Datasheet

2001-2012 Microchip Technology Inc. DS21468B-page 7
TC7662A
4.2 Output Ripple
ESR also affects the ripple voltage seen at the output.
The total ripple is determined by 2 voltages, A and B,
as shown in Figure 4-2. Segment A is the voltage drop
across the ESR of C
R
at the instant it goes from being
charged by C
P
(current flowing into C
R
) to being dis-
charged through the load (current flowing out of C
R
).
The magnitude of this current change is 2 x I
OUT
, hence
the total drop is 2 x I
OUT
x ESR
CR
volts. Segment B is
the voltage change across C
R
during time t
2
, the half of
the cycle when C
R
supplies current to the load. The
drop at B is I
OUT
x t
2
/C
R
volts. The peak-to-peak ripple
voltage is the sum of these voltage drops:
FIGURE 4-2: OUTPUT RIPPLE
4.3 Paralleling Devices
Any number of TC7662A voltage converters may be
paralleled to reduce output resistance (Figure 4-3).
The reservoir capacitor, C
R
, serves all devices, while
each device requires its own pump capacitor, C
P
. The
resultant output resistance would be approximately:
4.4 Cascading Devices
The TC7662A may be cascaded as shown (Figure 4-4)
to produce larger negative multiplication of the initial
supply voltage. However, due to the finite efficiency of
each device, the practical limit is 10 devices for light
loads. The output voltage is defined by:
V
OUT
= – n (V
IN
)
where n is an integer representing the number of
devices cascaded. The resulting output resistance
would be approximately the weighted sum of the
individual TC7662A R
OUT
values.
FIGURE 4-3: PARALLELING DEVICES LOWERS OUTPUT IMPEDANCE
FIGURE 4-4: INCREASED OUTPUT VOLTAGE BY CASCADING DEVICES
1
2 x f
PUMP
x C
R
V
RIPPLE
( + 2 x ESR
CR
x I
OUT
)
t
2
t
1
B
A
V
0
-(V
DD
)
R
OUT
=
R
OUT
(of TC7662A)
n (number of devices)
1
2
3
4
8
7
6
5
TC7662A
V
DD
1
2
3
4
8
7
6
5
TC7662A
R
L
C
2
C
1
"n"
"1"
+
C
1
1
2
3
4
8
7
6
5
V
DD
1
2
3
4
8
7
6
5
10μF
10μF
"n"
"1"
10μF
V
OUT
*
+
+
+
TC7662A
TC7662A
*V
OUT
= -nV
DD
10μF