Rainbow-electronics MAX684 Manual do Utilizador descarregar pdf (Página 8)

MAX682/MAX683/MAX684

In addition, the following two equations approximate

output ripple for each mode. In skip mode, output rip-

ple is dominated by ESR, and is approximately:

RIPPLE(SKIP)

≅ (2V

- V

OUT

)ESR

COUT

/ R

where ESR

COUT

is the ESR of the output filter capaci-

tance, and R

is the open-loop output transfer resist-

ance of the IC. R

is typically 0.8Ω for the MAX682,

1.6Ω for the MAX683, and 3Ω for the MAX684. In con-

stant-frequency mode, output ripple is dominated by

OUT

and is approximately:

RIPPLE(const-freq)

≅ I

OUT

/ (2 x f

OSC

x C

OUT

)

All capacitors must maintain a low (<100mΩ) equiva-

lent series resistance (ESR). Table 4 lists the manufac-

turers of recommended capacitors. Surface-mount

tantalum capacitors will work well for most applications.

Ceramic capacitors will provide the lowest ripple due to

their typically lower ESR.

If the source impedance or inductance of the input sup-

ply is large, additional input bypassing (2.2µF to 22µF)

may be needed. This additional capacitance need not

be a low-ESR type.

3.3V-Input to Regulated 5V-Output

Charge Pumps

8 _______________________________________________________________________________________

PART

(µF)

MAX682 2.2 47

MAX683 1 22

MAX684 0.47 10

OUT

RIPPLE

(mV)

100

(µF)

0.47

0.22

OUTPUT

(mA)

250

100

Table 2. Recommended Capacitor Values

for Quiescent Current (Skip Mode)

Table 3. Recommended Capacitor Values

for Smallest Size (Constant-Frequency

Mode, I

SSHHDDNN

= 22µA, 1MHz)

PART

(µF)

CERAMIC

OUT

(µF)

MAX682 1 2.2

MAX683 0.47 1

MAX684 0.22 0.47

OUT

RIPPLE

(mV)

(µF)

0.47

0.22

0.1

OUTPUT

(mA)

250

100

MANUFACTURER

PHONE

NUMBER

Sprague (603) 224-1961

AVX (803) 946-0690

VALUE

47µF to

10µF

47µF to

10µF

CXP

3.3V

CXN

SHDN

OUTIN

1µF

0.47µF

100k 100k

4.7µF

SKIP

GND PGND

MAX682

SKIP

CXP

CXN

OUT

5V/500mA

SHDN

GND PGND

Figure 6. Paralleling Two MAX682s

TDK (847) 390-4373

0.1µF to

2.2µF

Table 4. Recommended Capacitor

Manufacturers

OUT

CXN

CXP

SHDN

OUT

4 5

OFF

EXT

SKIP

GND PGND

MAX682

MAX683

MAX684

Figure 5. Standard Operating Circuit

TANTALUM

OUT

(µF)

4.7

2.2

CERAMIC

Ceramic

surface mount

DESCRIPTION

595D-series

tantalum

surface mount

TPS-series

surface mount

1 2 3 4 5 6 7 8 9 10 11 12

MAX682/MAX683/MAX684 1
Charge Pumps 1

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