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.
There are two open load detections schemes in the
NCV7691. These are OR’d conditions.
1. In normal regulation, the IC monitors the FB
voltage (typ 152 mV). When this voltage falls by
50% (to typ 76 mV), an open circuit is detected
and a current starts to charge FLTS to flag open
load, once FLTS voltage crosses the output
deactivation threshold the driver is switched off
resulting in a latched off−state. When regulating
via the NTC pin, the open load detection threshold
is V(NTC)/20.
2. During open load, the base current increases to try
and satisfy the regulation loop. Internal circuitry
monitors the base current. When the Base Current
Drive reaches the Output Source Current (typ 25
mA) threshold, an open circuit is flagged and the
driver is latched off.
Two schemes are used should the rise in base
current create a regulated voltage on the feedback
pin (FB). If this occurs scheme #1 would not
detect the open load.
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.
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
Table 1. OPEN LOAD DETECTION
Open Load
(VS > Open Load Disable Threshold)
FLTS BASE
No Open Load (with FLTS capacitor)
Normal Operation
(held low)
regulation
No Open Load Grounded regulation
FB = 1/2 regulation (with FLTS capacitor)
FLTS starts charging
Held low via internal pull−down resistor after
time−out.
BASE Current > 25 mA [typ] (with FLTS capacitor)
FLTS starts charging
Held low via internal pull−down resistor after
time−out.
FB = 1/2 regulation Grounded Actively held low.
BASE Current > 25 mA [typ] Grounded Actively held low.
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 kW [typ]) load
(and resultant long time to restart LED drive) eliminates
flickering effects.
