MAX8831 MAXIM [Maxim Integrated Products], MAX8831 Datasheet - Page 12

• Manufacturer Part Number: MAX8831
• Description: High-Efficiency, White LED Step-Up Converter with I2C Interface in 2mm x 2mm WLP
• Manufacturer: MAXIM [Maxim Integrated Products]

From shutdown, once any LED_ is enabled through the
I
quickly pulled to 1V by an internal pullup clamp. Since
the LED_ feedback node voltage is less than the regu-
lation threshold (0.35V typ), 40µA current is sourced
from the error amplifier (Figure 1) and further charges
C
verter starts switching at a reduced duty cycle. As
V
es. When V
stops sourcing current to C
the control loop achieves regulation as V
The V
load. A 2.5V upper limit to V
transient recovery and to allow maximum output for low
input voltages.
C
resistor whenever the step-up converter is turned off,
allowing the device to reinitiate soft-start when it is
enabled. See the Typical Operating Characteristics for
an example of soft-start operation.
The MAX8831 is considered OFF when V
V
above the V
enters the shutdown state and disables its internal ref-
erence. During shutdown, the MAX8831 holds all regis-
ters in reset, the step-up converter and all LED current
drivers are off, and supply current is reduced to 0.1µA
(typ). LX and LED1–LED5 are high impedance when
the step-up converter is off.
While the n-channel MOSFET is turned off, the step-up
regulator’s output is connected to IN through the exter-
nal inductor and Schottky diode.
With a valid supply voltage applied to V
than 1.6V) and with V
standby condition, whereby it is ready to accept I
commands. The step-up converter turns on when any
current regulator is enabled with an I
High-Efficiency, White LED Step-Up Converter
with I
Table 1. LED1, LED2 Programmable Current Levels and Register Values (continued)
12
2
COMP
C interface, the IC prepares for soft-start. C
UVLO
COMP
COMP
REGISTER VALUE
ILED_CNTRL
______________________________________________________________________________________
COMP
. Once V
rises, the step-up converter duty cycle increas-
threshold and V
is discharged to GND through a 20kΩ internal
0x2A
0x26
0x27
0x28
0x29
LED_
where the IC exits soft-start depends on the
UVLO
2
COMP
C Interface in 2mm x 2mm WLP
reaches 0.35V (typ), the error amplifier
threshold, and with V
Off, Shutdown, and Standby
IN
reaches 1.25V, the step-up con-
I
above V
LED_
DD
5.25
5.45
5.95
5.6
5.8
COMP
COMP
is below 1.6V. With V
(mA)
UVLO
, soft-start ends, and
is imposed to aid in
2
C command.
, the IC enters a
DD
REGISTER VALUE
IN
LED_
Soft-Start
ILED_CNTRL
DD
is below the
low, the IC
(greater
COMP
0x51
0x52
0x53
0x54
0x55
settles.
2
IN
is
C
The MAX8831 includes two fault-detection comparators
on each LED_ input to detect an open or shorted LED
condition. One comparator monitors LED_ voltage and
indicates an open LED_ fault when V
100mV. The other comparator detects when LED_ volt-
age rises above V
fault. The fault detection comparators are enabled only
when the corresponding LED_ current regulator is
enabled. Once a fault is detected, the two comparators
provide a single bit output (1 = fault, 0 = no fault) to the
STAT1 register (bits 0–4), corresponding to the appro-
priate LED regulator.
A debounce time of 16ms (typ) is applied from when a
fault condition is detected. At the end of the 16ms
debounce time, the status is latched in to the status
register and the respective current regulator is dis-
abled. If an open LED condition occurs on a current
regulator that is included in the adaptive output voltage
Figure 3. LED3, LED4, LED5 String Current vs. ILED3, ILED4,
ILED5 Control Register Value
I
LED_
14.45
14.65
14.2
14.9
14
(mA)
ILED3, ILED4, ILED5 CONTROL REGISTER VALUE (INTEGER)
5
4
3
2
1
0
ILED4, ILED5 CONTROL REGISTER VALUE
0
LED3, LED4, LED5 CURRENT vs. ILED3,
4
OUT
Open/Shorted LED Detection
REGISTER VALUE
8
ILED_CNTRL
- 0.7V, indicating a shorted LED
12
0x7C
0x7D
0x7E
0x7F
—
16
20
24
LED_
28
I
LED_
falls below
24.45
25.25
24.7
25
—
(mA)