Application Information

SIMPLE NEGATIVE VOLTAGE CONVERTER

The main application of LM2664 is to generate a negative

supply voltage. The voltage inverter circuit uses only two

external capacitors as shown in the Basic Application Cir-

cuits. The range of the input supply voltage is 1.8V to 5.5V.

The output characteristics of this circuit can be approximated

by an ideal voltage source in series with a resistance. The

voltage source equals −(V+). The output resistance R

function of the ON resistance of the internal MOSFET

switches, the oscillator frequency, the capacitance and ESR

of C

discharging C

the effect of the ESR of the pumping capacitor C

multiplied by four in the output resistance. The output ca-

pacitor C

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

counts once in the output resistance. A good approximation

of R

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

shutdown mode. While in normal operating mode, the pin is

connected to V+.

CAPACITOR SELECTION

As discussed in the Simple Negative Voltage Converter

section, the output resistance and ripple voltage are depen-

dent on the capacitance and ESR values of the external

capacitors. 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

www.national.com

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