- SLG59M1558V, Datasheet, Dialog Semiconductor.
- SLG59M1709V, Datasheet, Dialog Semiconductor.
- AN-1068, GreenFET and High Voltage GreenFET Load Switch Basics, Application Note, Dialog Semiconductor.
Author: Andrii Hrypa
Portable devices constitute the fastest growing segment of modern electronics in number and type. When portable devices are charging, current flows 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. It follows that such a system needs an active component which 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. There are, in general, two ways to deal with this system-level issue. The first one is to use dedicated bidirectional load switches available on the market, but at a hefty price. Another way is to construct such an active component using discrete or integrated MOSFETs, at lower cost. This is the approach we will describe.
MOSFETs have a body diode, so it blocks voltage only in one direction. To block voltages in both directions, an additional series diode of opposite polarity can be used, but 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 load switch (BLS) 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 BLS 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 application note will describe BLSs constructed with common-drain connected pairs of Dialog unidirectional GreenFET load switches, both P and N-channel. In these circuits, the GreenFET load switches' source terminals are respectively connected to power supply and load.
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 GreenFET load switches in the way shown in Figure 2, a simple and very small bidirectional load switch can be obtained.
Let's assume that terminal "A" is the input terminal of the BLS 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 and the voltage appears at terminal "B". This power-up operation is illustrated in Figure 3.
BLS turn off operation waveforms with and without capacitive loads are presented in Figure 4 and Figure 5, respectively.
BLS behavior for reverse load-current operation is the same.
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 load 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 GreenFET load switches, a common drain connection is selected as was shown in Figure 1a. The resulting design of this BLS is shown in Figure 6.
ON and VDD pins of U1 and U2 are connected in parallel, respectively, so both ICs can be turned on and off simultaneously. The BLS's output voltage follows a linear ramp set by a capacitor connected to each CAP pin. A larger capacitor value at the CAP pin produces a slower ramp, reducing inrush current from capacitive loads.
By applying 5 V at terminal "A", which corresponds to the U1's VOUT pins (U1-[8:12]), imparts a voltage at the node common to U1 and U2. This takes place because of the U1's body diode, which in this case is forward biased. Since there is a voltage drop across this diode, the voltage at the node common to both ICs (U1-[3:7] and U2-[3:7]) is approximately 4.4 V. This voltage is blocked by U2 because its ON pin asserted LOW. When the ON pin is asserted HIGH, both GreenFET load switches power up and the voltage now appears at terminal "B". During power up, a small transition, illustrated in Figure 7, can be observed. It is caused by internal linear ramp control circuit operation.
Turn-off operation waveforms with and without capacitive loads are presented in Figure 8 and Figure 9, respectively.
The reverse operational behavior of the BLS from terminal "B" to terminal "A" remains the same as in the forward direction.
Constructing a bidirectional load switch using the SLG59M1709V and similar GreenFET load switches retains all protection features. Thus, an GreenFET load switch's Active Current Limit, Short Circuit Protection, Thermal Shutdown Protection, and Inrush Current Control can easily protect important components upstream and downstream of the GreenFET load switch as well as the load switch itself.
For example, when the BLS's output voltage is greater than 300 mV, output current is initially limited to U2's Active Current Limit (IACL) threshold which is 8 A (from the SLG59M1709V's spec sheet). When a load current overload is detected, U2's ACL monitor increases U2's FET resistance to keep the current from exceeding the U2’s IACL threshold. During active current-limit operation, the output voltage is also reduced by IACL x RDSON(ACL). However, if the load-current overload condition persists where the die temperature rises because of the increased FET resistance, the U2’s internal Thermal Shutdown Protection circuit will be activated. If the die temperature exceeds 125 °C, U2's FET is shut OFF completely, thereby allowing the die to cool. When U2's die cools to 100 °C, U2's FET is turned back on. This process may repeat while the output current overload condition persists. Operation of both Active Current Limit protection as well as Thermal Shutdown Protection is illustrated in Figure 10.
When the BLS's output voltage is less than 300 mV (a short circuit condition), the U2's internal Short-circuit Current Limit (SCL) monitor limits the FET current to approximately 500 mA (the ISCL threshold). Observed BLS behavior is shown in Figure 11. (the ISCL threshold). Observed BLS behavior is shown in Figure 11.
In this application note, the SLG59M1558V and SLG59M1709V Dialog GreenFET load switches were configured for bidirectional applications. Additionally, it is also possible to use other P- and N-channel Dialog GreenFET load switches provided that those load switches do not include Reverse-Current Blocking, Reverse-Voltage Blocking, and VOUT Discharge features. Dialog load switches featuring Active Current Limiting and Short Circuit Current Limiting Modes with Thermal Shutdown Protection, and Inrush Current Control are all excellent candidates for constructing high-performance, pcb space-efficient bidirectional load switches.
The list of GreenFET Dialog load switches which can be used for bidirectional load switching applications (at time of this writing) are listed below:
More detailed information can be found on https://www.dialog-semiconductor.com/products/load-switches