CAT4026_11 Datasheet, PDF (12/14 Pages) ON Semiconductor – 6-Channel LED Controller

CAT4026
APPLICATION INFORMATION
Operation with Open and Shorted LED
The CAT4026 can detect both open and shorted LED
strings through two diode−OR circuits connected
respectively to the VCS and SCA pins, as shown in the
application circuit in Figure 1.
Open LED
When one of the channel becomes open or disconnected,
its cathode voltage drops to zero pulled down by the current
sensing resistor (R1−R6). The lowest cathode voltage is
sensed through a diode at the VCS pin (VCS pin is around
0.6 V above the lowest cathode voltage). This causes the
CAT4026 current feedback pin (IFB) current to increase to
1 mA and the power supply to increase the anode voltage
VOUT until the OCA pin exceeds 1 V threshold and latches
on the FLT−OCA fault (the pin is pulled low). At that time,
the CAT4026 disables the open channel (corresponding
BASE pin voltage goes to GND) and will ignore that channel
until the driver is shutdown. The FLT−OCA pin remains low
until the CAT4026 goes to shutdown mode or is powered
down. The output voltage VOUT now returns to normal
operation level where the lowest cathode voltage is around
3.2 V (VCS pin around 3.6 V). The anode voltage is sensed
at the OCA pin through a resistor divider (Ra, Rb) as shown
in Figure 21.
Shorted LED
In some cases, the LED string voltage may be different
between different strings (channel voltage mismatch). This
can be due to LED forward voltage variation or some LEDs
becoming shorted in one of the string. One of the string
would have a total LED forward voltage lower than other
channels. In operation, the cathode voltage of the “shorted”
channel will be higher than the other channels causing more
power to be dissipated in the external transistor of that
channel. Therefore, it is useful to detect this condition and,
if needed, derate the LED channel current. The highest
cathode voltage is sensed at the SCA pin through a
diode−OR network. A zener in series with the diodes, shown
in Figure 21, allows to adjust the cathode threshold voltage.
Once the SCA pin sinks more than about 1.3 mA, the
FLT−SCA fault is triggered and the pin is pulled low.
Figure 20 shows a power−up waveform for a threshold
voltage at about 45 V. In this example, when the fault is
triggered, the LED current decreases from 100 mA to 20 mA.
Figure 18. Open LED at Power−up
Figure 20. Shorted LED Channel at Power−up
Figure 21 shows a partial application schematic relative to
the OCA and SCA fault detection.
Figure 19. Open LED at Power−up, Base Voltages
Figure 21. Schematic for Open/Short Detection
Unused LED Channels
For applications that require less than 6 LED channels, the
unused channel BASE and RSET pins should be left
floating. All the other used channels will operate normally.
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