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High-Voltage, Linear High-Brightness LED
Driver with Open-LED Fault Detect
C FLTS = FLTS FLTS
R CS ( ? ) = CS
Applications Information
FLTS Capacitor
If a fault condition is detected, a 2.8mA current source
charges C FLTS to memorize the fault condition and avoid
flashing during the subsequent power-up cycles. The
choice of the C FLTS is based on the following conditions:
U Fault hold time
U Board leakage and FLTS leakage
U Input supply voltage, as this affects the maximum
FLTS voltage
U The number of ICs connected together in case of a
multistring application
Use the following equation to calculate the FLTS capaci-
tor (C FLTS ):
I × t × N
(V FLTS ? V FLTH )
where I FLTS is the maximum leakage current through
FLTS, t FLTS is the desired fault retention period, V FLTS
is the typical voltage (minimum of 15V or the supply
voltage) to which C FLTS is charged during a fault event,
V FLTH is the minimum threshold to detect the fault
memory as logic-high, and N is the number of devices in
a multistring application.
Programming the LED Current
The MAX16839 uses a sense resistor to program the
output current sink. To program the LED current, connect
a sense resistor from the current-sense input (CS) to
GND with short traces. The value of the sense resistor
for a given desired current is calculated with the follow-
ing equation:
V (V)
I LED (A)
where V CS is 203mV (typ) (see the Electrical
Characteristics table).
Input Voltage
For proper operation, the minimum input voltage must
always be:
V IN(MIN) = V CS(MAX) + V FT(MAX) + ? V DO
where V CS(MAX) is the maximum voltage drop across the
sense resistor R CS , V FT(MAX) is the total forward voltage
of all series-connected LEDs, and D V DO is the maximum
dropout voltage of the regulator. If the device is oper-
ated below V IN(MIN) , the output current may not meet the
full regulation specification (see the Typical Operating
Characteristics ).
For applications that require more than 3 LEDs per string
and have slow supply rise time (> 14 F s), FLTS should, in
general, be connected to ground to avoid false trigger-
ing of the open-LED protection.
PWM Dimming
The MAX16839 includes a PWM dimming input (DIM)
to control the LED brightness. An application of up to a
10kHz signal is recommended at DIM for proper opera-
tion. DIM also functions as an active-high enable input.
Driving DIM low turns off the output. Driving DIM high or
leaving DIM unconnected turns on the output.
For V IN > undervoltage-lockout threshold, the MAX16839
activates a 30 F A (typ) pullup current at DIM. When V IN <
undervoltage-lockout threshold, the MAX16839 activates
a 1mA pulldown current.
Supply Chopped Dimming
The MAX16839 can do efficient supply chopped
dimming, as it is specially designed for very low startup
delay. The LED current is turned on in 4 F s (typ) after
the supply comes up. The LED current pulse-width
modulates in phase with the supply voltage. Minimize the
input filter capacitor after the chopper circuit for fast rise
and fall times of the input supply.
Power Dissipation
The power dissipation (P D ) of the MAX16839 is
determined from the following equation:
P D = ( V IN × I IN ) + ? ? ( V IN ? ∑ V LED ? V CS ) × ( I LED × DUTY ) ? ?
where:
V IN = Supply voltage
I IN = Supply current
C V LED = Total forward voltage for the LED string
I LED = LED current
V CS = 203mV drop across R CS
DUTY = PWM duty cycle
The MAX16839 thermal specifications are given accord-
ing to the JEDEC-51 guidelines. Good mechanical/
thermal design practices must be applied to help
maintain the device junction temperature below the
absolute maximum ratings at all times.
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