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Output Ripple
A
B
0
V
t
2
t
1
–V+
Paralleling Devices
+
-
1
2
3
4
8
7
6
5
TL7660
"1"
1
2
3
4
8
7
6
5
TL7660
"n"
+
-
+
+
-
V+
C
1
C
2
R
L
C
1
TL7660
CMOS VOLTAGE CONVERTER
SCAS794 – JUNE 2006
APPLICATION INFORMATION (continued)
ESR also affects the ripple voltage seen at the output. The total ripple is determined by two voltages, A and B,
as shown in Figure 4 . Segment A is the voltage drop across the ESR of C
2
at the instant it goes from being
charged by C
1
(current flow into C
2
) to being discharged through the load (current flowing out of C
2
). The
magnitude of this current change is 2 × I
OUT
, hence the total drop is 2 × I
OUT
× eSR
C2
V. Segment B is the
voltage change across C
2
during time t
2
, the half of the cycle when C
2
supplies current to the load. The drop at
B is I
OUT
× t
2
/C
2
V. The peak-to-peak ripple voltage is the sum of these voltage drops:
V
RIPPLE
≈ (1/(2f
PUMP
C
2
) + 2(ESR
C2
)) × I
OUT
Again, a low ESR capacitor results in a higher performance output.
Figure 4. Output Ripple
Any number of TL7660 voltage converters may be paralleled to reduce output resistance (see Figure 5 ). The
reservoir capacitor, C
2
, serves all devices, while each device requires its own pump capacitor, C
1
. The resultant
output resistance would be approximately:
R
OUT
= R
OUT
(of TL7660)/n (number of devices)
Figure 5. Paralleling Devices
10
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