Rainbow-electronics MAX863 Manual de usuario descargar pdf (Pagina 15)

MAX863

Dual, High-Efficiency, PFM, Step-Up

DC-DC Controller

______________________________________________________________________________________ 15

MAX863

EXT2

CS2

OUT2

= 12V

FLYBACK OR SEPIC

OUTPUT

OUT1

= 3.3V, 600mA

= 2.0V TO 11V OR V

OUT2

N1B

IRF7301

0.1µF

115k

10pF

1µF

100µF

20V

≤0.1Ω

50mΩ

N1A

50mΩ

100k

330µF

10V

≤0.1Ω

MBRS340T3

10µH, 2.5A

CTX10-4

MBRS340T3

10µH

100µF

10V

≤0.1Ω

100µF

10V

≤0.1Ω

10µF

ON/OFF

FB2

SHDN1

EXT1

CMPSH-3C

CS1

LBI

LOW-BATTERY

DETECTOR OUTPUT

LBO

SENSE1

FB1

PGND

BOOT

GND

SHDN2

REF

82pF

Figure 9. 3-Cell to 3.3V Step-Up/Step-Down Logic Supply with 12V for Flash Memory or Analog Functions

__________Applications Information

Low-Input-Voltage Operation

When the voltage at V

falls and EXT1 or EXT2

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.

Starting Up Under Load

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.

________________Application Circuits

Bootstrapped 5V Logic and

24V LCD Bias Supply

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

OUT1

can be set to 3.3V if low threshold

MOSFETs are used.

1 2 ... 10 11 12 13 14 15 16

DC-DC Controller 1

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Rainbow-electronics MAX863 Manual de usuario Pagina 15

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