Datasheet

cycle, S5 and S7 are open, S6 and S8 are closed,
and the charge on C2 is transferred to C4, generat-
ing the negative supply. The eight switches are CMOS
power MOSFETs. S1, S2, S4, and S5 are P-channel
switches, while S3, S6, S7, and S8 are N-channel
switches.
E󰀩ciency Considerations
Theoretically, a charge-pump voltage multiplier can
approach 100% efficiency under the following conditions:
● The charge-pump switches have virtually no offset
and extremely low on-resistance
● Minimal power is consumed by the drive circuitry
● The impedances of the reservoir and pump
capacitors are negligible
For the MAX680/MAX681, the energy loss per clock cycle
is the sum of the energy loss in the positive and negative
converters as below:
LOSS
TOT
= LOSS
POS
+ LOSS
NEG
= ½ C1 [(V+)
2
– (V+)(V
CC
)] +
½ C2 [(V+)
2
– (V-)
2
]
There will be a substantial voltage difference between
(V+ - V
CC
) and V
CC
for the positive pump, and between
V+ and V-, if the impedances of pump capacitors C1 and
C2 are high relative to their respective output loads.
Larger C3 and C4 reservoir capacitor values reduce
output ripple. Larger values of both pump and reservoir
capacitors improve efficiency.
Maximum Operating Limits
The MAX680/MAX681 have on-chip zener diodes that
clamp V
CC
to approximately 6.2V, V+ to 12.4V, and
V- to -12.4V. Never exceed the maximum supply volt-
age: excessive current may be shunted by these diodes,
potentially damaging the chip. The MAX680/MAX681
operate over the entire operating temperature range with
an input voltage of +2V to +6V.
Applications
Positive and Negative Converter
The most common application of the MAX680/MAX681
is as a dual charge-pump voltage converter that provides
positive and negative outputs of two times a positive
input voltage. For applications where PC board space
is at a premium, the MAX681, with its capacitors inter-
nal to the package, offers the smallest footprint. The
simple circuit shown in Figure 3 performs the same
function using the MAX680 with external capacitors C1
and C3 for the positive pump and C2 and C4 for the
negative pump. In most applications, all four capaci-
tors are low-cost, 10µF or 22µF polarized electrolytics.
When using the MAX680 for low-current applications,
1µF can be used for C1 and C2 charge-pump capac-
itors, and 4.7µF for C3 and C4 reservoir capacitors.
C1 and C3 must be rated at 6V or greater, and C2 and C4
must be rated at 12V or greater.
Figure 3. Positive and Negative Converter
MAX680
C1
22µF
C3
22µF
V+ OUT
V- OUT
V
CC
IN
GND
C4
22µF
C2
22µF
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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