+5V to ±10V Voltage Converters
cycle, S5 and S7 are open, S6 and S8 are closed, and
the charge on C2 is transferred to C4, generating 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.
________________________Applications
Positive and Negative Converter
The most common application of the MAX680/MAX681
is as a dual charge-pump voltage converter that pro-
vides positive and negative outputs of two times a posi-
tive input voltage. For applications where PC board
space is at a premium, the MAX681, with its capacitors
internal 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 capacitors
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 capaci-
tors, 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.
MAX680/MAX681
__________Efficiency Considerations
Theoretically, a charge-pump voltage multiplier can
approach 100% efficiency under the following con-
ditions:
• 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 capaci-
tors 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
=
1
⁄
2
C1
[
(V+)
2
– (V+)(V
CC
)
] +
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 capaci-
tors C1 and C2 are high relative to their respective out-
put loads.
Larger C3 and C4 reservoir capacitor values reduce
output ripple. Larger values of both pump and reservoir
capacitors improve efficiency.
1
⁄
2
C1
22µF
1
2
C2
22µF
3
4
C1-
C2+
C2-
V-
MAX680
V+
C1+
V
CC
GND
8
7
6
5
GND
C4
22µF
V- OUT
C3
22µF
V
CC
IN
V+ OUT
________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.
Figure 3. Positive and Negative Converter
_______________________________________________________________________________________
5
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