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Important Points in the Design of a Power Supply Using a Linear RegulatorOvercurrent Protection（OCP） and Thermal Shutdown（TSD） of Linear Regulator IC

2025.01.22

table of contents

・Overcurrent Protection（OCP） of Linear Regulator ICs
・Thermal Shutdown （TSD） of Linear Regulator ICs
・Input/Output Equivalent Circuit of a Linear Regulator IC

The BDxxIC0 series is provided with overcurrent protection (OCP) and thermal shutdown (TSD) functions. This article explains the former function; the latter function is explained in the next article.

Overcurrent Protection（OCP） of Linear Regulator ICs

An overcurrent protection (OCP) circuit is provided in a linear regulator IC in order to protect the IC from destruction by the overcurrent that flows when the output is shorted to GND. This protection function is intended to protect the IC from destruction, and is not intended to protect an IC or applied equipment to which power is being supplied. When such equipment must be protected, it is necessary to install a fuse or some other current-limiting device.

The characteristic of the overcurrent protection function in the BDxxIC0 series is shown in the diagram below; from the shape of the curve, it is known as a foldback characteristic. Point A in the diagram is the current value at which overcurrent protection is activated; the reference value is about 2 A. There is some variation in the overcurrent detection value, but the lower limit is never lower than the maximum value of the recommended output current (1 A for the BDxxIC0 series) indicated on the data sheet. Of course if a current limit below the maximum output current were to be applied, the product specifications would not be satisfied.

As shown in the figure, when an overcurrent is detected (point A), the current foldback circuit is activated and the output voltage drops. As the output voltage drops, the current limiting operation is repeated, and point B is reached. The current at point B is the output short-circuit current. Because the power loss is small at point B and heat generation is also minimal, the IC can be protected from destruction. This state continues until the cause of the overcurrent is removed. There is automatic recovery of the output voltage when the overcurrent state is resolved.

Characteristics of overcurrent protection

During the interval from the time when the output current exceeds the maximum value of the recommended output current until the output current reaches the overcurrent detection value, the linear regulator operates normally, but because the maximum value of the recommended output current is exceeded, the electrical characteristics are not guaranteed. Moreover, if operation continues with the allowable dissipation exceeded, the thermal shutdown circuit is actuated and the output is turned off.

Thermal Shutdown （TSD） of Linear Regulator ICs

Thermal shutdown (TSD) is a function that protects a regulator IC from destruction by excessive heat when the temperature of the IC chip (the junction temperature) exceeds the maximum rating due to an output short-circuit or an increase in power losses in the IC. Similarly to the overcurrent protection, this function is not intended to protect an IC or applied equipment that receives power from overheating.

The thermal shutdown circuit in the BDxxIC0 series turns off the linear regulator output when the chip temperature exceeds about 175°C (reference value), shutting off the output current and halting heat generation to lower the temperature (see the diagram below). While there is some variation in the temperature at which an overheated state is detected, this temperature will never be lower than the absolute maximum rating for the junction temperature (in the case of the BDxxIC0 series, 150°C).

Thermal shutdown characteristics

When the chip temperature drops to 160°C or so, the output is again turned on and the supply of output current begins. The output is turned on and off repeatedly until the cause of the chip temperature rise is removed. The IC is not destroyed immediately if this state persists, but if it continues for a long period of time, degradation and destruction of the IC will occur. Please take measures to avoid such an occurrence.

Input/Output Equivalent Circuit of a Linear Regulator IC

Below are shown the adjustable output type and fixed output type input and output equivalent circuits for the BDxxIC0 series. From the equivalent circuits we get a general idea of the internal connections between pins and the basic structures of the output and input stage circuits. Of course, the actual circuits are more complex than this, but equivalent circuits may be useful for understanding circuit characteristics and behavior.

One difference between the adjustable output type and the fixed output type can be seen in the fact that the output voltage setting resistors are external resistors for the former, but are internal in the latter (R₁ and R₂). The values of the internal resistors are essentially the same as the external resistors of the adjustable output type IC.

Adjustable output type equivalent circuit

Fixed output type equivalent circuit

Equivalent circuits do not necessarily require study, but can be used to deepen understanding.

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