MAX871 MAXIM [Maxim Integrated Products], MAX871 Datasheet - Page 6

• Manufacturer Part Number: MAX871
• Description: Switched-Capacitor Voltage Inverters
• Manufacturer: MAXIM [Maxim Integrated Products]

Switched-Capacitor Voltage Inverters
Conversion losses occur during the charge transfer
between C1 and C2 when there is a voltage difference
between them. The power loss is:
To maintain the lowest output resistance, use capaci-
tors with low ESR (Table 1). The charge-pump output
resistance is a function of C1’s and C2’s ESR.
Therefore, minimizing the charge-pump capacitor’s
ESR minimizes the total output resistance.
Increasing the flying capacitor’s size reduces the out-
put resistance. Small C1 values increase the output
resistance. Above a certain point, increasing C1’s
capacitance has a negligible effect, because the out-
put resistance becomes dominated by the internal
switch resistance and capacitor ESR.
Increasing the output capacitor’s size reduces the out-
put ripple voltage. Decreasing its ESR reduces both
output resistance and ripple. Smaller capacitance val-
ues can be used with light loads if higher output ripple
can be tolerated. Use the following equation to calcu-
late the peak-to-peak ripple:
Bypass the incoming supply to reduce its AC impedance
and the impact of the MAX870/MAX871’s switching
Table 1. Low-ESR Capacitor Manufacturers
6
__________Applications Information
Surface-Mount
Tantalum
Surface-Mount
Ceramic
_______________________________________________________________________________________
1
P
V
PRODUCTION
CONV.LOSS
/
RIPPLE
2
METHOD
C2 V
=
RIPPLE
f
OSC
I
OUT
2
[
1
x C2
/
2
C1 V
2V
AVX
Matsuo
Sprague
AVX
Matsuo
MANUFACTURER
OUT RIPPLE
IN
2
Capacitor Selection
Input Bypass Capacitor
V
2
x I
Output Capacitor (C2)
Flying Capacitor (C1)
OUT
V
OUT
x ESR
]
2
x f
OSC
C
2
TPS series
267 series
593D, 595D series
X7R
X7R
SERIES
noise. The recommended bypassing depends on the cir-
cuit 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
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
spikes. A 0.1µF bypass capacitor is sufficient.
The most common application for these devices is a
charge-pump voltage inverter (Figure 1). This applica-
tion 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.
Two devices can be cascaded to produce an even
larger negative voltage (Figure 4). The unloaded output
voltage is normally -2 x V
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 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.
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
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OUT
constantly, except for short switching
PHONE
Combined Doubler/Inverter
IN
, but this is reduced slightly
Paralleling Devices
Cascading Devices
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Voltage Inverter
OUT
FAX
and zero.