LM2766 datasheet(6/9 Pages) NSC

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Circuit Description

The LM2766 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

2 and S4 are closed, C1 charges to the supply volt-

age V+. During this time interval, switches S

1 and S3 are

open. In the next time interval, S

2 and S4 are open; at the

same time, S

1 and S3 are closed, the sum of the input volt-

age V+ and the voltage across C

1 gives the 2V+ output volt-

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

load is added is determined by the parasitic resistance (R

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 LM2766 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

out is a

function of the ON resistance of the internal MOSFET

switches, the oscillator frequency, and the capacitance and

ESR of C

1 and C2. Since the switching current charging and

discharging C

1 is approximately twice as the output current,

the effect of the ESR of the pumping capacitor C

1 will be

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

pacitor C

2 is charging and discharging at a current approxi-

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

counts once in the output resistance. A good approximation

of R

out is:

where R

SW is the sum of the ON resistance of the internal

MOSFET switches shown in Figure 2. R

SW is typically 8Ω for

the LM2766.

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

1 is only needed to protect the device

from turning-on its own parasitic diode and potentially

latching-up. During start-up, D

1 will also quickly charge up

the output capacitor to V

IN minus the diode drop thereby de-

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

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.

Shutdown Mode

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

reduce the quiescent current to 0.1 µA. In normal operating

mode, the SD pin is connected to V+. The device can be

brought into the shutdown mode by applying to the SD pin a

voltage less than 20% of the V+ pin voltage.

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

Q(V+) is the quiescent power loss of the IC device,

and I

out is the conversion loss associated with the switch

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

out Rout), the output volt-

age ripple, and the converter efficiency. 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

DS101282-14

FIGURE 2. Voltage Doubling Principle

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