MAX870EUK(2004) Просмотр технического описания (PDF) - Maxim Integrated

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MAX870EUK
(Rev.:2004)

Switched-Capacitor Voltage Inverters

Maxim Integrated 

Switched-Capacitor Voltage Inverters

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:

VRIPPLE

=

IOUT

fOSC x C2

+

2

x

IOUT

x

ESRC2

Input Bypass Capacitor

Bypass the incoming supply to reduce its AC impedance

and the impact of the MAX870/MAX871’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

from the supply switches between 2 x IOUT 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 IOUT 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.

Cascading Devices

Two devices can be cascaded to produce an even

larger negative voltage (Figure 4). The unloaded output

voltage is normally -2 x VIN, 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.

The maximum load current and startup current of the

nth cascaded circuit must not exceed the maximum

output current capability of the (n-1)th circuit to ensure

proper stability.

Table 1. Low-ESR Capacitor Manufacturers

PRODUCTION

METHOD

Surface-Mount

Tantalum

Surface-Mount

Ceramic

MANUFACTURER

AVX

Matsuo

Sprague

AVX

Matsuo

SERIES

TPS series

267 series

593D, 595D series

X7R

X7R

PHONE

(803) 946-0690

(714) 969-2491

(603) 224-1961

(803) 946-0690

(714) 969-2491

FAX

(803) 626-3123

(714) 960-6492

(603) 224-1430

(803) 626-3123

(714) 960-6492

Table 2. Capacitor Selection for Minimum Output Resistance or Capacitor Size

PART

MAX870

MAX871

fOSC

125kHz

500kHz

CAPACITORS TO MINIMIZE

OUTPUT RESISTANCE

(RO = 23Ω, TYP)

C1 = C2

1µF

0.33µF

CAPACITORS TO MINIMIZE SIZE

(RO = 40Ω, TYP)

C1 = C2

0.33µF

0.1µF

6 _______________________________________________________________________________________

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