What Is a Load Switch IC?
A load switch IC is an integrated circuit that combines the function of driving a MOSFET and FET with low on-resistance to turn the power supply on and off with the function of outputting various protection functions and abnormal conditions to the external IC.
The use of load switch ICs allows the number of components to be reduced compared to the same functionality achieved by a combination of individual electronic components, thus saving space.
Uses of Load Switch ICs
Load switch ICs are used in power supply circuits within electronic devices. Since most load switch ICs have a current rating of 0.5A to 5A, they are often used in information and communication equipment such as PCs, PC peripherals, and mobile devices rather than in industrial equipment that operates motors, solenoids, and the like.
Since they are equipped with various protection functions, they are suitable for applications that require protection of peripheral circuits and harnesses in the event of load short-circuit failure or abnormal FET heat generation, or for applications where connected devices, such as USB, can be inserted with the power on.
Principle of Load Switch ICs
1. Power Supply to Load
Load switch ICs use an internal P-channel FET or N-channel FET to turn on or off the power supply to the load. The FET drive circuit inside the load switch IC controls the gate voltage of the FET and changes the resistance between the drain and source of the FET, thereby realizing the ON/OFF function of power supply to the load.
2. Overcurrent Protection
When the current output from the load switch IC exceeds the specified value, the power supply to the load is turned off. The load switch IC can be protected when the output terminal of the load switch IC is shorted to GND or when a current flows to the load.
For example, if an overcurrent flows to the FET for load drive, the overcurrent will continue to flow to the FET if there is no overcurrent protection function. As a result, the FET may fail or the wiring may be broken. However, with overcurrent protection, the power supply to the load is turned off, so the FET will not fail or the wiring will be broken.
3. Overheat Protection
When the temperature of the semiconductor junction inside the load switch IC exceeds the specified temperature, the power supply to the load is turned off. The load switch IC can be protected when the external environment generates abnormal heat or when the load current is higher than expected.
For example, if a load fails and a larger-than-expected current flows to the FET, the FET will continue to generate heat and fail if the over-temperature protection function is not provided. On the other hand, with over-temperature protection, the power supply to the load is turned off and the FET will not fail.
4. Low-Voltage Protection
If the power supply voltage input to the load switch IC falls below a specified value, the load switch IC will be turned off. If the power supply voltage input to the load-switch IC falls below a specified level, the power supply to the load is turned off. This function prevents load malfunctions when the power supply voltage drops due to a failure of the power supply circuit.
For example, if the power supply circuit fails and the supply voltage becomes lower than the guaranteed operating voltage, the load will malfunction because the power supply to the load will continue to be ON without the low-voltage protection function. On the other hand, with the low-voltage protection function, the power supply to the load is turned off and the load will not malfunction.
Other Information on Load Switch ICs
Load-Switch IC Terminals
The typical pins of load switch IC are VCC, GND, EN, FLG, and VOUT.
- VCC pin
The VCC pin is the power supply input pin of the load switch IC. The power supply line to be supplied to the load is connected to this pin, and a ceramic capacitor for bypass is connected between the VCC and GND pins. The ceramic capacitor for bypassing must be placed close to the pin to be effective. - VOUT pin
The VOUT pin is the power supply output pin and is connected to the power supply line of the load. Since the parasitic diode of the MOSFET in the load switch IC is disabled, there is no reverse current flow from VOUT to VCC. - EN pin
The EN pin is an input pin that controls the power supply output of the load-switch contact ON/OFF. Connecting the EN pin to the output port of the microcontroller allows the power supply to the load to be controlled.
Since the logic and voltage level of the EN pin varies depending on the load switch IC, check the datasheet and connect the appropriate logic and voltage. - FLG pin
The FLG pin is an output pin that indicates the status of the load switch IC. The voltage on the FLG pin changes when the load switch IC is normal or abnormal.
By connecting the FLG pin to the input port of the microcontroller, the state of the load switch IC can be monitored. Normally, since the FLG pin is an open drain output, connect an external pull-up resistor and leave the pin unconnected when the FLG pin is not used.