Circuit Description

The LM2767 contains four large CMOS switches which are

switched in a sequence to double the input supply voltage.

Energy transfer and storage are provided by external capaci-

tors. Figure 2 illustrates the voltage conversion scheme.

When S

age V+. During this time interval, switches S

open. In the next time interval, S

same time, S

age V+ and the voltage across C

age when there is no load. The output voltage drop when a

load is added is determined by the parasitic resistance (R

d-

s(on)

of the MOSFET switches and the ESR of the capacitors)

and the charge transfer loss between capacitors. Details will

be discussed in the following application information section.

Application Information

Positive Voltage Doubler

The main application of the LM2767 is to double the input

voltage. 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 2V+. The output resistance R

function of the ON resistance of the internal MOSFET

switches, the oscillator frequency, and the capacitance and

ESR of C

discharging C

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. R

for the LM2767.

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

oscillator frequency as well as the capacitance and ESR of

the output capacitor C

High capacitance, low ESR capacitors can reduce both the

output resistance and the voltage ripple.

The Schottky diode D

from turning-on its own parasitic diode and potentially

latching-up. During start-up, D

the output capacitor to V

creasing the start-up time. Therefore, the Schottky diode D

1

should have enough current carrying capability to charge the

output capacitor at start-up, as well as a low forward voltage

to prevent the internal parasitic diode from turning-on. A

Schottky diode like 1N5817 can be used for most applica-

tions. If the input voltage ramp is less than 10V/ms, a smaller

Schottky diode like MBR0520LT1 can be used to reduce the

circuit size.

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 allowable voltage

droop (which equals I

age ripple. Low ESR capacitors (

Table 1) are recommended

to maximize efficiency, reduce the output voltage drop and

voltage ripple.

TABLE 1. Low ESR Capacitor Manufacturers

Manufacturer

Phone

Website

Capacitor Type

Nichicon Corp.

(847)-843-7500

www.nichicon.com

PL & PF series, through-hole aluminum

electrolytic

AVX Corp.

(843)-448-9411

www.avxcorp.com

TPS series, surface-mount tantalum

Sprague

(207)-324-4140

www.vishay.com

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

Sanyo

(619)-661-6835

www.sanyovideo.com

OS-CON series, through-hole aluminum

electrolytic

Murata

(800)-831-9172

www.murata.com

Ceramic chip capacitors

Taiyo Yuden

(800)-348-2496

www.t-yuden.com

Ceramic chip capacitors

Tokin

(408)-432-8020

www.tokin.com

Ceramic chip capacitors

DS101274-14

FIGURE 2. Voltage Doubling Principle

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