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Trouble shooting - Additional LDO solves the problem

Trouble shooting-Additional LDO solves the problem

Checking circuit itself seemed to be OK, but a trouble is detected! As is often the case with such a trouble, an additional component is used.

Here, the actual trouble shooting example is introduced. It might be useful to choose a device not only for trouble shooting but also just a simple component's selection.

Case 1 Additional LDO solves the problem

LDOs are often used as power management ICs, they are applied to realize constant current circuits, etc.

LDOs are sometimes used for trouble-shooting because they are relatively reasonable cost, the number of external components is only two or less in most cases. How to select the LDO for trouble-shooting is usually same as the normal usage.

Background of the trouble

Original circuit: 5V PSU(Power supply unit) directly supplies the power to the post LSI, but the LSI's input voltage is limited up to 4.7V due to keep the post device's characteristics.

The voltage range for the post LSI is relatively wide, but high voltage is better than low voltage for the performance, therefore it is necessary to regulate close to 4.7V.


Cutomer's issue and condition confirmation

To choose the suitable device for this issue, some factors should be considered.

Check points Check points list
PSU's voltage tolerance: According to the customer 5V±10%. By this information, the input voltage of the regulation device must accept the range from 4.5V to 5.5V. Input voltage range
The target output voltage is less than input voltage, therefore, as the regulation device, an LDO is suitable. The output voltage of LDO has also its tolerance, therefore set output voltage should be less than 4.6V or around. Acceptable output voltage range
In this case, the minimum input voltage is 4.5V. Therefore, if the output voltage of LDO is more than the input voltage, by the load of the post device, output voltage becomes lower than the target output voltage. We should check the fact is acceptable or not. The drop voltage depends on the output current. According to the customer, the maximum output current is 100mA. Output current (Load current)
How about the PCB mount area space? In this case, the space is enough, but small size is better to add the device, however, they do not want to choose leadless type package. PCB space
PCB: 4-layer FR4, internal derating rule: 50% Maximum ambient temperature: 85℃ Power dissipation

Trouble was solved with using an additonal LDO, the RP130N461B-TR-FE.

How to select the adittional components for trouble-shooting is similar to how to select the ordinal components at usual design. By considering all necessary conditions such as Vin, Vout, Iout, Power dissipation of the package, trouble shooting could be perfect.



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