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Important Points in the Design of a Power Supply Using a Linear RegulatorProtection of Linear Regulator IC Terminals

2025.01.28

It is necessary to protect the terminals of linear regulator ICs, depending on the conditions and environment in which they are used. This article explains six possible cases and examples of protection circuits for each.

Protection of Linear Regulator IC Terminals

If a reverse voltage or overvoltage is applied to the terminals of an IC, the output voltage may not rise, or the IC may even be damaged. When the following conditions are conceivable, it is recommended that the terminals be protected appropriately.

  • 1. When the input and output voltage conditions are reversed → Reverse current bypass
  • 2. When the output load is inductive → Output reverse voltage protection
  • 3. Possibility of input polarities connected in reverse → Input reverse voltage protection
  • 4. When hot plugging possible → Hot plugging countermeasure (Input surge protection)
  • 5. When a load exists between disparate power supplies → Reverse current bypass
  • 6. Positive-negative power supply (Dual supply) → Output reverse voltage protection

1. When the Input and Output Voltage Conditions are Reversed → Reverse Current Bypass

In a circuit with a large output capacitor, when charge remains in the output capacitor even after the input power supply has been turned off, or when the input power supply turns off extremely fast, an inverted state may occur in which the output voltage is higher than the input voltage. In this case, a reverse current flows from the output to the input via a parasitic element within the IC. The parasitic element operation is not intentional, and therefore there is the possibility of element degradation or destruction.

As a countermeasure, a reverse current bypass diode (D1) may be connected externally to prevent reverse current from passing through the IC (left diagram below). However, when using the method in which the input line is open and the IC turned off (lower-right diagram), the reverse current consists of only the IC bias current and is so small that it does not cause parasitic element degradation or destruction, eliminating the need for a bypass diode.

Reverse current bypass diode/When the input is open

The bypass diode must be turned on in advance of the parasitic element in the IC. For MOSFET-type linear regulators, the turn-on voltage of the internal parasitic element is about 0.6 V, and so a bypass diode should be selected with a forward voltage VF lower than this.

The rated reverse voltage should be larger than the input/output voltage difference to be used (80% derating or less), and the rated forward current should be larger than the reverse current value (50% derating or less). Given these conditions, a rectifier diode or a Schottky barrier diode is recommended as the bypass diode.

However, Schottky barrier diodes generally have large reverse currents. If the reverse current is large, even when the output is turned off by the EN pin, a large diode leakage current (reverse current) flows from input to output; hence a diode with a small reverse current (roughly 1 µA or less) must be selected.

2. When the output load is inductive → Output reverse voltage protection

If the output load is inductive, the energy stored in the inductive load is discharged to ground at the instant the output voltage is turned off. There is an electrostatic breakdown prevention diode between the IC output pin and the GND pin; because a large current flows in this diode, IC may be destroyed. To prevent this, a Schottky barrier diode (D1) is connected in parallel with this electrostatic breakdown prevention diode (see below).

Current path for an inductive load (when output turns off)

Further, if the IC output pin and the load are connected by a long wire, the wire inductance may become an inductive load. Hence observe the waveform with an oscilloscope to ensure that no transient reverse voltage is occurring when the output is off.

In addition, when the load is a motor, the back emf of the motor may cause the same kind of current to flow, and so a diode is necessary.

3. Possibility of Input Polarities Connected in Reverse → Input Reverse Voltage Protection

When a power supply is connected to the input, if the positive and negative terminals are inadvertently connected in reverse, a large current will flow in the electrostatic breakdown prevention diode between the IC input pin and the GND pin, which may destroy the IC （See “Current path with input reverse-connected” below）. The simplest method to deal with such reverse connections is to connect either a Schottky barrier diode or a rectifier diode in series with the power supply, as described in “Reverse connection countermeasure 1” below.

Current path with input reverse-connected/Reverse connection countermeasure 1

When the input power supply is connected correctly, the forward voltage drop VF of the diode creates a power loss VF ×IO. Hence if the input power supply is a battery, this connection is not appropriate. The VF of a Schottky barrier diode is lower than that of a rectifier diode, and so the loss will also be somewhat smaller. Diodes emit heat, and thus a diode with a sufficiently large allowable dissipation should be selected. In the event of a reverse connection, a reverse current does flow in the diode, but it is very small.

The next method is to connect a diode in parallel with the power supply (Reverse connection countermeasure 2). Because the diode must be turned on sooner than the electrostatic breakdown prevention diode inside the IC, a Schottky barrier diode with a low VF should be used. With the correct connection, the operation is the same as when no diode is present. In the event of a reverse connection, all the currents from the power supply keep flowing through the diode, resulting in significant heat generation, and if the current capacity of the power supply is large, destruction may result. This countermeasure is intended to protect the circuit against inadvertent errors over a short length of time, or it is assumed that the power supply has an overcurrent protection circuit in the previous stage.

If still greater emphasis is to be placed on safety in this protection circuit, a fuse can be connected in series with the power supply, as in “Reverse connection countermeasure 3”. Fuse maintenance will then be necessary, but the circuit can be protected even more reliably.

Reverse connection countermeasure 2/Reverse connection countermeasure 3

The next “Reverse connection countermeasure 4” is to connect a P-channel MOSFET in series to the power supply. The diode between the drain and source of the MOSFET is a “body diode” (parasitic element). With the correct connections, the P-channel MOSFET is turned on, and so the voltage drop here is the product of the MOSFET on-resistance and the output current IO. This voltage drop is smaller than the voltage drop due to the diode in the “Reverse connection countermeasure 1”, resulting in smaller power losses. In the case of a reverse connection, the MOSFET does not turn on, and so current does not flow. If the MOSFET gate-source voltage exceeds the rated voltage (with derating considered), the gate-source voltage should be lowered by adding voltage dividing resistors between the gate and source as in “Reverse connection countermeasure 5”.

Reverse connection countermeasure 4/Reverse connection countermeasure 5

4. When Hot Plugging Possible → Hot Plugging Countermeasure (Input Surge Protection)

When wiring is connected to the input of the IC with the supply-side (input) power being on, a pulse waveform is generated due to the inductance component of the wiring and the metal contact of the connecting plug. If this surge voltage exceeds the absolute maximum rating of the IC, the IC may be destroyed. To prevent surge voltages from being applied to the IC input pin, a TVS (Transient Voltage Suppressor) diode (D1 in the diagram below) can be used to absorb the surge.　

Hot plugging countermeasures

5. When a Load Exists between Disparate Power Supplies → Reverse Current Bypass

When a load exists between different power supplies, the voltages of the power supplies do not rise and fall with the same timing, and so current flows through the load to the output terminal of the other power supply. In this case, a reverse voltage occurs between the input and output of the IC, and so reverse current bypass diodes (D1, D2 in the diagram below) are required.

Current path between different power supplies and diode insertion

6. Positive-Negative Power Supply (Dual Supply) → Output Reverse Voltage Protection

In a positive-negative power supply shown below, the speed of voltage rise is different between the positive and negative power supplies. Hence, when there is a load between the two, the power supply that has risen first draws current via the load from the output of the other power supply, resulting in a reverse voltage at the output. To prevent damage to the IC and avoid the possibility that an output voltage cannot rise, Schottky diodes (D1, D2) with low VF values must be connected between the outputs and GND to protect the outputs from reverse voltages.

Inserting a diode between positive-negative power supplies; current path when negative power supply regulator starts first

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