In nearly all system designs, it is critical to both manage the DC power rails as well as protect them against various internal and external fault modes. The challenge is complicated when there are multiple rails, as is increasingly the case in today’s systems including small, low-power, and battery-powered designs.
Management of the power rail(s) begins with a power management IC (PMIC) which directs the turn-on and turn-off of the flow of current to the rail(s) as needed. The PMIC is also responsible for managing the timing and sequencing among multiple rails. However, actual physical-level control of the power rail is the task of the load switch, a MOSFET-based arrangement that can be directed to allow current to pass or block it.
In addition to basics such as inrush-current slew-rate control and overtemperature protection, load switches are now increasingly required to include other functions and features such as controlled power down, quick output discharge, and true reverse-current blocking, all of which are difficult to implement using discrete FET-based designs.
To bypass this complexity, while reducing the cost and board space required for a discrete implementation, designers can choose load-switch ICs that incorporate the required capabilities in a single package with the switch. These integrated load switches solve or avoid many operational power-rail problems, and also help address many mobile or battery-powered design requirements.
This article will discuss the role of load switches, their basic functions, additional functionality, and advanced features which make them more than just relatively simple, electronically controlled on/off switches for power rails. The article will use three new load-switch ICs in the TCK12xBG series from Toshiba Electronic Devices & Storage Corporation (Toshiba) to illustrate these points and show how they are applied to meet the needs of the latest product designs.
The basics of load switches
A basic load switch has just four pins: input voltage, output voltage, enable and ground (Figure 1). When a logic-level control signal is applied to its ON/OFF control pin (which can be active high or active low), the device is enabled and the pass FET turns on. This allows current to flow from the input pin VIN to the output pin VOUT, thus delivering power to the load circuitry.
Figure 1: The load switch is a FET-based pass-through device which can allow/block the flow of current from a DC supply to its load via an electronic control signal. (Image source: Bill Schweber)
A load switch is more than just a packaged pass FET. At a minimum, it also includes control logic, the FET driver, level shifters, and various circuitry-protection functions such as overcurrent protection and backflow (also called reverse current) prevention, either of which can damage the system and its components. They can also implement other useful functions such as slew-rate control as the power rail is turned on and overtemperature protection.
In its simplest application, the load switch is used between a supply and the power rail of a single load to allow it to be turned on via the PMIC when needed, or put into a quiescent state to save power (Figure 2).
Figure 2: In its simplest application, the load switch is controlled by the PMIC and controls the flow of current to the load. (Image source: Toshiba)
The load switch has several key parameters which designers must assess. The three top-tier ones are the maximum input voltage and output current it can support, along with its “on” resistance. Other parameters that may also be critical, depending upon the application, include:
- Quiescent current (IQ): The current needed to power the load switch, without any current at its output.
- Shutdown (standby) current (ISD): The current flowing into VIN when the device is disabled.
- ON pin input-leakage current (ION): The current flowing into the ON/OFF control pin when it is enabled.
Low quiescent current and shutdown current are increasingly important in battery-powered applications such as wearables, smartphones, and IoT modules, where they have a large impact on battery life and run time.
The overcurrent protection feature of a load switch is not just for protection against clear failures such as a temporary or permanent short circuit at the load. It may also be needed to mitigate the outcome of an output voltage drop that occurs in some cases when a rail feeds several loads, and one load turns on more quickly (Figure 3). The sudden increase in current demand causes the output of the supply to momentarily droop below its nominal value. This delay, or recovery period, is determined by the load-transient performance of the supply and the load specifics.
Figure 3: A single load switch may supply multiple loads which may not ramp up and turn on simultaneously. (Image source: Toshiba)
In turn, this droop may cause the second load to not start up properly or behave erratically. For these reasons, the current-limiting feature of a load switch is useful as it moderates the output voltage drop induced by the increased demand for current by the first load.
Many systems need to ensure that their multiple loads are energized in a specific sequence, and with defined timing between each power rail going active. In these cases, multiple load switches are used under the control of the PMIC which manages their sequencing and relative timing (Figure 4).
Figure 4: By using multiple load switches, the sequencing and timing of the turn-on of the various loads can be controlled as needed for proper system operation. (Image source: Bill Schweber)
Reverse current blocking
The reverse current blocking of a load switch is just what the name implies: it prevents current from flowing backward when the voltage on the output side becomes higher than the input side.
This can occur due to two common situations. First, the power supply, such as a car battery, may inadvertently be connected backwards as a result of accidental grazing of the battery terminals by the disconnected cables, or even by making a mistake when reconnecting them. It may even be something as basic as an average user inserting batteries backwards.
The second situation is somewhat less obvious. Consider the case where two supplies of different voltages are multiplexed to a load (Figure 5). The voltage on the shared output side can become higher than the voltage on the input side of the lower voltage supply. In this scenario, current can flow from the higher voltage side to the lower voltage side, damaging the lower voltage supply.
Figure 5: Problems of reverse current flow can occur even when multiplexed supplies are connected via their own load switches. (Image source: Toshiba)
There are three ways to deal with reverse current blocking:
- The simplest way is to add a diode in series with the output. However, the voltage drop across the diode (0.6 volts to 0.8 volts for a standard silicon diode) lowers the supplied rail voltage, and the diode must have a power rating sufficient to dissipate the associated heat.
- The second way is to use a MOSFET in series with the rail, but it’s on resistance (RON) also causes a voltage drop, and it has thermal dissipation which must be accommodated.
- The third option is to use a load switch with a reverse current blocking function that implements the needed backflow prevention countermeasure without tradeoffs.
The discharge function
Normally, an automatic discharge function connects VOUT and GND when the power multiplexer is turned off. There are many benefits to having this quick output discharge:
- The output is not left floating and is always in a known state.
- Downstream modules are always turned off completely.
However, there are situations where quick output discharge is not desirable:
- If the output of the load switch is connected to a battery, quick output discharge can cause the battery to drain when the load switch is disabled via the ON pin.
- If two load switches are being used in a two-input, one-output multiplexer (where the outputs are tied together), power would constantly be wasted through the quick-output discharge, as current will be flowing through the internal resistor to ground whenever the load switch is disabled via the ON pin.
Therefore, when configuring the power multiplexer with the load switch IC, it is necessary to select a load switch that does not have a discharge function. This is where a load switch feature called true reverse current blocking is needed. It prevents reverse current flow from the output terminal to the input terminal regardless of the ON/OFF status of the load switch.
A load switch with this function compares the input voltage VIN with the output voltage VOUT in the IC, and the backflow prevention circuit activates when VOUT>VIN (Figure 6).
Figure 6: True reverse current blocking prevents current flow to the input terminal from the output terminal regardless of whether the load switch is ON or OFF. (Image source: Toshiba)
There are additional subtleties associated with true reverse current blocking and the automatic discharge function; they are discussed in more detail in the Toshiba application note “Overcurrent protection function and reverse current prevention function of the load switch IC.”
New ICs target high-growth applications
Load switches are not new, but they are increasingly tailored to the requirements of specific applications. This is clearly demonstrated by the Toshiba TCK12xBG family of next-generation load switches which comprises three devices: the TCK126BG, TCK127BG, and TCK128BG (Figure 7).
Figure 7: The internal block diagram of devices in the TCK12xBG family shows their functional simplicity; shown is the TCK128BG. (Image source: Toshiba)
The three ICs, which are rated for operation from 1.0 to 5.5 volts and current to 1 A, are very similar with some modest distinct differences to optimally match them to specific application priorities and needs. Many of their specifications are superior to their predecessors and available competitive devices.
Most dramatic is the reduction in quiescent current (IQ) from 110 nanoamps (nA) down to a mere 0.8 nA, for a reduction of 99.9%, or a little over two orders of magnitude. In addition, standby current is just 13 nA. Typical on-resistance RON is 46 mΩ at 5.0 volts, 58 mΩ at 3.3 volts, 106 mΩ at 1.8 volts, and 210 mΩ at 1.2 volts.
Other attributes of these load switches go beyond electrical specifications. They are also far smaller than other available units from Toshiba and other suppliers in the same voltage/current class. They are available in a four-lead WCSP4G package measuring 0.645 × 0.645 × 0.465 mm, with a ball pitch of 0.35 mm. This represents a 34% footprint reduction from predecessor load switches in a 0.79 × 0.79 × 0.55 mm package with a pitch of 0.4 mm (Figure 8).
Figure 8: The smaller size of the TCK12xBG devices compared to their predecessors yields a 34% reduction in required circuit board space. (Image source: Toshiba, modified by author)
This small size provides designers with significant savings in board space, a feature that is critical for ultra-compact applications such as wearable devices. In addition, the package has a 25 micrometer (μm) backside coating that reduces physical impact and damage and prevents chipping.
The three load switches in the family feature built-in slew-rate-control drivers with a rise time of 363 microseconds (µs) at 3.3 volts. The differences among the switches are in the presence or absence of the quick output discharge function, and the active state of the ON/OFF pin (Figure 9).
Figure 9: The three load switches in the TCK12xBG family differ in the pairing of the Quick Output Discharge function and whether the control line is active high or active low. (Image source: Toshiba)
Load switches with highly integrated functionality are critical if designers are to meet demand for lower power consumption, smaller footprint, and lower cost for small, battery-powered devices, such as wearables and smartphones, as well as IoT devices. As shown, the TCK12xBG family of load switches from Toshiba feature low quiescent current and smaller size, have integrated elements to meet functional and protection requirements, and simplify design.
- Toshiba “Load Switch Training Module”