When there’s a need to replace an IC in PCB circuit design, we’d like to share some helpful tips to ensure a smoother process, enabling designers to achieve better results in their PCB designs.
1. **Direct Substitution**
Direct substitution refers to replacing the original IC with another without any modifications, ensuring that the main performance and specifications of the device remain unaffected after the swap.
The principle of replacement is as follows: the function, performance parameters, package type, pin configuration, pin count, and pin spacing of the replacement IC must match those of the original. “Matching function” not only means the same functional capabilities, but also the same logic polarity. This includes ensuring that the input and output levels, voltage, and current amplitudes are identical.
Performance parameters refer to the IC’s key electrical characteristics (or characteristic curves), maximum power dissipation, maximum operating voltage, frequency range, and various input/output signal impedances that should closely align with those of the original IC. If using a substitute with lower power, adding a heatsink may be necessary.
2. **Substitution with the Same Type of IC**
Replacing an IC with another of the same type is generally a reliable solution. However, caution is needed during installation to ensure the IC is oriented correctly; otherwise, the integrated circuit may be damaged when powered on. Some single-inline power amplifier ICs may share the same model, function, and characteristics, but their pin layout may differ. For example, the dual-channel power amplifier IC, ICLA4507, has both “normal” and “reverse” pin configurations. The markings (such as color dots or notches) for the starting pin may vary, and different suffixes (like “R” or no suffix) can indicate a change in pin arrangement. For instance, M5115P and M5115RP are variants of the same IC.
3. **Substitution of ICs with the Same Prefix Letter and Different Numbers**
1. **Direct Substitution**
As long as the pin functions of the substitute ICs are identical, slight differences in the internal PCB circuit and electrical parameters do not prevent direct substitution. For example, the ICLA1363 and LA1365 ICs can be used interchangeably in audio applications. The LA1365 includes an additional Zener diode inside pin 5 compared to the ICLA1363, but otherwise, they are identical in function.
Typically, the prefix letter indicates the manufacturer and the type of PCB circuit. When the numbers following the prefix are the same, most ICs can be directly substituted. However, there are exceptions. In some cases, even with the same numbers, the functions may be entirely different. For instance, the HA1364 is an audio IC, while the uPC1364 is a color decoding IC. Similarly, the NJM4558 is an 8-pin operational amplifier, while the 14-pin CD4558 is used in digital circuits, so they are not interchangeable. Therefore, substitution should always depend on the pin functions.
Some manufacturers offer unpackaged IC chips that are processed and branded with the factory’s name, while others release improved versions of existing ICs to enhance certain parameters. These products are often identified by different model numbers or suffixes. For example, AN380 and uPC1380 are directly interchangeable, and similarly, AN5620, TEA5620, DG5620, etc., can also be substituted for one another.
2. **Indirect Substitution**
Indirect substitution refers to replacing an IC that cannot be directly swapped by slightly modifying the peripheral PCB circuit. This may involve altering the pin arrangement or adding/removing components to make the replacement feasible.
Substitution principle: The IC used for replacement may differ in appearance or function from the original, but the core functions and characteristics should remain similar. The performance of the original circuit should not be adversely affected after the replacement.
1. **Substitution of ICs with Different Packages**
When IC chips of the same type come in different packages, the pin configuration of the new IC needs to be adjusted to match the original device’s pinout. For instance, the CA3064 and CA3064E have the same internal characteristics but differ in packaging: the former comes in a radial pin circular package, while the latter is a dual in-line package. These ICs can be connected according to their pin functions. Similarly, the AN7114, AN7115, LA4100, and LA4102 ICs are similar in package form, with the lead and heatsink positioned 180 degrees apart. The AN5620 (16-pin dual in-line with heatsink) and TEA5620 (18-pin dual in-line) can be used interchangeably by grounding the 9th and 10th pins of the TEA5620.
2. **Substitution of ICs with Different Pin Functions**
If the PCB circuit has the same function but different pin assignments, the substitution can be made based on the specific parameters and instructions for each IC. For example, in a TV, the AGC and video signal output may have different polarities. A simple inverter can be used at the output terminal to facilitate the replacement.
3. **Substitution of ICs with Similar Packages but Different Pin Functions**
This type of substitution requires modifying the peripheral PCB circuit and adjusting the pin arrangement. This process demands solid theoretical knowledge, comprehensive documentation, and practical experience.
4. **Unused Pins Should Not Be Grounded Unnecessarily**
Some pins in the equivalent PCB circuits are not marked and may be unused or reserved for future functions. These “empty” pins should not be grounded without proper verification, as they may serve as alternate connections or internal links.
5. **Combination Substitution**
Combination substitution involves reassembling undamaged components from multiple ICs of the same model to create a functional replacement for a damaged IC. This approach is particularly useful when the original IC is unavailable. However, the replaced IC must have accessible interface leads.
The key to indirect substitution lies in understanding the electrical parameters, internal equivalent circuits, pin functions, and component connections of the ICs being substituted. Exercise caution during actual implementation.
(1) Ensure that the integrated PCB circuit pin numbering sequence is correct.
(2) Modify the peripheral PCB components accordingly to match the characteristics of the substituted IC.
(3) Ensure the power supply voltage is compatible with the replacement IC. If the original voltage is high, reduce it; if low, verify that the replacement IC can operate at this level.
(4) After substitution, measure the quiescent current of the IC. If the current is significantly higher than normal, it may indicate self-excitation in the PCB circuit, requiring decoupling and adjustment. If the gain differs from the original, adjust the feedback resistor value.
(5) Verify that the input and output impedances of the IC match the original PCB circuit after substitution. Also, check the drive capability.
(6) When making changes, use the existing pin holes and leads on the original PCB to maintain neat external wiring and prevent signal interference or high-frequency oscillation.
(7) It’s advisable to connect a DC current meter in series with the power supply Vcc loop before powering on, to monitor changes in the total current of the circuit.
6. **Replacing ICs with Discrete Components**
Sometimes, discrete components can be used to replace a damaged IC and restore functionality. Before replacing, it is important to understand the internal workings of the IC, the voltage levels of each pin, the waveform diagrams, and the operating principles of the peripheral circuit. Key considerations include:
(1) Whether the signal can be extracted from the work C and connected to the input terminal of the peripheral circuit.
(2) Whether the signal processed by the peripheral circuit can be sent to the next stage for further processing. Signal matching must not disrupt the main parameters or performance of the circuit.
For example, if the audio amplifier section is damaged, discrete components may be used to replace it by examining the typical circuit configuration and identifying the faulty component.
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