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www.fairchildsemi.com AN-9758 Open-Load Detection in Smart Switches 1. Introduction The new family of smart high-side MOSFETs consists of five-terminal and seven-terminal high-side devices. Fairchild’s smart high-side switches are PowerTrench® MOSFETs with integrated protection circuitry. This family features efficient power MOSFETs with active clamp and integrated protection functions, including: over-temperature, current limitation, over-current and reverse battery. The smart high-side switches include protection functions such as under-voltage, over-voltage, short-circuit, and overtemperature shutdown, as shown in Figure 1. Some external components are needed to operate a device as shown in Figure 2. This family features a control input and a diagnostic pin. An internal charge pump circuit allows the MOSFET to be driven in a high-side configuration without additional external components. This application note explains the features of the high-side family, helps the designer select components, and provides suggestions on how to detect open-load in both OFF and ON state in automotive systems. The open-load detection circuit can be implemented with external resistors. The purpose of open-load detection is to provide the vehicle driver an alert about the state of the system. For example, if headlamps were to fail at night or during a foggy day; it creates a dangerous situation. The open-load detection functions during OFF state and issues a warning when headlamps fail. Figure 1. Block Diagram of Smart High-Side Switches Figure 2. Typical Application Circuit for Open-Load Detection © 2012 Fairchild Semiconductor Corporation Rev. 1.0.0 • 9/19/12 www.fairchildsemi.com AN-9758 APPLICATION NOTE 2. Possible Solution Two types of open-load detection circuits are discussed: during OFF state and during ON state. 2.1. Open-Load Detection during OFF State The first method for open-load detection is to use resistors to detect a voltage showing up at external resistor during OFF state. There are cases where open-load detection is requested also when the load is not activated. In this case, the micro-controller is able to detect the open-load as soon as it occurs during OFF state. Figure 4. Open Load Detection During OFF State Current flows into R1, R2, and R3.The output is connected to R2 and R3 and a certain voltage is shown at output. The voltage of Diag1 is almost zero. Therefore, the open-load condition equation for applying voltage to Diag2 pin can be approximated as: Rtotal 1 R2 R3 Vload (Bat R1 ) /(R1 Rtotal 1 ) The smart high-side switches can detect normal condition and open-load condition by the voltage of Diag1 and Diag2. However, an external passive resistor; like R1, R2, and R3; is needed to detect open-load state. To reduce leakage current during OFF state, high-value resistors are used. Assume the application circuit is applied to the conditions: In normal condition, the load is connected to GND and extremely low current (beside the output leakage) flows into the load. The voltage of Diag1 and Diag2 is almost zero. The normal condition equation for applying voltage to Diag2 pin can be approximated as: Vload (Bat Rtotal 2 ) /(R1 Rtotal 2 ) FDDS10H04A_F085A Rload is OPEN VBAT is 12 V. In this case, voltage shown at R3 can be expressed as: Rtotal1 30k 10k 40k Vload (12V 40k) /(100k 40k) 3.43V (4) VR 3 (Vload 10k) /(10k 30k) 0.86V Rtotal 1 R2 R3 Rtotal 2 (Rtotal 1 Rload ) / Rtotal 1 Rload ) (3) VR 3 (Vload R3 ) / Rtotal 1 Figure 3. Normal Operating during OFF State The result of open-load condition during OFF state is that VDIAG1 is almost zero and VDIAG2 is 0.86 VBAT 12 VBAT. (1) VR 3 (Vload R3 ) / Rtotal 1 Assume the application circuit is applied to the conditions: FDDS10H04A_F085A Rload is 6 Ω VBAT is 12 V. In this case, voltage at R3 can be expressed as: Rtotal 1 30k 10k 40k Rtotal 2 ( 40k 6 ) / 40k 6 ) 5.98 Vload (12V 5.98 ) /(100k 6 ) 3mV (2) VR 3 (Vload 10k ) /(10k 30k ) 1mV Figure 5. Normal and Open-Load during OFF State As the result; in normal condition, VDIAG1 and VDIAG2 are almost zero. If the load is disconnected during OFF state, an external resistor pulls up the output so that the open-load condition is detected via VDAIG2 and VDIAG1. © 2012 Fairchild Semiconductor Corporation Rev. 1.0.0 • 9/19/12 www.fairchildsemi.com 2 AN-9758 APPLICATION NOTE 2.2. Open-Load Detection during ON State The second method for open-load detection is to use the resistor network and check the voltage levels at Diag1 and Diag2 during ON state. Figure 8. Normal Operating during ON State The smart high-side switches can detect normal condition and open-load condition with voltage of Diag1 and Diag2. However, an external passive resistor; like R1, R2, and R3; is needed to detect open-load state. Figure 6. Timing Diagram During OFF State Even if the MOSFET is turned off, a certain voltage appears at Diag2 in the case of an open-load, as described timing diagram in Figure 6. In normal condition, VOUTPUT is almost the same as VBAT. The voltage of Diag1 and Diag2 is not zero. The equation for applying voltage to the Diag2 pin can be approximated as: Figure 7 provides a flow chart for a possible method to detect with a software strategy. With this software routine, it is possible to distinguish between normal condition and open-load condition during OFF state. If VBAT is taken into consideration, a VDIAG2 range from 0.4 V to 1.9 V detects open-load. VDIAG2 is 0.64 V at 9 VBAT and 1.14 V at 16 VBAT in case of open-load state. Rtotal 1 R2 R3 Vload (VBAT VD S ) (5) VR 3 (Vload R3 ) / Rtotal 1 where VDS is the voltage drop between drain and source. Assume the application circuit is applied to the conditions: FDDS10H04A_F085A Rload is 6 Ω VBAT is 12 V. In this case, the voltage at R3 can be expressed as: R total 1 30k 10k 40k Vload (12V Max. 0.065V ) 11.9V VR 3 (Vload 10k ) /(10k 30k ) 2.98V (6) VIS (12V / 6 ) / 10000 1k 0.2V VR 3 (Vload 10k )(10k 30k ) 2.98V As the result, VDIAG1 is 0.2 V and VDIAG2 is 2.98 V under normal condition. If the load is disconnected during ON state, an external resistor pulls up the output so the open-load condition is detected by VDIAG2 and VDIAG1. Figure 7. Open Load Detection During Off State © 2012 Fairchild Semiconductor Corporation Rev. 1.0.0 • 9/19/12 www.fairchildsemi.com 3 AN-9758 APPLICATION NOTE Figure 9. Open-Load Detection during ON State Output current flows into R2 and R3. The output is connected to R2 and R3 and battery voltage is shown at output. The voltage of Diag1 is almost zero. The open-load condition equation for applying voltage to the Diag2 pin can be approximated as: Rtotal 1 R2 R3 Vload (VBAT VD S ) (7) Figure 11. Timing Diagram during ON State VR 3 (Vload R3 ) / Rtotal 1 Even if the MOSFET is turned on, a certain voltage is not reached at Diag1 in open-load condition, as shown in Figure 11. where VDS is the voltage drop between drain and source. Assume the application circuit is applied to the conditions: Figure 12 provides a flow chart for a possible method to detect with a software strategy. FDDS10H04A_F085A Rload is 6 Ω VBAT is 12 V. In this case, voltage at R3 can be expressed as: Rtotal 1 30k 10k 40k Vload (12B Max. 0.065V ) ~ 11.9V ) VR 3 (Vload 10k ) /(10k 30k ) 2.98V ) (8) VIS ~ 0V VR 3 (Vload 10k ) /(10k 30k ) 2.98V ) As the result of those equations, VDIAG1 is 0 V and VDIAG2 is 2.98 V in open-load condition. Figure 12. Open-Load Detection during ON State With this software routine, it is possible to distinguish between normal condition and open-load condition during ON state. If VBAT is taken into consideration, the VDIAG2 range is from 2.2 V to 5V to detect open-load. VDIAG2 is 2.25 V at 9 VBAT and 4 V at 16 VBAT in open-load condition. Figure 10. Normal and Open-Load during ON State © 2012 Fairchild Semiconductor Corporation Rev. 1.0.0 • 9/19/12 www.fairchildsemi.com 4 AN-9758 APPLICATION NOTE 3. Conclusion In this application note, a method for the detection of openload conditions for smart high-side switches during ON and OFF state was explained and a software strategy proposed. The measured leakage currents during OFF state are shown in Table 1. Table 1. Total Leakage Current of MOSFET and External Component (12 V Condition) Total External R1 R2 R3 MOSFET Leakage Load Components (kΩ) (kΩ) (kΩ) Leakage Current Normal in OFF State 100 30 10 6Ω 0.8 µA 120 µA Open-Load in 100 OFF State 30 10 Open 0.8 µA 86 µA Figure 13. Logic Diagram of Open Load State A software strategy is required to distinguish between ON state of open-load and OFF state of open-load. The best solution for open-load detection is determined by the requirements of the application. For better understanding of the open-load detection process, refer to logic diagram in Figure 13. References [1] AN-8039 — Using the FDDS100H06_F085 in Automotive Systems [2] FDDS10H04A_F085A and FDBS09H04A_F085A Datasheets Related Resources FDDS10H04A — Smart High-Side Switch FDBS09H04A — Smart, Protected, High-Side Switch DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. © 2012 Fairchild Semiconductor Corporation Rev. 1.0.0 • 9/19/12 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. www.fairchildsemi.com 5