Rainbow-electronics MAX1595 Manual de usuario descargar pdf (Pagina 5)

MAX1595

Regulated 3.3V/5.0V Step-Up/Step-Down

Charge Pump

_______________________________________________________________________________________ 5

exceeds the available battery voltage, the use of a

charge pump such as the MAX1595 provides high effi-

ciency, small size, and constant light output with chang-

ing battery voltages. If the output is used only to light

LEDs, the output capacitor can be greatly reduced. The

frequency modulation of the LED intensity is not dis-

cernible to the human eye, and the smaller capacitor

saves both size and cost.

Adding two Schottky diodes and two capacitors imple-

ments a tripler and allows the MAX1595_ _ _50 to regu-

late a current of 75mA with a supply voltage as low as

2.3V (Figure 3).

Capacitor Selection

The MAX1595 requires only three external capacitors

(Figure 4). Their values are closely linked to the output

current capacity, oscillator frequency, output noise con-

tent, and mode of operation.

Generally, the transfer capacitor (C

) will be the smallest,

and the input capacitor (C

) is twice as large as C

Higher switching frequencies allow the use of the smaller

and C

. The output capacitor (C

OUT

) can be any-

where from 5-times to 50-times larger than C

. Table 1

shows recommended capacitor values.

In addition, the following equation approximates output

ripple:

RIPPLE

≅ I

OUT

/ (2 x f

OSC

x C

OUT

)

Table 2 lists the manufacturers of recommended capaci-

tors. Ceramic capacitors will provide the lowest ripple

due to their typically lower ESR.

Power Dissipation

The power dissipated in the MAX1595 depends on out-

put current and is accurately described by:

DISS

= I

OUT

(2V

- V

OUT

)

DISS

must be less than that allowed by the package

rating.

Layout Considerations

All capacitors should be soldered in close proximity to

the IC. Connect ground and power ground through a

short, low-impedance trace. The input supply trace

should be as short as possible. Otherwise, an additional

input supply filter capacitor (tantalum or electrolytic) may

be required.

AOUT

INPUT

2.3V

1µF

SHDN

GNDPGND

OUT

MAX1595_ _ _50

0.22µF

CXP

CXN

1µF

OUTPUT

REGULATED 5V

75mA

1µF 0.22µF

OUT

0.47µF

= 1µF

= 0.1µF

SHDN

GND

CXP CXN

PGND

AOUT

MAX1595_ _ _50

100Ω 100Ω 100Ω

Figure 3. Regulated Voltage Tripler

Figure 2. White LED Bias Supply

0.22µF

1µF

OUT

1µF

OUT

CXN

CXP

SHDN

OUT

OFF

GND

PGND

AOUT

MAX1595

Figure 4. Standard Operating Circuit

1 2 3 4 5 6 7 8

Charge Pump 1

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