Simple Load Switch

A load switch is an electronic switch. It works like a relay to power supply rails in systems, but it has no moving parts.

Load switches are increasingly found in today’s portable electronic devices, providing a simple and smart method for the system to make the appropriate power management decisions based on which peripherals or sub-circuits are in use.

Since the load switch is controlled by the system, and connects or disconnects a voltage rail to a specific load, the setup provides a simple means to power a load when it is in demand and allows the system to maximize performance.

This post will provide the fundamental basics of what a load switch is , when it should be used, and how it can be implemented in a system.

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Basic Load Switch

A load switch is comprised of two main elements – the pass transistor and the on/off control segment. The pass transistor is most commonly a P-channel or N-channel MOSFET that passes the voltage supply to a specified load when the pass transistor is on.

The figure below shows an example load switch circuit for a P-channel pass transistor (PMOS Transistor). Needless to say, the selection of a P-channel or N-channel load switch depends on the specific needs of the application.

A logic signal from the system power management control circuitry turns the load switch on and off via a small-signal NMOS transistor Q1. That is, when EN input is LOW, Q1 is off and the gate of the pass transistor is pulled up to VIN through the resistor R1.

When the EN input is HIGH, Q1 turns on, the pass transistor gate is pulled to GND, and the load switch turns on. As long as the input voltage rail is higher than the threshold voltage of the PMOS pass transistor, it will turn on when EN is HIGH without the need of an additional voltage source.

Note at this point that this is a high-side load switch that connects or disconnects a power source to a load and the switch is controlled by an external enable signal.

A high-side load switch sources current to a load, while a low-side load switch connects or disconnects the load to ground, thus sinking current from the load.

Thus, a load switch is essentially a high-side solid-state switch wired in series between a power source and a load.

Since it is desirable to minimize the voltage drop and power dissipation, a low-on-resistance MOSFET (usually an enhancement mode device) is mainly used as the pass transistor.

As pointed before, two MOSFETs act like a switching element in the load switch, one of them being a P-channel device, and the other an N-channel device.

The gate-drive circuit of the load switch contains a gate drive transistor and a few passive components that level-shifts the input control/enable signal to a value that will fully turn on the MOSFET, reducing the on-resistance to its lowest possible level.

Either a P-channel or N-channel MOSFET can be used as the pass element, depending on the purpose of the load switch, but an N-channel MOSFET provides lower on-resistance than the P-channel MOSFET.

However to get the lower on-resistance value, a Charge-Pump (voltage booster) circuit is needed to raise the drive voltage applied to the N-channel Pass MOSFET’s gate (see below figure).

Moreover, turn-on speed plays an important role in the behaviour of the load switch. Note that when turning on a sub-system without any slew rate control, the input rail may sag because of the inrush current that can happen from quickly charging a load capacitor (the faster the device switches on, the higher the inrush current will be).

The inrush current (momentary flow of a large current much greater than the steady-state current) causes a dip in the input supply voltage that can adversely impact the functionality of the entire system. Likewise, excessively large inrush current can cause system failures or malfunctions.

This issue, however, can be solved by controlling the rise time of the output voltage, simply by adding a few more passive components as shown below. Recall, a fast device turn-on creates an inrush current, and a softer turn-on scales down that current spike. Even so, precaution must be taken when slowing down the device turn-on.

Integrated Load Switch

Although load switches can be constructed with electromechanical relays and discrete semiconductors, integrated load switches (Load Switch ICs) can be used for bettered power

management to control the power supply to the load.

A miniaturized smart integrated load switch consumes less power and it offers a variety of built-in protection features (that are often difficult to implement with discrete components) such as undervoltage lockout, overcurrent protection, thermal shutdown, etc.

There are many models but the DML3009LDC single-channel load switch from Diodes Incorporated (https://www.diodes.com/assets/Datasheets/DML3009LDC.pdf) is one such example of smart integrated load switches. Below you can see its functional block diagram taken from the official datasheet.

And, here are some other similar integrated devices worth mentioning:

TCK22946G (https://www.lcsc.com/datasheet/lcsc_datasheet_2202031800_TOSHIBA-TCK22946G-LF_C2149798.pdf)
NX3P2902B (https://www.nxp.com/docs/en/data-sheet/NX3P2902B.pdf)
TPS22810 (https://www.ti.com/lit/ds/symlink/tps22810-q1.pdf)
TPS22936C (https://www.ti.com/lit/ds/symlink/tps22963c.pdf)
TPS22918 (https://www.ti.com/lit/ds/symlink/tps22918.pdf)
BD2201GUL (https://fscdn.rohm.com/en/products/databook/datasheet/ic/power/power_switch/bd2200gul-e.pdf)

Practical Power Switch Circuit

As a final step, here is a practical example of an integrated power switch circuit that is enough to quench the thirst of an avid experimenter. Try it out and have fun!

The STMPS2171 (https://www.st.com/resource/en/datasheet/stmps2141.pdf) enhanced single channel power switch (U1) incorporates a 90mW N-channel MOSFET high-side power switch for power distribution. The power switch is controlled by a TTL logic enable (EN) input. It is intended for applications where heavy capacitive loads and short-circuits are likely to be encountered.