Input Equivalent Circuits

As parts of [How to Read Power Supply IC Datasheets], thus far we have discussed [Datasheet Cover Page], [Block Diagram], [Absolute Maximum Ratings and Recommended Operating Conditions], [Key to Electric Characteristics], [How to Interpret Property Graphs and Waveforms], [Application Circuit Examples] and [Component Selection]. In this section we discuss internal equivalent circuits that pertain to the input pins for an IC. Although not many manufacturers opt to disclose “input equivalent circuits”, ROHM’s datasheets do provide such information. Because this information can be extremely helpful in understanding the behavior of IC input pins, we are pleased to discuss it even though the topic is not something that is covered in all datasheets.

Input Equivalent Circuit

The pins not only for power ICs but for virtually any IC are governed by specification values, such as maximum ratings and bias current levels. Aside from specification values, datasheets often provide voltage/current characteristic in the form of a graph. These values are mostly dependent upon the internal circuit to which a given pin is connected. In particular, because input pins are used to connect resistors and capacitors as well as the output from other devices, knowledge of specific bias currents and input circuits for an input pin helps the engineer to design circuits with pin-point accuracy.

Information on input or equivalent circuits can cast light on the following properties:
　- Voltage/current characteristics of a pin
　- Bias current (which may be referred to as a sink/source, in/out current and in several other expressions)
　- Protective structure

We now discuss the power supply IC BD9A300MUV as an example. It is clear from the input equivalent circuit for the FB pin that gates for the input transistors contain 20kΩ resistors as protective features. Also, because of the presence of a parasitic diode, it is clear that the negative voltage of the maximum rating (from -0.3V to +7V）represents a value which is the forward voltage of the diode plus some margin, and that when the FB pin exceeds the VF for the diode, a forward current flows to the FB pin.

For the MODE pin, a 10μA（typ）, 20μA（max） ＠5V current flowing into the pin is defined. In terms of the input equivalent circuit, because the MODE pin is connected to AGND through 10Ω＋500 kΩ resistors, it is clear that of necessity an approximately 10μA current flows into the pin. Conversely, including the 10 kΩ resistor which is connected to the transistor gate, in the application example the pin is directly connected to the input voltage, and it can be understood that the internal structure permits such a connection.

Although bias currents for pins are specified as specification values, in relative terms, because in most cases detailed specifications and explanations do not exist for protective structures (not to be confused with protective functions, such as over-voltage protection), information on equivalent circuits can be extremely helpful in the design and specific evaluation processes.

Also, questions also arise, such as “What if a voltage is applied to the pin before the IC is powered on?”. In the case of an FB pin, if the voltage applied is a negative voltage, a current flows, as noted above. However, if a bias relationship arises in which some kind of parasitic path is formed in the transistor, it is clear that by virtue of the provision of a 20 kΩ resistor basically there is no possibility of a large current being induced. Thus, equivalent circuits can provide supplementary information in examining the question as to what results can be expected if conditions not stated in the datasheets are imparted.

The equivalent circuit described in the datasheet for the power supply IC is an input equivalent circuit, and it is easy to understand that this information is extremely helpful. That being said, since an equivalent circuit is no more than just that, in a situation where a critical decision must be made, it would be helpful to ascertain the information with the manufacturer. Also, if the information provided does not include equivalent circuits, the manufacturer must be contacted. In such a case, the information would be provided by the manufacturer on a case by case basis.