Capacitor Selection

As discussed in the

Positive Voltage Doubler section, the

output resistance and ripple voltage are dependent on the

capacitance and ESR values of the external capacitors. The

output voltage drop is the load current times the output resis-

tance, and the power efficiency is

Where I

and I

L

on-resistance, the two external capacitors and their ESRs.

The selection of capacitors is based on the specifications of

the dropout voltage (which equals I

age ripple, and the converter efficiency. Low ESR capacitors

(Table 1) are recommended to maximize efficiency, reduce

the output voltage drop and voltage ripple.

Low ESR Capacitor Manufacturers

Manufacturer

Phone

Capacitor Type

Nichicon Corp.

(708)-843-7500

PL & PF series, through-hole aluminum electrolytic

AVX Corp.

(803)-448-9411

TPS series, surface-mount tantalum

Sprague

(207)-324-4140

593D, 594D, 595D series, surface-mount tantalum

Sanyo

(619)-661-6835

OS-CON series, through-hole aluminum electrolytic

Murata

(800)-831-9172

Ceramic chip capacitors

Taiyo Yuden

(800)-348-2496

Ceramic chip capacitors

Tokin

(408)-432-8020

Ceramic chip capacitors

Other Applications

Paralleling Devices

Any number of LM2681s can be paralleled to reduce the out-

put resistance. Each device must have its own pumping ca-

pacitor C

shown in Figure 3. The composite output resistance is:

Cascading Devices

Cascading the LM2681s is an easy way to produce a greater

voltage (A two-stage cascade circuit is shown in Figure 4).

The effective output resistance is equal to the weighted sum

of each individual device:

R

Note that, the increasing of the number of cascading stages

is pracitically limited since it significantly reduces the effi-

ciency, increases the output resistnace and output voltage

ripple.

DS100965-19

FIGURE 3. Lowering Output Resistance by Paralleling Devices

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