NCV7691GEVB NCV7691 Current Controller with Automotive LED Lamps in Sequencing Application Evaluation Board NCV7691GEVB Basic Demonstration Open Circuit Detection Figure 3 provides a representation of the display feature The NCV7691 utilizes the voltage feedback network to of this evaluation board. the FB pin for detection of open circuit conditions to be 1. When the board is powered, 3 sets of LED strings reported at the FLTS pin. An open circuit condition using are initialized in the off state. this board can be simulated by removing Jumpers J1, J2, and st 2. The 1 string of LEDs turns on. J3. The open circuit is indicated at the onboard LED bar, nd 3. After a delay period, the 2 string of LEDs turns but J5 may be removed to route the signal off the board if st on while the 1 string remains illuminated. desired. Detection of open circuit conditions require rd 4. After a similar delay period, the 3 string of LEDs jumpers J8, J9, and J10 to be connected when using the st nd turns on while the 1 and 2 string remain onboard LED array LED loads. illuminated. The low current setup with the LED array using supplied 5. All 3 strings remain illuminated for an extended board resistor values for R3, R7, and R8 of 8.2 will period of time. illuminate each segment of the LED array at 18 mA. The 6. The progression is repeated as all 3 sets of LED drive capability of the NCV7691 to the base of the external strings transition to the off state. transistor has a value per the datasheet between 14 mA and 30 mA. An open circuit condition might fail to be detected if all of the drive current is directed to the base of the BCP56 device to the emitter of the BCP56 device through to the sense resistors on the FB pins (R3, R7, R8). e.g. If the drive capability of the NCV7691 is 18 mA, and 18 mA is provided to the base, the voltage at the FB pin will be at the regulation voltage (150 mV) with or without an open circuit event. The voltage generated on the FB pin will be high enough for the system to think the device is regulating and no open circuit exists. This is why three jumpers are provided on the demo board (J8, J9, J10). Connecting these jumpers provides a load directly to the base drive node to eliminate some of the base drive so that an open circuit condition will be detected. Figure 3. NCV7691 Sequencing