Datasheet

MAX870/MAX871
Switched-Capacitor Voltage Inverters
_______________________________________________________________________________________ 7
Paralleling Devices
Paralleling multiple MAX870s or MAX871s reduces the
output resistance. Each device requires its own pump
capacitor (C1), but the reservoir capacitor (C2) serves
all devices (Figure 5). Increase C2’s value by a factor
of n, where n is the number of parallel devices. Figure 5
shows the equation for calculating output resistance.
Combined Doubler/Inverter
In the circuit of Figure 6, capacitors C1 and C2 form the
inverter, while C3 and C4 form the doubler. C1 and C3
are the pump capacitors; C2 and C4 are the reservoir
capacitors. Because both the inverter and doubler use
part of the charge-pump circuit, loading either output
causes both outputs to decline toward GND. Make sure
the sum of the currents drawn from the two outputs
does not exceed 40mA.
Heavy Output Current Loads
Under heavy loads, where higher supply is sourcing cur-
rent into OUT, the OUT supply must not be pulled above
ground. Applications that sink heavy current into OUT
require a Schottky diode (1N5817) between GND and
OUT, with the anode connected to OUT (Figure 7).
Layout and Grounding
Good layout is important, primarily for good noise per-
formance. To ensure good layout, mount all compo-
nents as close together as possible, keep traces short
to minimize parasitic inductance and capacitance, and
use a ground plane.
MAX870
MAX871
“n”
MAX870
MAX871
“1”
2
1
V
OUT
C2
2
+V
IN
C1
C2
C1
33
44
551
V
OUT
= -nV
IN
…
…
Figure 4. Cascading MAX870s or MAX871s to Increase
Output Voltage
MAX870
MAX871
“n”
MAX870
MAX871
“1”
2
1
V
OUT
C2
2
+V
IN
C1
C1
3
3
44
5
51
V
OUT
= -V
IN
R
OUT
=
R
OUT
OF SINGLE DEVICE
NUMBER OF DEVICES
…
…
Figure 5. Paralleling MAX870s or MAX871s to Reduce Output
Resistance
MAX870
MAX871
2
1
V
OUT
= (2V
IN
) -
(V
FD1
) - (V
FD2
)
C2
+V
IN
C1
3
4
5
V
OUT
= -V
IN
C4
D1
D1, D2 = 1N4148
C3
D2
Figure 6. Combined Doubler and Inverter
MAX870
MAX871
4
1
GND
OUT
Figure 7. High V- Load Current