Datasheet

Detailed Description
The MAX681 contains all circuitry needed to implement a
dual charge pump. The MAX680 needs only four capac-
itors. These may be inexpensive electrolytic capacitors
with values in the 1µF to 100µF range. The MAX681
contains two 1.5µF capacitors as C1 and C2, and two
2.2µF capacitors as C3 and C4. See Typical Operating
Characteristics.
Figure 2a shows the idealized operation of the positive
voltage converter. The on-chip oscillator generates a 50%
duty-cycle clock signal. During the first half of the cycle,
switches S2 and S4 are open, S1 and S3 are closed, and
capacitor C1 is charged to the input voltage V
CC
. During
the second half-cycle, S1 and S3 are open, S2 and S4
are closed, and C1 is translated upward by V
CC
volts.
Assuming ideal switches and no load on C3, charge is
transferred onto C3 from C1 such that the voltage on C3
will be 2V
CC
, generating the positive supply.
Figure 2b shows the negative converter. The switches
of the negative converter are out of phase from the
positive converter. During the second half of the clock
cycle, S6 and S8 are open and S5 and S7 are closed,
charging C2 from V+ (pumped up to 2V
CC
by the posi-
tive charge pump) to GND. In the first half of the clock
Figure 2. Idealized Voltage Quadrupler: a) Positive Charge Pump; b) Negative Charge Pump
Figure 1. Test Circuit
V
CC
a) b)
S1
S3
8kHz
C1+
C1
C3
C1-
S2
S4
S5
S6
S7 S8
C2-
GND
V-
R
L
-
R
L
+
C2+
C4
C2
GNDV
CC
I
L
-
V+
GND
I
L
+
V+
I
L
+
R
L
+
R
L
-
I
L
-
MAX680
C1
4.7µF
V
CC
IN
C3
10µF
V+ OUT
V- OUT
GND
C4
10µF
C2
4.7µF
C1-
8
7
6
5
C2+
V-
V+
1
2
3
4
C1+
V
CC
GND
C2-
MAX680/MAX681 +5V to ±10V Voltage Converters
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