AND9039/D Using SENSEFET[ with CAT2300 in Load Switch Applications Prepared by: Ann Starks ON Semiconductor http://onsemi.com APPLICATION NOTE Introduction The SENSEFET is a five−terminal device, as shown in Figure 1. The main MOSFET and mirror MOSFET share common gate and drain connections, but have separate sources. A Kelvin connection applied to the main source terminal provides lossless current measurement capability, and reduces measurement error. It is often necessary to closely monitor the current through critical paths in order to make appropriate power management decisions and to maximize the performance of the system. One common current monitoring method is series resistor current sensing. For this method, a low value precision resistor is placed in the current path and the differential voltage measured across the resistor is used to make power management decisions. While this method is simple to implement, the efficiency of the circuit is reduced due to the power dissipation across the sense resistor. The need for a precision current sense methodology with minimal impact to efficiency and board space has led ON Semiconductor to develop a new, low RDS(on) SENSEFET product portfolio that utilizes an internal mirror MOSFET to monitor the current through the load switch without impacting system efficiency. SENSEFET The SENSEFET is essentially comprised of two matched MOSFETs: the main current−carrying power MOSFET and a much smaller sensing MOSFET. The device structure is fairly simple. A small number of source cells is isolated from the rest and connected to a separate sense pin, creating a matched mirror MOSFET. The ratio of source cells to sense cells is very large, and when current flows through the main MOSFET a much smaller current is produced through the mirror MOSFET. The ratio of current through the main MOSFET to the current through the mirror MOSFET is called the current ratio, IRATIO. A typical IRATIO is 400:1. I RATIO + I SOURCE I SENSE Figure 2. SENSEFET Equivalent Circuit Figure 2 shows a simplified equivalent circuit for the SENSEFET device. When Source and Sense are held at the same potential, the IRATIO is a known constant. Therefore, one could measure the sense current to accurately calculate the load current. If a measurement resistor is inserted between the source and sense terminals as illustrated in Figure 2, a fraction of the load current is sampled, but the source and sense terminals are no longer at the same potential and IRATIO is no longer a known constant. To minimize this effect, the measurement resistor value must be less than 10% of the mirror MOSFET’s on resistance (RMEAS < 100 mW). Therefore, placing a measurement resistor in this configuration is not practical. In order to use a larger resistor (eq. 1) Figure 1. SENSEFET Device Symbol © Semiconductor Components Industries, LLC, 2011 August, 2011 − Rev. 0 1 Publication Order Number: AND9039/D AND9039/D completely separate from the Sense trace and must begin directly at the Sense pin of the SENSEFET itself. value without changing the IRATIO, additional components must be used, as described in the next section. Integrated Solution ON Semiconductor has implemented a novel two−chip solution for lossless current sensing that provides a scalable voltage proportional to the load current. Figure 3 shows the NTMFS4854NS SENSEFET paired with the ON Semiconductor integrated measurement circuit CAT2300. Figure 4. PCB Trace Connection of Sense and KS An external resistor RMEAS is placed between the IMEAS pin of the CAT2300 and ground, producing a voltage that is proportional to the sense current flowing through the mirror MOSFET. This voltage, VMEAS, is calculated as: V MEAS + I SENSE @ R MEAS (eq. 2) The load current can be determined by combining Equations 1 and 2: I LOAD + I RATIO @ V MEAS R MEAS (eq. 3) Measurement Resistor Selection The value of RMEAS depends on several factors, including the application’s voltage rail and load current range. Equation 4 gives the maximum value that RMEAS can be for a given application. VK is the Kelvin voltage of the SENSEFET terminal and IMAX is the maximum load current used in the application. The IRATIO is specified in the SENSEFET device datasheet. Figure 3. CAT2300 SENSEFET Measurement Circuit The CAT2300 is a SENSEFET controller and current monitoring device for load switch applications, and is available in a low profile 2 mm x 3 mm TDFN package. The CAT2300 can monitor load currents from 1 A to 25 A for power supply rails ranging from 0.9 V − 1.5 V. It is designed for the dual purpose of lossless current sensing and control of the SENSEFET turn−on and turn−off. The gate of the SENSEFET is controlled by the logic level EN pin of the CAT2300. A logic high turns on the SENSEFET and a logic low turns the SENSEFET off. The gate turn−on time can be adjusted by adding a pull−up resistor between VDD and gate (faster turn−on) or by adding capacitance between the SENSEFET gate and source (softer turn−on). The current sense circuit is comprised of an amplifier with a MOSFET follower stage. The Kelvin signal is used as a voltage reference, and the op−amp / follower stage accurately tracks the sense current by holding Sense at the same potential as Kelvin. An additional Kelvin Sense (KS) pin is included in the CAT2300 design as a Kelvin connection for the mirroring MOSFET, in order to eliminate any voltage drops in the Sense trace on a printed circuit board (PCB). Figure 4 illustrates the correct connection of the Sense and KS board traces. To have a true Kelvin connection, the KS trace must be a dedicated connection, R MEAS + V K * 0.1 + I RATIO @ I SENSE V K * 0.1 I MAX (eq. 4) As an example, let’s assume that the NTMFS4854NS SENSEFET is used in a 1.5 V power rail application, and the maximum load is 10 A. VK at maximum load current can be estimated using the maximum RDS(on) value specified in the SENSEFET datasheet, as shown in Equation 5. V K + V IN * I MAX @ R DS(on)MAX (eq. 5) The NTMFS4854NS, has a typical IRATIO of 399 and a maximum RDS(ON) of 3.9 mW at 4.5 VGS. Using Equation 5: VK = 1.5 V − (10 A) * (3.9 mW) = 1.461 V. Therefore, the maximum voltage producible at the IMEAS pin of the CAT2300 is estimated to be 1.461 V − 0.1 V at 10 A. The maximum value of RMEAS can be calculated by combining Equations 4 and 5. This value must not be exceeded. ǒ1.461 * 0.1Ǔ + 54.3 W R MEAS + 399 10 (eq. 6) For best measurement results across the input voltage range, select a resistor value for RMEAS that produces 0.8 V on the CAT2300 ISENSE pin at the maximum load current. Equation 3 therefore becomes: http://onsemi.com 2 AND9039/D R MEAS + V MEAS @ I RATIO I LOAD + 0.8 V @ I RATIO I LOAD To have a true Kelvin connection, the KS trace must be a dedicated connection, completely separate from the Sense trace and must begin directly at the Sense pin of the SENSEFET itself (Figure 6). (eq. 7) Therefore R MEAS + 0.8 V @ 399 10 A + 31.9 W Experimental Results The NTMFS4854NS SENSEFET and CAT2300 two−chip solution was tested in a load switch circuit under the conditions shown in Table 1. for this example. To obtain the best measurement accuracy, use a resistor with a tolerance less than 1%. A standard resistor value of 31.6 W would be used in this example. Table 1. LOAD SWITCH CIRCUIT TEST CONDITIONS Layout Considerations It is important that good layout practices be implemented to minimize the losses in the traces of the PCB. Place the CAT2300 as close to the SENSEFET as possible, minimize PCB trace lengths, and use proper Kelvin connections. A PCB with a separate ground plane is recommended for the integrated solution. Input Voltage 1.5 V VDD Voltage 5V Load Current 1 − 10 A RMEAS 31.6 W Figure 5. Suggested Part Placement Figure 7. IRATIO vs. Load Current of 1 − 10 A Figure 5 shows a suggested placement of the SENSEFET, CAT2300 and measurement resistor. Make sure that the PCB traces for VIN and VOUT are thick enough to handle the maximum load current of the specific application. Figure 7 shows measured IRATIO over the load current range. The measured IRATIO loses accuracy for currents below 1 A due to the input offset voltage of the CAT2300 internal op−amp. As discussed in previous sections, the source and sense terminals must be held at the same potential to have a constant IRATIO. The input offset voltage creates a voltage difference between the two terminals, introducing error to the measurement. This error is most prominent for currents below 1 A. Discrete Solution for Measurement Needs Below 1 A For applications that require high measurement accuracy for load currents below 1 A, a discrete implementation with a low offset precision op−amp is recommended. Figure 6. KS Layout Connection http://onsemi.com 3 AND9039/D As can be seen in Figure 9, the measured IRATIO is constant down to 100 mA of load current. The precision op−amp used in this example had a typical input offset voltage of 8 mV and maximum of 50 mV. The input voltage offset of the amplifier is very important, and IRATIO accuracy improves with lower input voltage offset. Conclusion ON Semiconductor has created a two−chip lossless current sensing solution for the load switch application that pairs a SENSEFET device with the CAT2300 integrated measurement circuit. An external measurement resistor provides design flexibility for various load switch applications, and the load current can be monitored without impacting the device performance or efficiency of the load switch circuit. With this circuit implementation, the IRATIO remains a known constant across the load current range, down to 1 A. If it is required to accurately measure currents below 1 A, it is recommended to use a discrete measurement circuit with a precision op−amp. The sense current is given as a voltage value that can be fed to an analog−to−digital converter or similar IC to provide more accurate measurements for power management decisions. Contact ON Semiconductor for an evaluation board. Figure 8. Discrete Measurement Circuit Figure 8 shows an implementation of a discrete measurement circuit using a precision op−amp. The measurement circuit was tested using a 1.5 V input voltage rail across a load of 0 A to 10 A. References 1. “Current Sensing Power MOSFETS.” Application Note # AND8093/D. ON Semiconductor. www.onsemi.com 2. H. Massie. “Current Sensing Power MOSFET Use in DC−DC Converters.” Application Note # AND8210/D. ON Semiconductor. www.onsemi.com Figure 9. IRATIO vs. Load Current for Discrete Solution http://onsemi.com 4 AND9039/D SENSEFET is a registered trademark of Semiconductor Components Industries, LLC (SCILLC). ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. 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