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High-Efficiency, White LED Step-Up Converter
with I 2 C Interface in 2mm x 2mm WLP
Table 2. LED3, LED4, and LED5
Programmable Current Levels and
Register Values
Open Schottky Diode Detection
The MAX8831 detects an open external Schottky diode
by sensing V OUT before turning on the step-up convert-
er. If V OUT is above 0.8V (typ), the MAX8831 allows the
step-up converter to turn on. If V OUT is less than 0.8V
ILED_CNTRL
ILED_CNTRL
(typ), indicating that the external Schottky diode is
REGISTER
VALUE
0x00
0x01
0x02
0x03
0x04
0x05
0x06
0x07
0x08
0x09
0x0A
0x0B
0x0C
0x0D
0x0E
0x0F
I LED_ (mA)
0.05
0.1
0.15
0.20
0.25
0.30
0.35
0.45
0.55
0.65
0.80
0.95
1.10
1.25
1.40
1.60
REGISTER
VALUE
0x10
0x11
0x12
0x13
0x14
0x15
0x16
0x17
0x18
0x19
0x1A
0x1B
0x1C
0x1D
0x1E
0x1F
I LED_ (mA)
1.75
1.90
2.10
2.25
2.40
2.60
2.80
3.00
3.25
3.50
3.70
3.90
4.15
4.40
4.70
5.00
open, the MAX8831 is put into standby state, the
ON/OFF control register bits for the LEDs are set low,
and the step-up converter stops switching. Bit 2
(OSDD) of the STAT2 register is updated to a 1 to indi-
cate that an open Schottky diode event has occurred.
Thermal-Shutdown Protection
When the junction temperature exceeds +160°C (typ),
the ON/OFF control register bits for all LEDs are reset
to low and the MAX8831 enters standby mode and the
step-up converter stops switching. Bit 1 (TSD) of the
STAT2 register is updated to a 1 to indicate that ther-
mal shutdown has occurred.
System States and Fault Handling
The MAX8831 implements two fault registers (STAT1,
STAT2) to provide users with fault indication through
the I 2 C interface.
The STAT1 register indicates a fault condition for each
LED_ string, whether a shorted or open LED_ fault has
occurred. In the event of an LED_ fault, the correspond-
ing bit in the STAT1 register is latched and the ON/OFF
control bit for that current regulator is cleared. An I 2 C
regulation, the output voltage starts to rise. Depending
on the converter output voltage and load condition, the
output voltage can reach the OVP threshold before the
16ms debounce timer expires. In this case, the convert-
er disables all current regulators, forces the IC into
standby mode, and the status register indicates only an
OVP fault.
If the BOOST_CNTL register is programmed to power
LED3, LED4, or LED5 from an alternate source, only
open LED detection is enabled for LED3, LED4, or
LED5.
Output Overvoltage Protection
The MAX8831 protects the LEDs from excessive volt-
age by initiating overvoltage protection (OVP) when
V OUT rises above 28V (min). When OVP occurs, the
MAX8831 turns off the LED current regulators by reset-
ting all ON/OFF control register bits to 0, causing the IC
to enter standby status and turn off the step-up con-
verter. Bit 0 (OVP) of the STAT2 register is updated to a
1 to indicate that an OVP event has occurred. The step-
up converter automatically restarts after an OVP event
when V OUT decreases below 25V (typ).
read of the STAT1 register causes all STAT1 bits to be
cleared and the corresponding string to be reenabled.
If the fault is persistent, then the corresponding bit in
the STAT1 register is set again. All open/short fault
monitors are subject to a 16ms blanking period to
ensure that the MAX8831 does not respond to a false
fault occurrence.
The second status register, STAT2, reports the follow-
ing global system faults: output overvoltage-protection
detection (OVP), thermal-shutdown detection (TSD),
and open Schottky diode detection (OSDD). If a TSD,
OVP, or OSDD fault occurs, the IC enters standby
mode, the step-up converter stops switching, and all
the current regulators are shut down by clearing their
ON/OFF control bits. Once standby occurs, the
MAX8831 does not transition back to the ON state until
the STAT2 register is read, clearing the fault indication,
and an I 2 C command enabling one or more current
regulators is received.
See Figure 4 for a state diagram of the MAX8831.
______________________________________________________________________________________
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