Datasheet

The MAX680/MAX681 are not voltage regulators: the
output source resistance of either charge pump is approx-
imately 150Ω at room temperature with V
CC
at 5V. Under
light load with an input V
CC
of 5V, V+ will approach +10V
and V- will be at -10V. However both, V+ and V- will droop
toward GND as the current drawn from either V+ or V-
increases, since the negative converter draws its power
from the positive converter’s output. To predict output
voltages, treat the chips as two separate converters and
analyze them separately. First, the droop of the negative
supply (V
DROP-
) equals the current drawn from V- - (I
L-
)
times the source resistance of the negative converter
(RS-):
V
DROP -
= I
L-
x RS-
Likewise, the positive supply droop (V
DROP+
) equals the
current drawn from the positive supply (I
L+
) times the
positive converter’s source resistance (RS+), except that
the current drawn from the positive supply is the sum of
the current drawn by the load on the positive supply (I
L+
)
plus the current drawn by the negative converter (I
L-
):
(V
DROP+
) = I
L+
x RS+ = (I
L+
+ I
L-
) x RS+
The positive output voltage will be:
V+ = 2V
CC
– V
DROP+
The negative output voltage will be:
V- = (V+ – V
DROP
) = - (2V
CC
– V
DROP+
– V
DROP-
)
The positive and negative charge pumps are tested and
specified separately to provide the separate values of
output source resistance for use in the above formulas.
When the positive charge pump is tested, the negative
charge pump is unloaded. When the negative charge
pump is tested, the positive supply V+ is from an external
source, isolating the negative charge pump.
Calculate the ripple voltage on either output by noting
that the current drawn from the output is supplied by the
reservoir capacitor alone during one half-cycle of the
clock. This results in a ripple of:
V
RIPPLE
= ½ I
OUT
(
1
/f
PUMP
)(
1
/CR)
For the nominal f
PUMP
of 8kHz with 10µF reservoir
capacitors, the ripple will be 30mV with I
OUT
at 5mA.
Remember that in most applications, the positive charge
pump’s I
OUT
is the load current plus the current taken by
the negative charge pump.
Figure 4. Paralleling MAX680s For Lower Source Resistance
MAX680
22µF
22µF
C1-
8
7
6
5
C2+
V-
V+
1
2
3
4
C1+
V
CC
GND
C2-
MAX680
22µF
22µF
22µF
V+ OUT
V- OUT
V
CC
IN
GND
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
www.maximintegrated.com
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