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Analog/Mixed Signal

Load Switch IC Provides One Basic Function

20 February 2015

Load switches are the all-electronic equivalent of the on/off toggle switch: via a simple control signal they switch many amps of current to a load. They are needed to power-up/down circuit boards, sub-circuits, and any system functions which are not needed at a given time, to save power and reduce thermal load. Their primary parameters are current capacity, on-resistance, size, and of course, price.

While they aren't glamorous, they are essential building-block "nut and bolt" components needed in many applications including industrial equipment, medical devices, and servers. Their functional simplicity is their virtue, as they don't confuse the system software, they don't have complex functional sequences, and they don't demand constant attention from the operating code.

The recently introduced MIC95410 high-side load switch from Micrel Inc. is an example of high performance, small package, and low price which eases the design challenge that engineers face, as shown in Figure 1. Controlled by a single TTL-level digital line, it turns on/off voltage rails which can range from sub- one V up to 5.5 V, handling currents up to 7 A with on-resistance RDS(on) of just 6.6 mΩ (typical), Micrel said.

Figure 1

Connection of the Micrel MIC95410 high-side load switch is simple, and it does not need system or software-based initialization; the user just provides a power supply, TTL-level control signal, and output load connections to be switched.

The MIC95410's operating voltage bias supply can be between 2.7 V and 9 V. Both the OFF-state current from the bias supply and the power-switch OFF-state leakage current are below 1 μA. Users can take advantage of an extra feature if desired, where the switch provides control of turn-on slew rate in order to limit inrush currents to the input supply voltage.

This slew-rate limiting, implemented via an external capacitor, minimizes system transients on the supply rail which can cause hard-to-trap malfunctions and erratic operation of the associated circuitry. The IC's internal N-channel MOSFET switch, as shown in Figure 2, provides two user benefits: it eliminates the need for a separate MOSFET (thus reducing board space needs and BOM cost) and also ensures that overall performance of the switch's control and power-handling functions are determined by the vendor's specification rather than the subtleties of a user-selected MOSFET.

Figure 2

Within the load switch are the control-line interface, charge pump, MOSFET driver, and N-channel MOSFET to act as the electronic equivalent of an on/off toggle switch.

Even a functionally simple component can suffer due to inadequate board layout, and the load switch is no exception; it has its own board-layout guidelines for optimum electrical and thermal behavior. To ensure the part meets its specifications, Micrel provides a detailed example of suitable layout and routing for the top and bottom layers of the PC board, along with some variations on the basic example.

Both size and price of the switch are small: it is packaged as a 10-lead, 1.2 mm × 2.0 mm QFN device, priced at $0.45 (1000 pieces).

Questions or comments on this story? Contact: [email protected]

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