MAX16814_15 Datasheet, PDF (15/25 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
overtemperature shutdown. An open-drain LED fault
flag output (FLT) goes low when an open-LED string
is detected, a shorted LED string is detected, and
during thermal shutdown. FLT is cleared when the fault
condition is removed during thermal shutdown and
shorted LEDs. FLT is latched low for an open-LED
condition and can be reset by cycling power or toggling
the EN pin. The thermal shutdown threshold is +165NC
and has 15NC hysteresis.
Open-LED Management and Overvoltage
Protection
On power-up, the MAX16814 detects and discon-
nects any unused current-sink channels before entering
soft-start. Disable the unused current-sink channels
by connecting the corresponding OUT_ to LEDGND.
This avoids asserting the FLT output for the unused
channels. After soft-start, the MAX16814 detects open
LED and disconnects any strings with an open LED from
the internal minimum OUT_ voltage detector. This keeps
the DC-DC converter output voltage within safe limits
and maintains high efficiency. During normal operation,
the DC-DC converter output regulation loop uses the
minimum OUT_ voltage as the feedback input. If any
LED string is open, the voltage at the opened OUT_
goes to VLEDGND. The DC-DC converter output voltage
then increases to the overvoltage protection threshold
set by the voltage-divider network connected between
the converter output, OVP input, SGND. The overvolt-
age protection threshold at the DC-DC converter output
(VOVP) is determined using the following formula:
V_OVP=
1.23
×
₁₊
R1
R2 ^
(see
the
Typical
Operating
Circuit)
where 1.23V (typ) is the OVP threshold. Select R1 and
R2 such that the voltage at OUT_ does not exceed
the absolute maximum rating. As soon as the DC-DC
converter output reaches the overvoltage protection
threshold, the PWM controller is switched off set-
ting NDRV low. Any current-sink output with VOUT_ <
300mV (typ) is disconnected from the minimum voltage
detector.
Connect the OUT_ of all channels without LED
connections to LEDGND before power-up to avoid OVP
triggering at startup. When an open-LED overvoltage
condition occurs, FLT is latched low.
Short-LED Detection
The MAX16814 checks for shorted LEDs at each rising
edge of DIM. An LED short is detected at OUT_ if the
following condition is met:
VOUT_ > VMINSTR + 3 x VRSDT
where VOUT_ is the voltage at OUT_, VMINSTR is
the minimum current-sink voltage, and VRSDT is the
programmable LED short detection threshold set at
the RSDT input. Adjust VRSDT using a voltage-divider
resistive network connected at the V_CC output, RSDT
input, and SGND.
Once a short is detected on any of the strings, the LED
strings with the short are disconnected and the FLT
output flag asserts until the device detects that the
shorts are removed on any of the following rising edges
of DIM. Connect RSDT directly to VCC to always disable
LED short detection.
Applications Information
DC-DC Converter
Three different converter topologies are possible with
the DC-DC controller in the MAX16814, which has the
ground-referenced outputs necessary to use the con-
stant current-sink drivers. If the LED string forward volt-
age is always more than the input supply voltage range,
use the boost converter topology. If the LED string
forward voltage falls within the supply voltage range, use
the boost-buck converter topology. Boost-buck topology
is implemented using either a conventional SEPIC con-
figuration or a coupled-inductor boost-buck configura-
tion. The latter is basically a flyback converter with 1:1
turns ratio. 1:1 coupled inductors are available with tight
coupling suitable for this application. Figure 6 shows
the coupled-inductor boost-buck configuration. It is also
possible to implement a single inductor boost-buck con-
verter using the MAX15054 high-side FET driver.
The boost converter topology provides the highest
efficiency among the above mentioned topologies. The
coupled-inductor boost-buck topology has the advan-
tage of not using a coupling capacitor over the SEPIC
configuration. Also, the feedback loop compensation for
SEPIC becomes complex if the coupling capacitor is not
large enough. A coupled-inductor boost-buck is not suit-
able for cases where the coupled-inductor windings are
not tightly coupled. Considerable leakage inductance
requires additional snubber components and degrades
the efficiency.
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