Rainbow Electronics MAX16823 - Manual (Page 6)

MAX16823
High-Voltage, 3-Channel Linear High-Brightness
LED Driver with Open LED Detection
6 _______________________________________________________________________________________
Detailed Description
The MAX16823 three-channel current regulator operates
from a 5.5V to 40V input voltage range and delivers up
to 70mA per channel to one or more strings of HBLEDs.
The output currents are programmable using external
current-sense resistors in series with the LEDs. Three
DIM inputs allow a wide range of independent pulsed
dimming in addition to providing the on and off control
of the outputs. The MAX16823 offers an LEDGOOD
output that indicates an open-circuit condition when
one or more LED channels are open.
Integrated pass elements minimize external compo-
nents while providing ±5% output current accuracy.
Additional features include a 3.4V (±5%) voltage regula-
tor with 4mA output current capability, short-circuit and
thermal protection.
The MAX16823 uses a feedback loop to linearly control
the current from each output. The voltage across each
sense resistor is compared to a fixed reference voltage
and the error is amplified to drive the internal power
pass device for a particular channel. See the
Block
Diagram . The regulation point is factory-set at 203mV.
The regulated current is adjusted by the value of
R
CS__ .
The MAX16823 is a constant-current LED driver inter-
nally optimized for driving the impedance range
expected from one or more HBLEDs.
3.4V Regulator (REG)
The MAX16823 includes a fixed 3.4V voltage regulator
that delivers up to 4mA of load current for auxiliary
applications throughout the 5.5V to 40V input voltage
range. Connect a 0.1µF compensation capacitor from
REG to ground. Shorting REG to ground disables the
thermal shutdown.
Thermal Protection
The MAX16823 enters a thermal-shutdown mode in the
event of overheating. This typically occurs in overload or
output short-circuit conditions. When the junction temper-
ature exceeds T
J = +155°C, the internal thermal protec-
tion circuitry turns off the series pass elements. The
MAX16823 recovers from thermal-shutdown mode once
the junction temperature drops by 23°C. The part will
therefore protect itself by low-frequency thermal cycling
in the event of a short-circuit or overload condition.
Applications Information
Programming the LED Current
The MAX16823 uses sense resistors to set the output
current for each channel. To set the LED current for a
particular channel, connect a sense resistor across the
corresponding current-sense input (CS_) and GND. For
optimal accuracy, connect the low-side of the current-
sense resistors to the IC’s ground pin with short traces.
The value needed for the sense resistor for a given cur-
rent can be calculated with the equation below:
where V
CS_
is 203mV and I
OUT_
is the desired LED
current (I LED ).
Input-Voltage Considerations
For proper operation, the minimum input voltage must
always be:
where V
FT(MAX)
is the total forward voltage of all series-
connected LEDs. The minimum operating voltage of
the device is 5.5V.
LEDGOOD
The MAX16823 features an active-low, open-drain
LEDGOOD output that goes low either when one or
more LED channels are open or when a signal at any of
the dimming inputs remains low for a period greater
than the programmed LEDGOOD delay time t
DELAY .
Program t DELAY with a capacitor connected between
LGC and ground. The LEDGOOD output goes low after
the programmed delay period t
DELAY has elapsed. Use
the following formula to set the delay period:
where C LGC is the capacitor connected between LGC
and ground. The LEDGOOD output goes low during
the thermal shutdown.
t
C F
DELAY LGC
= × 257 900 , ( ) ( ) Ω
V V V V
IN MIN CS MAX FT MAX DO MAX ( ) _( ) ( ) ( )
≥ + +
R
V V
I A
CS
CS
OUT
_
_
_
( )
( )
( )
Ω =