Switched-Capacitor Voltage Inverters

6

_______________________________________________________________________________________

Conversion losses occur during the charge transfer

between C1 and C2 when there is a voltage difference

between them. The power loss is:

__________Applications Information

Capacitor Selection

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.

Flying Capacitor (C1)

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.

Output Capacitor (C2)

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:

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 cir-

cuit configuration and on where the load is connected.

When the inverter is loaded from OUT to GND, current

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

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 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 and values.

Cascading Devices

Two devices can be cascaded to produce an even

larger negative voltage (Figure 4). The unloaded output

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.

V

=

I

f

x C2

RIPPLE

OUT

OSC

+ 2

2

x I

x ESR

OUT

C

P

C1 V

V

C2 V

2V

V

x f

CONV.LOSS

IN

2

OUT

2

RIPPLE

2

OUT RIPPLE

OSC

[

]

=

−







 +

−









1 2

1 2

/

/

Table 1. Low-ESR Capacitor Manufacturers

Matsuo

AVX

MANUFACTURER

(714) 969-2491

(803) 946-0690

(800) 282-4975

PHONE

(603) 224-1961

(619) 661-6835

Sprague

Sanyo

(603) 224-1430

(619) 661-1055

(714) 960-6492

(803) 626-3123

FAX

Surface-mount, 595D series

Through-hole, OS-CON series

Surface-mount, 267 series

Surface-mount, TPS series

DEVICE TYPE

USA

Japan

81-7-2070-6306

81-7-2070-1174