MAX16814 Datasheet, PDF (15/25 Pages) Maxim Integrated Products – Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller

Integrated, 4-Channel, High-Brightness LED
Driver with High-Voltage DC-DC Controller
thermal shutdown. FLT is cleared when the fault condi-
tion 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 disconnects
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 chan-
nels. 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 mini-
mum 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 con-
verter output, OVP input, SGND. The overvoltage protec-
tion 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 con-
verter output reaches the overvoltage protection thresh-
old, the PWM controller is switched off setting 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 connec-
tions to LEDGND before power-up to avoid OVP trigger-
ing 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 pro-
grammable LED short detection threshold set at the
RSDT input. Adjust VRSDT using a voltage-divider resis-
tive network connected at the VCC 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 out-
put 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 for-
ward 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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