MAX16838B Datasheet, PDF (14/21 Pages) Maxim Integrated Products – Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller

MAX16838B
Integrated, 2-Channel, High-Brightness LED Driver
with High-Voltage Boost and SEPIC Controller
Connect a 4.7Ω resistor from V_CC to DRV to power the
rest of the chip from the V_CC pin with the 5V internal
regulator. Bypass DRV to PGND with a minimum of 1µF
ceramic capacitor as close as possible to the device. For
input voltage range of 4.5V to 5.5V, connect IN to V_CC.
LED Current Control (ISET)
The device features two identical constant-current
sources used to drive multiple HB LED strings. The
current through each of the channels is adjustable
between 20mA and 150mA using an external resistor
(R_ISET) connected between ISET and SGND. Select
R_ISET using the following formula:
R= _ISET
1512 _(Ω)
I _{OUT _}
where I_{OUT_} is the desired output current for both
channels in amps.
For single-channel operation, connect channel 1 and
channel 2 together. See Figure 2.
LED Dimming Control
The device features LED brightness control using an
external PWM signal applied at DIM. The device accepts
a minimum pulse width of 500ns. Therefore, a 10,000:1
dimming ratio is achieved when using a PWM frequency
of 200Hz. Drive DIM high to enable both LED current
sinks and drive DIM low to disable both LED current sinks.
BOOST
CONVERTER
OUTPUT
40mA TO 300mA
MAX16838B
OUT1
OUT2
Figure 2. Configuration for Higher LED String Current
The duty cycle of the PWM signal applied to DIM also
controls the DC-DC converter’s output voltage. If the
turn-on duration of the PWM signal is less than 20
oscillator clock cycles (DIM pulse width decreasing), then
the boost converter regulates its output based on feed-
back from the OV input. During this mode, the converter
output voltage is regulated to 95% of the OV threshold
voltage. If the turn-on duration of the PWM signal is great-
er than or equal to 24 oscillator clock cycles (DIM pulse
width increasing), then the converter regulates its output
such that the minimum voltage at OUT_ is 1V.
When the DIM signal crosses the 20 r 24 oscillator clock
cycle boundary, the control loop of the MAX16838B expe-
riences a discontinuity due to an internal mode transition,
which can cause flickering (the boost output voltage
changes as described in the previous paragraph). To
avoid flicker, the following is recommended:
●● Avoid cross the 20 or 24 oscillator clock cycle boundary.
●● Do not set the OVP level higher than 3V above the
maximum LED operating voltage.
●● Optimize the compensation components so that
recovery is as fast as possible. If the loops phase
margin is less 45°, the output voltage can ring during
the 20 or 24 oscillator clock cycle boundary, which
can contribute to flicker.
Fault Protections
The device’s fault protections include cycle-by-cycle
current limiting, DC-DC converter output overvoltage
protection, open-LED detection, short-LED detection, and
overtemperature detection. An open-drain LED fault flag
output (FLT) goes low when an open-LED/short-LED or
overtemperature condition is detected.
Open-LED Management and Overvoltage Protection
The device monitors the drains of the current sinks (OUT_)
to detect any open string. If the voltage at any output
falls below 300mV and the OV threshold is triggered (i.e.,
even with OUT_ at the OV voltage the string is not able to
regulate above 300mV), then the device interprets that
string to be open, asserts FLT, and disconnects that string
from the operation loop. The device features an adjustable
overvoltage-threshold input (OV). Connect a resistor-divid-
er from the switching converter output to OV and SGND
to set the overvoltage-threshold level. Use the following
formula to program the overvoltage threshold:
V_O=_V
1.23V
×

¹

+
R2 _OV
R1_OV



Open-LED detection is disabled when PWM pulse width
is less than 20 switching clock cycles (DIM pulse width
decreasing).
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