MAX828/MAX829
Switched-Capacitor Voltage Inverters
6 _______________________________________________________________________________________
Flying Capacitor (C1)
Increasing the flying capacitor’s size reduces the output
resistance. Small C1 values increase the output resis-
tance. Above a certain point, increasing C1’s capaci-
tance has a negligible effect, because the output
resistance becomes dominated by the internal switch
resistance and capacitor ESR.
Output Capacitor (C2)
Increasing the output capacitor’s size reduces the output
ripple voltage. Decreasing its ESR reduces both output
resistance and ripple. Smaller capacitance values can
be used with light loads if higher output ripple can be
tolerated. Use the following equation to calculate the
peak-to-peak ripple:
Input Bypass Capacitor
Bypass the incoming supply to reduce its AC impedance
and the impact of the MAX828/MAX829’s switching noise.
The recommended bypassing depends on the circuit
configuration and on where the load is connected.
When the inverter is loaded from OUT to GND, current
from the supply switches between 2 x I
OUT
and zero.
Therefore, use a large bypass capacitor (e.g., equal to
the value of C1) if the supply has a high AC impedance.
When the inverter is loaded from IN to OUT, the circuit
draws 2 x I
OUT
constantly, except for short switching
spikes. A 0.1µF bypass capacitor is sufficient.
Voltage Inverter
The most common application for these devices is a
charge-pump voltage inverter (Figure 1). This application
requires only two external components—capacitors C1
and C2—plus a bypass capacitor, if necessary. Refer to
the Capacitor Selection section for suggested capacitor
types and values.
Cascading Devices
Two devices can be cascaded to produce an even
larger negative voltage (Figure 4). The unloaded output
voltage is normally -2 x V
IN
, but this is reduced slightly
by the output resistance of the first device multiplied by
the quiescent current of the second. When cascading
more than two devices, the output resistance rises dra-
matically. For applications requiring larger negative
voltages, see the MAX864 and MAX865 data sheets.
Paralleling Devices
Paralleling multiple MAX828s or MAX829s 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. The
equation for calculating output resistance is also shown
in Figure 5.
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.
