PCB circuit; Check its driving ability;

(6) When changing, make full use of the foot holes and leads on the original PCB board, and the external leads shall be neat and avoid crossing before and after, so as to check and prevent the PCB circuit from self-excitation, especially to prevent the high-frequency self-excitation;

(7) Connect the current ammeter in series in the Vcc circuit of the power supply before powering on, and observe whether the change of the total current of the integrated PCB circuit is normal from large to small in the resistance value of the buck resistance.

6. Replace IC with discrete components

Sometimes a damaged part of an IC can be replaced by a discrete component to restore its function. Before replacement, we 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 composed with peripheral components. Consideration should also be given to:

(1) Whether the signal can be taken out from work C 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 internal circuit of the integrated PCB circuit for reprocessing (the signal matching during connection shall not affect its main parameters and performance). If the AMPLIFIER IC is damaged, from the typical application of PCB circuit and internal PCB circuit, by sound amplifier, frequency identification and frequency amplification stage, available signal input method to find the damaged part, if the audio amplifier part is damaged, can be replaced by discrete components.