MAX863 Maxim, MAX863 Datasheet - Page 9

• Manufacturer Part Number: MAX863
• Description: Dual / High-Efficiency / PFM / Step-Up DC-DC Controller
• Manufacturer: Maxim

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
parator and one-shot timing circuitry turn on. The low-
voltage oscillator is enabled by connecting the BOOT
pin to V
ed to V
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
between 1.5V and 2.7V, such as during power-up and
low-battery conditions.
Figures 2 and 4 show standard applications in boot-
strapped and non-bootstrapped modes. In boot-
Figure 4b. Adjustable Non-Bootstrapped Typical Operating Circuit
DD
Bootstrapped/Non-Bootstrapped Modes
OUT1
DD
rises above 2.7V. At this point, the error com-
. When BOOT is high, V
.
100 F
0.1
Low-Voltage Start-Up Oscillator
V
16V
10pF
82pF
C1
OUT1
C8
C9
V
IN
_______________________________________________________________________________________
= 12V
= 2.7V TO 11V
R8
1M
1%
R9
115k
1%
Dual, High-Efficiency, PFM, Step-Up
LOW-BATTERY
DETECTOR OUTPUT
MBRS340T3
DD
R7
100k
D1
10 H
must be connect-
N1.A
2A
L1
(BOOT Pin)
R1
50m
R5
DD
BOOT
CS1
FB1
SENSE1
EXT1
LBO
C3
100 F
20V
0.1
is
V
DD
MAX863
PGND
LBI
strapped mode, the IC is powered from the output (V
is connected to OUT1, BOOT is connected to V
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
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
exceed 11V. If BOOT is high, V
ed to OUT1.
In non-bootstrapped mode, the IC is powered by a
direct connection from the input voltage to V
the voltage swing applied to the gate of the external
MOSFET is derived from V
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.
C2
0.1 F
SHDN1
SHDN2
EXT2
GND
CS2
REF
FB2
R6
100m
C4
100 F
20V
0.1
R2
ON/OFF
N1.B
IRF7301
DC-DC Controller
L2
10 H
1A
C7
0.1 F
MBRS140
D2
1%
1M
56k
1%
R3
R4
DD
, the external MOSFET on-
V
C6
15pF
C10
270pF
OUT2
DD
= 24V
C5
22 F
35V
0.1
must be connect-
DD
DD
must not
. Since
DD
DD
DD
to
).
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