Rainbow Electronics MAX684 - Manual (Page 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:
V RIPPLE(SKIP)
≅
(2V IN - V OUT )ESR COUT / R TX
where ESR COUT is the ESR of the output filter capaci-
tance, and R TX is the open-loop output transfer resist-
ance of the IC. R TX is typically 0.8
Ω
for the MAX682,
1.6
Ω
for the MAX683, and 3
Ω
for the MAX684. In con-
stant-frequency mode, output ripple is dominated by
C
OUT and is approximately:
V RIPPLE(const-freq)
≅
I OUT / (2 x f OSC x C OUT )
All capacitors must maintain a low (<100m
Ω
) 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
C IN
(µF)
MAX682 2.2 47
MAX683 1 22
MAX684 0.47 10
V
OUT
RIPPLE
(mV)
100
100
100
C X
(µF)
1
0.47
0.22
OUTPUT
(mA)
250
100
50
Table 2. Recommended Capacitor Values
for Quiescent Current (Skip Mode)
Table 3. Recommended Capacitor Values
for Smallest Size (Constant-Frequency
Mode, I
S SH HD DN N
= 22µA, 1MHz)
PART
C
IN
(µF)
CERAMIC
C OUT
(µF)
MAX682 1 2.2
MAX683 0.47 1
MAX684 0.22 0.47
V OUT
RIPPLE
(mV)
80
80
80
C
X
(µF)
0.47
0.22
0.1
OUTPUT
(mA)
250
100
50
MANUFACTURER
PHONE
NUMBER
Sprague (603) 224-1961
AVX (803) 946-0690
VALUE
47µF to
10µF
47µF to
10µF
CXP
3.3V IN
CXN
SHDN
OUT IN
1 µ F
1 µ F
0.47 µ F 0.47 µ F
100k 100k
4.7 µ F
SKIP
GND PGND
MAX682 MAX682
SKIP
CXP
CXN
OUT
5V/500mA
IN
SHDN
GND PGND
Figure 6. Paralleling Two MAX682s
TDK (847) 390-4373
0.1µF to
2.2µF
Table 4. Recommended Capacitor
Manufacturers
C X
C IN C OUT
OUT
CXN
CXP
SHDN
IN
OUT
7
4 5
6
3
2
1
8
ON
OFF
R EXT
SKIP
GND PGND
MAX682
MAX683
MAX684
IN
V
ON
Figure 5. Standard Operating Circuit
TANTALUM
C OUT (µF)
10
4.7
2.2
CERAMIC
Ceramic
surface mount
DESCRIPTION
595D-series
tantalum
surface mount
TPS-series
surface mount