The typical value for the input capacitor is 1µF, and the
typical value for the output capacitor is 1µF. Higher
value capacitors can reduce input and output ripple,
but at the expense of size and higher cost.
Diode Selection
The high switching frequency of the MAX8831
demands a high-speed rectification diode for optimum
efficiency. A Schottky diode is recommended due to its
fast recovery time and low forward voltage drop.
Ensure that the diode’s average and peak current rat-
ing exceeds the average output current and peak
inductor current. In addition, the diode’s reverse break-
down voltage must exceed V
OUT
.
Compensation Network Selection
The step-up converter is compensated for stability
through an external compensation network from COMP
to GND. The compensation capacitor is typically
0.22µF for most applications. Note that higher C
COMP
values increase soft-start duration, as well as the time
delay between enabling the step-up converter to initiat-
ing soft-start.
Combining BLINK Timer
and RAMP Functions
When using the ramp functionality of LED3, LED4, and
LED5 in combination with the blink timer, it is recom-
mended to keep the ramp-up timer shorter than the
blink ON timer and the ramp-down timer shorter than
the blink OFF timer. See Figure 14. Failing to comply
with this restriction results in LED_ current not reaching
maximum value during blink ON time, and LED_ current
not returning to minimum current before turning off dur-
ing the blink OFF time. The blink ON and blink OFF
timers (t
ON_BLINK
and t
OFF_BLINK
) are programmed
according to the following equations as guidance:
Where t
LED_RU
is the LED_ ramp-up time, t
LED_RD
is
the LED_ ramp-down time, and LED_CODE is the deci-
mal equivalent of the ILED_CNTL register value of
Table 2.
Using the LED3, LED4, and LED5
BOOST_CNTRL Bit
The default setting of the BOOST_CNTL bits (low)
include the LED3, LED4, and LED5 current regulators
in the step-up converter minimum voltage select feed-
back circuit. This is intended for multi-LED strings pow-
ered from the step-up converter. For single LED
indicator lights, set the respective BOOST_CNTL bit
high, connect the LED anode to the battery or other
voltage source, and connect the LED cathode to the
respective LED_ input. Ensure the voltage source is
high enough to satisfy VF of the LED plus 150mV (cur-
rent regulator dropout voltage). If BOOST_CNTL bits
are set to high for LED3, LED4, and LED5 and LED1
and LED2 are not enabled, the step-up converter does
not turn on when LED3, LED4, or LED5 is enabled.
