Rainbow-electronics MAX669 Manual do Utilizador descarregar pdf (Página 10)

MAX668/MAX669

1.8V to 28V Input, PWM Step-Up

Controllers in µMAX

10 ______________________________________________________________________________________

MAX669

LDO

CS+

REF

FREQ

SYNC/

SHDN

PGND

GND

EXT

= 1.8V to 12V

0.22µF

0.1µF

1µF

100k

0.02Ω

218k

24.9k

220pF

MBRS340T3

68µF

20V

68µF

20V

0.1µF

OUT

= 12V @ 0.5A

68µF

20V

4.7µH

IRF7401

Figure 2. MAX669 High-Voltage Bootstrapped Configuration

MAX669

LDO

CS+

REF

FREQ

SYNC/

SHDN

PGND

GND

EXT

= 1.8V to 5V

0.22µF

1µF

100k

0.02Ω

75k

24.9k

220pF

MBRS340T3

68µF

10V

68µF

10V

0.1µF

OUT

= 5V @ 1A

68µF

10V

4.7µH

FDS6680

IRF7401

Figure 3. MAX669 Low-Voltage Bootstrapped Configuration

Bootstrapped Operation

With bootstrapped operation, the IC is powered from

the circuit output (V

OUT

). This improves efficiency

when the input voltage is low, since EXT drives the FET

with a higher gate voltage than would be available from

the low-voltage input. Higher gate voltage reduces the

FET on-resistance, increasing efficiency. Other (unde-

sirable) characteristics of bootstrapped operation are

increased IC operating power (since it has a higher

operating voltage) and reduced ability to start up with

high load current at low input voltages. If the input volt-

age range extends below 2.7V, then bootstrapped

operation with the MAX669 is the only option.

With V

connected to V

OUT

, as in Figure 2, EXT volt-

age swing is 5V when V

is 5.2V or more, and V

0.2V when V

is less than 5.2V. If the output voltage

does not exceed 5.5V, the on-chip regulator can be

disabled by connecting V

to LDO (Figure 3). This

eliminates the LDO forward drop and supplies maxi-

mum gate drive to the external FET.

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 16 17 18

MAX668/MAX669 1
Controllers in µMAX 1

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