effect of the ESR of the pumping capacitor C

plied by four in the output resistance. The output capacitor

C

equal to the output current, therefore, its ESR only counts

once in the output resistance. A good approximation of R

out

is:

where R

MOSFET switches shown in

Figure 2.

High capacitance, low ESR capacitors will reduce the output

resistance.

The peak-to-peak output voltage ripple is determined by the

oscillator frequency, the capacitance and ESR of the output

capacitor C

Again, using a low ESR capacitor will result in lower ripple.

Shutdown Mode

A shutdown (SD ) pin is available to disable the device and

reduce the quiescent current to 1µA. Applying a voltage less

than 20% of V+ to the SD pin will bring the device into shut-

down mode. While in normal operating mode, the pin is con-

nected to V+.

Capacitor Selection

As discussed in the

Simple Negative Voltage Converter sec-

tion, the output resistance and ripple voltage are dependent

on the capacitance and ESR values of the external capaci-

tors. The output voltage drop is the load current times the

output resistance, 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 LM2664s 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:

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