ICL7660, ICL7660A
t
2
t
1
B
0
V
A
-(V+)
FIGURE 14. OUTPUT RIPPLE
V+
1
2
C
1
3
4
ICL7660
ICL7660A
“1”
8
7
6
5
C
1
1
2
3
4
ICL7660
ICL7660A
“n”
8
7
6
5
C
2
R
L
-
+
FIGURE 15. PARALLELING DEVICES
V+
1
2
10µF
+
ICL7660
ICL7660A
“1”
8
7
6
5
10µF
+
1
2
ICL7660
ICL7660A
“n”
8
7
6
5
-
10µF
+
V
OUT
=
-
nV+
-
3
4
-
3
4
10µF
-
+
FIGURE 16. CASCADING DEVICES FOR INCREASED OUTPUT VOLTAGE
Typical Applications
Simple Negative Voltage Converter
The majority of applications will undoubtedly utilize the ICL7660
and ICL7660A for generation of negative supply voltages.
Figure 13 shows typical connections to provide a negative
supply negative (GND) for supply voltages below 3.5V.
The output characteristics of the circuit in Figure 13A can be
approximated by an ideal voltage source in series with a
resistance as shown in Figure 13B. The voltage source has
a value of -V+. The output impedance (R
O
) is a function of
the ON resistance of the internal MOS switches (shown in
Figure 12), the switching frequency, the value of C
1
and C
2
,
and the ESR (equivalent series resistance) of C1 and C2. A
good first order approximation for R
O
is:
R
O
≅
2(R
SW1
+ R
SW3
+ ESR
C1
) +
2(R
SW2
+ R
SW4
+ ESR
C1
) +
R
O
≅
2(R
SW1
+ R
SW3
+ ESR
C1
) +
1
(f
PUMP
) (C1)
(f
PUMP
=
f
OSC
2
+ ESR
C2
, R
SWX
= MOSFET switch resistance)
1
(f
PUMP
) (C1)
Combining the four R
SWX
terms as R
SW
, we see that:
R
O
≅
2 (R
SW
) +
+ 4 (ESR
C1
) + ESR
C2
RSW, the total switch resistance, is a function of supply
voltage and temperature (See the Output Source Resistance
graphs), typically 23Ω at 25°C and 5V. Careful selection of
C
1
and C
2
will reduce the remaining terms, minimizing the
output impedance. High value capacitors will reduce the
1/(f
PUMP
•
C
1
) component, and low ESR capacitors will
lower the ESR term. Increasing the oscillator frequency will
reduce the 1/(f
PUMP
•
C1) term, but may have the side effect
of a net increase in output impedance when C
1
> 10µF and
there is no longer enough time to fully charge the capacitors
8
FN3072.7
October 10, 2005
INTERSIL [ INTERSIL CORPORATION ]
