MAX8831 MAXIM [Maxim Integrated Products], MAX8831 Datasheet - Page 10

• 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]

The other three regulators (LED3, LED4, LED5) are suit-
able for keyboard backlight functions or for driving sig-
nal indicators, and are programmable up to 5.0mA
using a 32-step logarithmic dimming scheme. The low-
current regulators (LED3, LED4, LED5) can be operat-
ed from the step-up converter or from a separate low-
voltage source.
The I
current regulators, including: on/off state, LED current,
ramp-up/ramp-down timers, and blink rate timers
(LED3, LED4, LED5). The MAX8831 I
addresses are factory set at 0x9A/0x9B (contact the
factory for other address options).
The IC features several protective features, including:
open/short LED fault detection, output overvoltage pro-
tection, thermal shutdown, and open Schottky diode
detection. The status of each fault is monitored continu-
ally for readback through the I
The MAX8831’s fixed-frequency, current-mode, step-up
controller automatically chooses the lowest active LED_
voltage to complete the feedback loop (Figure 1).
Specifically, the difference between the lowest LED_
voltage and the 350mV reference is integrated by the
error amplifier. The resulting error signal is compared to
the external switch current plus slope compensation to
terminate the switch on-time. As the load changes, the
error amplifier sources or sinks current to COMP to
adjust the required peak inductor current. The slope-
compensation signal is added to the current-sense sig-
nal to improve stability at high duty cycles.
At light loads, the MAX8831 automatically skips pulses
to improve efficiency and to prevent overcharging the
output capacitor. In SKIP mode, the inductor current
ramps up for a minimum on-time of 20ns (typ), then dis-
charges the stored energy to the output. The switch
remains off until another pulse is needed to step-up the
output voltage.
When the MAX8831 is programmed by the I
to use an alternate supply voltage for the LED3, LED4,
or LED5 string (see the Low-Current Regulators (LED3,
LED4, LED5) section), internal logic masks that LED_
input and it is not used to regulate the step-up convert-
er output.
The MAX8831 contains two low-dropout (200mV max),
25.25mA linear current regulators (LED1, LED2) that
can each drive up to 9 series LEDs (depending on LED
string forward voltage) for display backlighting func-
High-Efficiency, White LED Step-Up Converter
with I
10
______________________________________________________________________________________
2
C interface controls all operational aspects of the
High-Current Regulators (LED1, LED2)
Fixed-Frequency Step-Up Controller
2
C Interface in 2mm x 2mm WLP
2
C interface.
2
C write/read
2
C interface
tions. Each high-current regulator is independently
enabled and is programmable from 50µA to 25.25mA in
128 logarithmic steps (Table 1, Figure 2) using the I
interface. Additionally, the I
ramp-up and ramp-down timers for each regulator to
one of eight different timing settings. See the MAX8831
I
high-current regulators.
The MAX8831 also contains three low-dropout (150mV
max), 5.0mA linear current regulators (LED3, LED4,
LED5) that can each drive up to 9 series LEDs for key-
pad backlighting or signal indicator functions. Each
current regulator is independently enabled, and is pro-
grammable from 50µA to 5.0mA in 32 logarithmic steps
(Table 2, Figure 3) using the I
ramp-up and ramp-down timers are programmable for
LED3, LED4, and LED5, with eight possible timing set-
tings. The individual blink ON and blink OFF timers for
each low-current regulator are also programmable, or
these features can be disabled. See the MAX8831 I
Registers section for details.
The LED3, LED4, and LED5 low-current regulators can
be powered from an alternate external source. By pro-
gramming the BOOST_CNTL register, internal logic
masks that LED_ input and it is not used to regulate the
step-up converter output.
Figure 2. LED1, LED2 String Current vs. I
Register Value
2
C Registers section for details on I
25
20
15
10
5
0
ILED1, ILED2 CONTROL REGISTER VALUE (INTEGER)
0
ILED2 CONTROL REGISTER VALUE
LED1, LED2 CURRENT vs. ILED1,
20
40
Low-Current Regulators
60
2
C interface programs the
80
(LED3, LED4, LED5)
2
C interface. Individual
100
LED1
120
2
C control of the
, I
140
LED2
Control
2
2
C
C