MAX863 Maxim, MAX863 Datasheet - Page 15

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

When the voltage at V
approaches the MOSFET gate-to-source threshold volt-
age, the MOSFET may operate in its linear region and
dissipate excessive power. Prolonged operation in this
mode may damage the MOSFET if power dissipation
ratings are inadequate. This effect is more significant in
non-bootstrapped mode, but can occur in boot-
strapped mode if the input voltage drops so low that it
cannot support the load and causes the output voltage
to collapse. To avoid this condition, use logic-level or
low-threshold MOSFETs.
Figure 9. 3-Cell to 3.3V Step-Up/Step-Down Logic Supply with 12V for Flash Memory or Analog Functions
__________Applications Information
V
V
FLYBACK OR SEPIC
OUT1
IN
330 F
= 2.0V TO 11V OR V
0.1
10V
OUTPUT
= 3.3V, 600mA
C1
______________________________________________________________________________________
Low-Input-Voltage Operation
LOW-BATTERY
DETECTOR OUTPUT
DD
100k
R7
OUT2
MBRS340T3
10 H, 2.5A
falls and EXT1 or EXT2
Dual, High-Efficiency, PFM, Step-Up
CTX10-4
D1
T1
50m
10 F
C9
N1A
R1
R5
R6
100 F
0.1
10V
C3
SENSE1 FB1
CS1
LBO
EXT1
LBI
CMPSH-3C
PGND
MAX863
D3
The Typical Operating Characteristics show the
Bootstrapped-Mode Minimum Start-Up Input Voltage
vs. Output Current graph. The MAX863 is not intended
to start up under full load in bootstrapped mode with
low input voltages.
The circuit in Figure 8 operates from two AA or AAA
cells, and generates 5V (up to 750mA) for logic and
24V (up to 35mA) for an LCD bias supply. OUT1 is
used to bootstrap the MAX863 for better MOSFET gate
drive. V
MOSFETs are used.
________________Application Circuits
GND
V
DD
SHDN1
SHDN2
BOOT
EXT2
CS2
REF
FB2
100 F
0.1
10V
C4
1 F
C2
OUT1
50m
ON/OFF
R2
DC-DC Controller
C7
0.1 F
can be set to 3.3V if low threshold
N1B
IRF7301
L2
10 H
2A
MBRS340T3
Bootstrapped 5V Logic and
D2
115k
1%
1M
1%
R3
Starting Up Under Load
R4
24V LCD Bias Supply
V
C6
10pF
C8
82pF
OUT2
= 12V
C5
100 F
20V
0.1
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