Datasheet

14 Maxim Integrated
High-Voltage HB LED Drivers with
Integrated High-Side Current Sense
MAX16833/MAX16833B–MAX16833D
Thermal Protection
The ICs feature thermal protection. When the junction
temperature exceeds +160NC, the ICs turn off the external
power MOSFETs by pulling the NDRV low and DIMOUT
high. External MOSFETs are enabled again after the junc-
tion temperature has cooled by 10°C. This results in a
cycled output during continuous thermal-overload condi-
tions. Thermal protection protects the ICs in the event of
fault conditions.
Short-Circuit Protection
Boost Configuration
In the boost configuration, if the LED string is shorted
it causes the (ISENSE+ to ISENSE-) voltage to exceed
300mV. If this condition occurs for R 1Fs, the ICs activates
the hiccup timer for 8192 clock cycles during which:
U
NDRV goes low and DIMOUT goes high.
U
The error amplifier is disconnected from COMP.
U FLT is pulled to SGND.
After the hiccup time has elapsed, the ICs retry. During
this retry period, FLT is latched and is reset only if there is
no short detected after 20Fs of retrying.
Buck-Boost Configuration
In the case of the buck-boost configuration, once an
LED string short occurs the behavior is different. The
ICs maintain the programmed current across the short.
In this case, the short is detected when the voltage
between ISENSE+ and IN falls below 1.5V. A buck-boost
short fault starts an up counter and FLT is asserted only
after the counter has reached 8192 clock cycles con-
secutively. If for any reason (V
ISENSE+
- V
IN
> 1.5V),
the counter starts down counting, resulting in FLT being
deasserted only after 8192 consecutive clock cycles of
(V
ISENSE+
- V
IN
> 1.5V) condition.
Exposed Pad
The ICs’ package features an exposed thermal pad on
its underside that should be used as a heatsink. This
pad lowers the package’s thermal resistance by provid-
ing a direct heat-conduction path from the die to the
PCB. Connect the exposed pad and GND to the system
ground using a large pad or ground plane, or multiple
vias to the ground plane layer.
Applications Information
Setting the Overvoltage Threshold
The overvoltage threshold is set by resistors R5 and R11
(see the Typical Operating Circuits). The overvoltage
circuit in the ICs is activated when the voltage on OVP
with respect to GND exceeds 1.23V. Use the following
equation to set the desired overvoltage threshold:
V
OV
= 1.23V (R5 + R11)/R11
Programming the LED Current
Normal sensing of the LED current should be done on
the high side where the LED current-sense resistor is
connected to the boost output. The other side of the LED
current-sense resistor goes to the source of the p-channel
dimming MOSFET if PWM dimming is desired. The LED
current is programmed using R7. When V
ICTRL
> 1.23V,
the internal reference regulates the voltage across R7 to
200mV:
LED
200mV
I
R7
=
The LED current can also be programmed using the
voltage on ICTRL when V
ICTRL
< 1.2V (analog dimming).
The voltage on ICTRL can be set using a resistive divider
from the REF output in the case of the MAX16833B/
MAX16833D. The current is given by:
ICTRL
LED
V
I
R7 6.15
=
×
where:
( )
REF
ICTRL
V R8
V
R8 R9
×
=
+
where V
REF
is 1.64V and resistors R8 and R9 are in
ohms. At higher LED currents there can be noticeable
ripple on the voltage across R7. High-ripple voltages can
cause a noticeable difference between the programmed
value of the LED current and the measured value of the
LED current. To minimize this error, the ripple voltage
across R7 should be less than 40mV.