11
Rev. 9-22-00
SP6830/6831 Low Power Voltage Inverters With Shutdown Copyright 2000 Sipex Corporation
and P
is the the power output. Both efficiencies
are provided to designers for comparison.
APPLICATION INFORMATION
For the following applications, C1 = C2 = C3 =
3.3
μ
F for the SP6830
and C1 = C2 = C3 = 1.0
μ
F
for the SP6831.
Capacitor Selection
Low ESR capacitors are needed to obtain low
output resistance. Refer to
Table 1
for some
suggested low ESR capacitors. The output
resistance of the SP6830/6831 devices is a
function of the ESR of C1 and C2. This output
resistance can be determined by the equation
previously provided in the
Efficiency
section:
R
OUT
≈
4 x (2 x R
SWITCHES
+ ESR
C1
) +
ESR
C2
+
1
f
OSC
x C1
,
where R
is the circuit's output resistance,
R
is the internal resistance of the MOSFET
switches, ESR
and ESR
are the ESR of their
respective capacitors, and f
OSC
is the oscillator
frequency. This term with f
is derived from an
ideal switched-capacitor circuit as seen in
Figure
19
.
Minimizing the ESR of C1 and C2 will minimize
the total output resistance and will improve the
efficiency.
Flying Capacitor
Decreasing flying capacitor, C1, values will
increase the output resistance of the SP6830/
6831 devices while increasing C1 will reduce the
output resistance. There is a point where
increasing C1 will have a negligible effect on the
output resistance due to the the domination of the
output resistance by the internal MOSFET switch
resistance and the total capacitor ESR.
Output Capacitor
Increasing output capacitor, C2, values will
decrease the output ripple voltage. Reducing the
ESR of C2 will reduce both output ripple voltage
and output resistance. If higher output ripple can
be tolerated in designs, smaller capacitance values
for C2 should be used with light loads. The
following equation can be used to calculate the
peak-to-peak ripple voltage:
V
RIPPLE
= 2 x I
OUT
x ESR
C2
+
I
OUT
f
OSC
x C2
.
Input Bypass Capacitor
The bypass capacitor at the input voltage will
reduce AC impedance and the impact of any of
the SP6830/6831 device's switching noise. It is
recommended that for heavy loads a bypass
capacitor approximately equal to the flying
capacitor, C1, be used. For light loads, the value
of the bypass capacitor can be reduced.
Table 1. Suggested Low ESR Surface Mount Capacitors
T
R
R
A
E
P
B
X
M
E
U
P
N
I
S
/
R
E
#
R
E
U
N
T
O
C
H
A
P
F
E
U
L
N
E
A
T
M
R
E
B
M
U
N
T
R
A
P
/
E
C
E
N
G
A
A
T
C
T
L
O
A
P
V
A
C
R
z
S
H
k
E
0
X
0
1
A
M
@
R
O
P
T
C
Y
T
E
A
P
A
Z
S
C
E
0
3
8
6
P
S
/
T
E
M
6
E
K
9
6
0
0
3
6
8
0
2
0
*
5
3
3
B
4
9
4
T
V
0
2
/
F
μ
3
5
/
B
e
s
a
u
a
C
T
m
0
3
8
6
P
S
/
E
1
U
G
4
2
A
R
P
3
0
S
0
4
2
5
3
0
0
X
5
3
3
D
5
9
5
V
5
3
/
F
μ
3
0
/
C
e
s
a
u
a
C
T
m
0
3
8
6
P
S
/
K
6
0
D
T
0
0
1
8
4
8
K
5
3
3
A
1
R
5
X
6
1
2
3
C
V
0
1
/
F
μ
3
4
0
R
5
X
/
6
0
2
1
1
3
8
6
P
S
/
X
9
4
V
A
1
1
4
4
4
8
K
5
0
1
C
Z
5
0
8
0
V
0
1
/
F
μ
1
4
0
R
7
X
/
5
0
8
0
1
3
8
6
P
S
/
E
6
6
M
E
K
9
6
0
0
3
8
C
A
R
K
5
0
1
C
5
0
8
0
C
V
0
1
/
F
μ
1
5
0
R
7
X
/
5
0
8
0
1
3
8
6
P
S
/
K
6
0
D
T
0
0
1
8
4
8
K
5
0
1
A
1
R
5
X
2
1
0
2
C
V
0
1
/
F
μ
1
5
0
R
5
X
/
5
0
8
0