CAT4026_11 Datasheet, PDF (10/14 Pages) ON Semiconductor – 6-Channel LED Controller

CAT4026
External adjustment of LED Anode supply voltage is
controlled by the IFB current sink in conjunction with an
external feedback circuit. The external circuit should be
configured so that 1 mA drive signal will achieve the desired
necessary dynamic adjustment range for expected worst
case maximum LED string operating voltage range
A linear transconductance relationship exists for the drive
current (1 mA/V) for Cathode operation between 2.5 V and
3.5 V.
C1
Connect a capacitor of 1 nF and a 10 kW resistor from the
C1 pin to the LED Anode voltage. Capacitor voltage rating
must be greater than the highest LED anode voltage.
C3
Connect pin to GND.
SCA
The SCA pin is used to detect a severe mismatch in LED
string voltage, such as the occurrence of an Anode−Cathode
short. The SCA pin is connected to each LED cathode via a
diode array and a voltage level translator. The threshold
voltage of the detector can be adjusted by using an external
Zener diode.
A conduction level of 1.5 mA into the SCA pin will trigger
a FAULT condition. The FAULT condition will be cleared
upon the conduction current level falling below 0.5 mA and
normal operation will resume.
OCA
The OCA input is used to detect and protect against
abnormally high LED Anode condition. An external
resistive divider connected to the OCA pin, from the LED
Anode voltage, will trigger a FLT−OCA condition once the
OCA input level exceeds 1.0 V. Any open−LED channel will
automatically be disabled and removed from the feedback
loop when OCA is triggered. This method provides an
auto−recovery feature for the system to resume normal
operation ensuring only the ‘good’ LED channels are
included in the feedback loop.
If the open−LED function is not used, the OCA pin should
be tied to GND.
VCS
The VCS pin is connected to each LED cathode via a
diode array. This pin detects the lowest LED cathode voltage
and sets the feedback signaling to allow the SMPS to adjust
the LED Anode voltage to the appropriate levels for
optimum efficiency (3 V operating point for the minimum
cathode voltage on any string). An external high voltage
diode array such as BAS21LT is recommended.
VA
The VA output pin is optional and allows the user to power
an external feedback control circuit for setting the common
LED Anode operating voltage level.
This output is a buffered voltage signal, which tracks 50%
of the internal reference being used to control and set the
nominal operating level of the lowest LED Cathode string
voltage. An internal source impedance of 250 W is present
on this output and the nominal voltage is set to 1.8 V
(thermal compensation exists to cancel out the external
sensing diode temperature coefficient present on the VCS
pin).
VC
The VC pin is a buffered voltage signal, which tracks 50%
of the voltage level present at the VCS input pin (i.e. the VC
voltage is determined by the lowest operating Cathode
voltage present on any LED string).
This signal provides a convenient feedback control
method for systems which use standalone converters to
generate the LED Anode supply voltage (as opposed to a
current feedback option). An external suitable resistive
divider, at the VC pin, can be used to directly control the
feedback input of the standalone converter.
During shutdown mode, the VC pin is forced into high
impedance mode, while during normal operation an output
source impedance of 360 W is present on the VC pin.
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