Introduction 

Portable devices constitute the fastest growing segment of modern electronics in number and type. When portable devices are charging, current is allowed to flow INTO the battery. When the charging adapter is removed and the portable device is powered up, current flows FROM the battery to power the device. In these charging/discharging scenarios, an active component is needed that can conduct current in both directions. Furthermore, this component should have an ability to block positive and negative voltages to prevent battery discharge when the device is turned off and unplugged. There are, in general, two ways to address this system-level issue. The first one is to use dedicated bidirectional power switches. While available on the market, dedicated bidirectional power switches are expensive. A better way is to construct such an active component using lower cost discrete or integrated MOSFETs. This is the approach we will describe here. 

Since all MOSFETs have a body diode, voltages are blocked only in one direction. To block voltages in both directions, an additional series diode of opposite polarity can be used. In this case, current flow is unidirectional. To achieve bidirectional current flow with ability to block voltages in both directions, a second MOSFET is needed. Thus, a bidirectional power switch (BPS) is a four-quadrant switch that can block positive and negative voltages in the OFF state and allows load currents in either direction in the ON state. 

Figure 1 shows common configurations which can be used for BPS realization using MOSFETs: a) - common drain N-channel MOSFETs; b) - common source N-channel MOSFETs; c) - common drain P-channel MOSFETs and d) - common source P-channel MOSFETs.

This project will describe BPSs constructed with common-drain connected pairs of lowvoltage Dialog unidirectional integrated power switches (IPS), both P and N-channel. In these circuits, the IPSs' source terminals are respectively connected to power supply and load. A commonsource connection will be used for high-voltage unidirectional IPSs. And in this case, power supply and load are connected to drain terminals.

A Very Low-Power, Bidirectional Power Switch Using Two SLG59M1558V IPSs

The SLG59M1558V is a 28.5 mΩ, 1.0 A rated P-channel MOSFET controlled by a single ON pin and is packaged in an ultra-small 1.0 x 1.0 mm package. Serially connecting these two IPSs in the way shown in Figure 2, a simple, very small, and inexpensive bidirectional power switch can be realized.

Let's assume that terminal "A" is the input terminal of the BPS and the ON pin is asserted LOW. The "B" terminal has a 20 Ω load resistor and 10 µF load capacitor. When 5 V is applied at terminal "A" this voltage is blocked from U2-3 because the ON pin is asserted LOW. When the ON pin becomes asserted HIGH, U1 and U2 close simultaneously then 5V appears at terminal "B". This power-up operation is illustrated in Figure 3.

BPS turn off operation waveforms with and without capacitive loads are presented in Figure 4 and Figure 5, respectively.

BPS behavior for reverse load-current operation is the same.

A Feature-Rich, High-Performance Bidirectional Power Switch Using Two SLG59M1709V IPSs

Operating from a 2.5 V to 5.5 V power supply and fully specified over the -40 °C to 85 °C temperature range, the SLG59M1709V is a high-performance 4 mΩ, 4 A single-channel nFET integrated power switch with adjustable inrush current control which is achieved by adjusting the VOUT slew rate with an external capacitor. Using a proprietary MOSFET design, the SLG59M1709V achieves a stable 4 mΩ RDSON across a wide input/supply voltage range. Incorporating two-stage current protection as well as thermal protection, the SLG59M1709V is designed for all 0.8 V to 5.5 V power rail applications. 

For this type of Dialog IPS, a common drain connection is selected as was shown in Figure 1a. The resulting design of this BPS is shown in Figure 6.

The ON and...

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