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NCV7691 Datasheet(PDF) 9 Page - ON Semiconductor |
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NCV7691 Datasheet(HTML) 9 Page - ON Semiconductor |
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9 / 18 page  NCV7691 www.onsemi.com 9 Open Load Detection Faulted output strings due to open load conditions sometimes require the complete shutdown of illumination within an automotive rear lighting system. The NCV7691 provides that feature option. When an open load is detected, the output turns off, and can be turned back on again by a toggle of the PWM pin or a power down of the supply (VS). If the open load feature is not used, FLTS should be tied to GND. Grounding FLTS disables open load detection. Short circuit detection and thermal shutdown functions remain active but are not reported externally. The BASE pin is actively held low in this case. System Voltage and Overvoltage Fold−back Low voltage system operation is typically limited by head room in the LED string. Because of this limitation, detection of open loads is inactive below VS = typ 8.5 V (Open Load Disable voltage). There is also an upper limitation. The current roll off feature of the part resets the loop at a lower reference voltage and consequential lower current for VS above the Overvoltage Fold−back threshold on VS, (typ 19.5 V). The open load Detection circuitry is inactive for VS above this Overvoltage Fold−back threshold voltage. Open Load Timing The timing for open load detection is programmed using the FLTS pin. The NCV7691 device regulates a 152 mV reference point (Figure 8 on the feedback pin (FB)). When the voltage decreases (half of the FB Regulation Voltage) or the base current reaches the internal 25 mA (typ) limit for 42 ms the timer associated with the FLTS pin starts by charging the capacitor with a 2 mA current source. When the voltage on FLTS exceeds the output Deactivation Threshold (1.15 V (typ)), the BASE pin is pulled low and is held low by an internal pulldown resistor. A 42 msec blanking time during power up ensures there is enough time for power−up to eliminate false open−load detections. The slow FLTS discharge (600 k W [typ]) load (and resultant long time to restart LED drive) eliminates flickering effects. FLTS Interface Figure 9 shows an open−drain logic level FET serving as a buffer to the microprocessor. Figure 10 shows the proper wired “OR” connection for applications which require all channels to latch−off with an open load condition. An open load condition will be reported back to the microprocessor regardless of which channel it occurs on. Note the NCV7691 device uses a feature which allows any channel to charge the FLTS capacitor due to its definition at a charge current value much higher than the discharge value (2 mA versus 600 k W [typ]). Additional NCV7691 Single Current Controller devices device may share the same common FLTS capacitors in systems requiring multiple ICs. Figure 8. Open Load Detection Circuitry FLTS GND FB BASE C2 0.1uF R1 1 ohm VS FLTS Clamp (V) 2mA Blanking Timer (42 usec) 1.15V Output Drive Output Deactivation Threshold FLTS Charge Current BCP56 600 kW VS Monitoring VS Open Load Disable VS Open load can be disabled by connecting FLTS to GND. Detect Figure 9. Open Drain Output Interface to Microprocessor FLTS GND C1 0.1uF + − 600k 2mA 1.15V Output Deactivation Threshold FLTS Charge Current FLTS pull−down resistor BSS138 to microprocessor FLTS Clamp (V) Figure 10. FLTS Wired OR to Microprocessor FLTS GND C1 0.1uF + − 2mA 1.15V Output Deactivation Threshold FLTS Charge Current BSS138 to microprocessor Note – Only one timing capacitor and interface transistor are required for system operation. FLTS GND NCV7691 NCV7691 600k FLTS pull−down resistor FLTS Clamp (V) |
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