6
FN3072.7
October 10, 2005
Detailed Description
The ICL7660 and ICL7660A contain all the necessary
circuitry to complete a negative voltage converter, with the
exception of 2 external capacitors which may be inexpensive
10µF polarized electrolytic types. The mode of operation of
the device may be best understood by considering Figure
12, which shows an idealized negative voltage converter.
Capacitor C
1
is charged to a voltage, V+, for the half cycle
when switches S
1
and S
3
are closed. (Note: Switches S
2
and S
4
are open during this half cycle.) During the second
half cycle of operation, switches S
2
and S
4
are closed, with
S
1
and S
3
open, thereby shifting capacitor C
1
negatively by
V+ volts. Charge is then transferred from C
1
to C
2
such that
the voltage on C
2
is exactly V+, assuming ideal switches and
no load on C
2
. The ICL7660 approaches this ideal situation
more closely than existing non-mechanical circuits.
In the ICL7660 and ICL7660A, the 4 switches of Figure 12
are MOS power switches; S
1
is a P-Channel device and S
2
,
S
3
and S
4
are N-Channel devices. The main difficulty with
this approach is that in integrating the switches, the
substrates of S
3
and S
4
must always remain reverse biased
with respect to their sources, but not so much as to degrade
their “ON” resistances. In addition, at circuit start-up, and
under output short circuit conditions (V
OUT
= V+), the output
voltage must be sensed and the substrate bias adjusted
accordingly. Failure to accomplish this would result in high
power losses and probable device latchup.
This problem is eliminated in the ICL7660 and ICL7660A by a
logic network which senses the output voltage (V
OUT
) together
with the level translators, and switches the substrates of S
3
and
S
4
to the correct level to maintain necessary reverse bias.
FIGURE 9. OUTPUT VOLTAGE AS A FUNCTION OF OUTPUT
CURRENT
FIGURE 10. SUPPLY CURRENT AND POWER CONVERSION
EFFICIENCY AS A FUNCTION OF LOAD
CURRENT
NOTE:
6. These curves include in the supply current that current fed directly into the load R
L
from the V+ (See Figure 11). Thus, approximately half the
supply current goes directly to the positive side of the load, and the other half, through the ICL7660/ICL7660A, to the negative side of the load.
Ideally, V
OUT
∼ 2V
IN
, I
S
∼ 2I
L
, so V
IN
x I
S
∼ V
OUT
x I
L
.
NOTE: For large values of C
OSC
(>1000pF) the values of C
1
and C2 should be increased to 100µF.
FIGURE 11. ICL7660, ICL7660A TEST CIRCUIT
Typical Performance Curves (Test Circuit of Figure 11) (Continued)
T
A
= 25°C
V+ = 2V
+2
+1
0
-1
-2
SLOPE 150Ω
012345678
LOAD CURRENT I
L
(mA)
OUTPUT VOLTAGE
100
90
80
70
60
50
40
30
20
10
0
POWER CONVERSION EFFICIENCY (%)
P
EFF
I+
LOAD CURRENT I
L
(mA)
0 1.5 3.0 4.5 6.0 7.5 9.0
20.0
18.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0
SUPPLY CURRENT (mA) (NOTE 6)
T
A
= 25°C
V+ = 2V
1
2
3
4
8
7
6
5
+
-
C
1
10µF
I
S
V+
(+5V)
I
L
R
L
-V
OUT
C
2
10µF
ICL7660
C
OSC
+
-
(NOTE)
ICL7660A
ICL7660, ICL7660A
