MAX16814_13 Datasheet, PDF (12/25 Pages) Maxim Integrated Products – Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller

MAX16814
Integrated, 4-Channel, High-Brightness LED
Driver with High-Voltage DC-DC Controller
Detailed Description
The MAX16814 high-efficiency HB LED driver inte-
grates all the necessary features to implement a high-
performance backlight driver to power LEDs in small
to medium-sized displays for automotive as well as
general applications. The device provides load-dump
voltage protection up to 40V in automotive applications.
The MAX16814 incorporates two major blocks: a DC-DC
controller with peak current-mode control to implement
a boost, coupled-inductor boost-buck, or a SEPIC-type
switched-mode power supply and a 4-channel LED driv-
er with 20mA to 150mA constant current-sink capability
per channel. Figure 1 is the simplified functional diagram
and Figure 2 shows the circuit used for typical operating
characteristics.
The MAX16814 features a constant-frequency peak
current-mode control with programmable slope com-
pensation to control the duty cycle of the PWM control-
ler. The high-current FET driver can provide up to 2A of
current to the external n-channel MOSFET. The DC-DC
converter implemented using the controller generates
the required supply voltage for the LED strings from
a wide input supply range. Connect LED strings from
the DC-DC converter output to the 4-channel constant
current-sink drivers that control the current through the
LED strings. A single resistor connected from the SETI
input to ground adjusts the forward current through all
four LED strings.
The MAX16814 features adaptive voltage control that
adjusts the converter output voltage depending on the
forward voltage of the LED strings. This feature mini-
mizes the voltage drop across the constant current-sink
drivers and reduces power dissipation in the device.
A logic input (EN) shuts down the device when pulled
low. The device includes an internal 5V LDO capable of
powering additional external circuitry.
All the versions of the MAX16814 include PWM dimming.
The MAX16814A_ and the MAX16814U_ versions, in par-
ticular, provide very wide (5000:1) PWM dimming range
where a dimming pulse as narrow as 1µs is possible at
a 200Hz dimming frequency. This is made possible by
a unique feature that detects short PWM dimming input
pulses and adjusts the converter feedback accordingly.
Advanced features include detection and string-dis-
connect for open-LED strings, partial or fully shorted
strings and unused strings. Overvoltage protection
clamps the converter output voltage to the programmed
OVP threshold in the event of an open-LED condition.
Shorted LED string detection and overvoltage protec-
tion thresholds are programmable using RSDT and OVP
inputs, respectively. An open-drain FLT signal asserts to
indicate open-LED, shorted LED, and overtemperature
conditions. Disable individual current-sink channels by
connecting the corresponding OUT_ to LEDGND. In this
case, FLT does not assert indicating an open-LED con-
dition for the disabled channel. The device also features
an overtemperature protection that shuts down the con-
troller if the die temperature exceeds +165NC.
Current-Mode DC-DC Controller
The peak current-mode controller allows boost, coupled-
inductor buck-boost, or SEPIC-type converters to gener-
ate the required bias voltage for the LED strings. The
switching frequency can be programmed over the 200kHz
to 2MHz range using a resistor connected from RT to
SGND. Programmable slope compensation is available
to compensate for subharmonic oscillations that occur at
above 50% duty cycles in continuous conduction mode.
The external MOSFET is turned on at the beginning of
every switching cycle. The inductor current ramps up
linearly until it is turned off at the peak current level set by
the feedback loop. The peak inductor current is sensed
from the voltage across the current-sense resistor RCS
connected from the source of the external MOSFET to
PGND. The MAX16814 features leading-edge blanking to
suppress the external MOSFET switching noise. A PWM
comparator compares the current-sense voltage plus the
slope compensation signal with the output of the transcon-
ductance error amplifier. The controller turns off the exter-
nal MOSFET when the voltage at CS exceeds the error
amplifier’s output voltage. This process repeats every
switching cycle to achieve peak current-mode control.
Error Amplifier
The internal error amplifier compares an internal feed-
back (FB) with an internal reference (REF) and regulates
its output to adjust the inductor current. An internal
minimum string detector measures the minimum current-
sink voltage with respect to SGND out of the 4 constant-
current-sink channels. During normal operation, this
minimum OUT_ voltage is regulated to 1V through
feedback. The error amplifier takes 1V as the REF and
the minimum OUT_ voltage as the FB input. The ampli-
fied error at the COMP output controls the inductor peak
current to regulate the minimum OUT_ voltage at 1V. The
resulting DC-DC converter output voltage is the highest
LED string voltage plus 1V.
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