MAX863
Dual, High-Efficiency, PFM, Step-Up
DC-DC Controller
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Low-Voltage Start-Up Oscillator
(BOOT Pin)
The MAX863 features a low-voltage start-up oscillator
that guarantees start-up in bootstrapped configuration
down to 1.5V. At these low supply voltages, the error
comparator and internal biasing of the IC are locked
out. The low-voltage oscillator switches the external
MOSFET with around 30% duty cycle until the voltage
at V
DD
rises above 2.7V. At this point, the error com-
parator and one-shot timing circuitry turn on. The low-
voltage oscillator is enabled by connecting the BOOT
pin to V
DD
. When BOOT is high, V
DD
must be connect-
ed to V
OUT1
.
Use the start-up oscillator in the bootstrapped configu-
ration only, since the MAX863 operates in an open-loop
state while the start-up oscillator is active. When using
a non-bootstrapped circuit configuration, connect
BOOT to GND to disable the start-up oscillator. This
prevents the output from rising too high when V
DD
is
between 1.5V and 2.7V, such as during power-up and
low-battery conditions.
Bootstrapped/Non-Bootstrapped Modes
Figures 2 and 4 show standard applications in boot-
strapped and non-bootstrapped modes. In boot-
strapped mode, the IC is powered from the output (V
DD
is connected to OUT1, BOOT is connected to V
DD
).
Bootstrapped-mode operation is useful for increasing
the gate drive to the MOSFETs in low-input-voltage
applications, since EXT1 and EXT2 swing from V
DD
to
GND. Increasing the gate-drive voltage reduces MOS-
FET on-resistance, which improves efficiency and
increases the load range. For supply voltages below
5V, bootstrapped mode is recommended. In boot-
strapped mode, the output connected to V
DD
must not
exceed 11V. If BOOT is high, V
DD
must be connect-
ed to OUT1.
In non-bootstrapped mode, the IC is powered by a
direct connection from the input voltage to V
DD
. Since
the voltage swing applied to the gate of the external
MOSFET is derived from V
DD
, the external MOSFET on-
resistance increases at low input voltages. The mini-
mum input voltage is 2.7V. For operation down to 4V,
use logic-level MOSFETs. For lower input voltages, use
low-threshold logic-level MOSFETs. When both output
voltages are set above 11V, non-bootstrapped mode is
mandatory.
