High-Voltage, 350mA, Adjustable Linear
High-Brightness LED Driver
MAX16835
6
Maxim Integrated
power series pass device (see the
Functional Diagram
).
The regulation point is factory set at (V
CS+
- V
CS-
) =
200mV ±3.5%. The regulated current is user defined by
the value of R
SENSE
.
The MAX16835 is a current controller internally opti-
mized for driving the impedance range expected from
one or more HB LEDs.
+5V Regulator
The MAX16835 includes a fixed +5V output regulator
that delivers up to 4mA of load current for low-power
applications throughout the +6.5V to +40V input volt-
age range. Connect a 0.1µF compensation capacitor
from V5 to ground. Shorting V5 to ground disables the
thermal shutdown. When EN is low, V5 is off.
Thermal Protection
The MAX16835 enters a thermal-shutdown mode in the
event of overheating. This typically occurs in overload or
output short-circuit conditions. When the junction tem-
perature exceeds T
J
= +159°C (typ), the internal ther-
mal-protection circuitry turns off the series pass device.
The MAX16835 recovers from thermal-shutdown mode
once the junction temperature drops by 24°C (typ). The
part therefore protects itself by thermally cycling in the
event of a short-circuit or overload condition. Shorting V5
to ground disables the thermal shutdown.
Applications Information
Programming the LED Current
The MAX16835 uses a sense resistor across CS+ and
CS- to set the LED current. The differential sense amplifier
connected across R
SENSE
provides ground-loop immu-
nity and low-frequency noise rejection. The LED current
is given by the equation below:
I
LED
= V
SENSE
/R
SENSE
Input Voltage Considerations
For proper operation, the minimum input voltage must
always be:
V
IN(MIN)
≥ V
SENSE(MAX)
+ V
FT(MAX
+ ∆V
DO(MAX)
where V
FT(MAX)
is the total forward voltage of all series-
connected LEDs. The minimum operating voltage of the
device is +6.5V. The device operates below +6.5V;
However, the output current may not meet the full regu-
lation specification.
Low-Frequency PWM
at the Output
The MAX16835 provides pulsed or chopped current
dimming. Generally, HB LEDs are binned to match at
their full-rated current; however, LEDs from the same
bin exhibit poor matching at currents other than full-
rated current. To achieve uniformity, HB LED manufac-
turers recommend PWM pulsing of the LED current at
their full-rated value. There are two methods for pro-
ducing a PWM output. One method is by pulsing the
enable input (EN) while having a constant voltage at IN.
The other method is to connect EN to IN and pulse both
EN and IN. Both methods generate a regulated-ampli-
tude PWM current (variable duty cycle) that can pro-
vide control over the LED brightness (see Figures 1
and 2).
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