LM2665M6X datasheet(6/8 Pages) NSC

Nome de Peças	LM2665M6X

Descrição Electrónicos	Switched Capacitor Voltage Converter

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Fabricante Electrônico	NSC [National Semiconductor (TI)]

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Application Information (Continued)

voltage source equals 2V+. The output resistance R

out is a

function of the ON resistance of the internal MOSFET

switches, the oscillator frequency, the capacitance and ESR

of C

1 and C2. Since the switching current charging and dis-

charging C

1 is approximately twice as the output current, the

effect of the ESR of the pumping capacitor C

1 will be multi-

plied by four in the output resistance. The output capacitor

C

2 is charging and discharging at a current approximately

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.

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

oscillator frequency, the capacitance and ESR of the output

capacitor C

2:

High capacitance, low ESR capacitors can reduce both the

output resistance and the voltage ripple.

The Schottky diode D

1 is only needed for start-up. The inter-

nal oscillator circuit uses the OUT pin and the GND pin. Volt-

age across OUT and GND must be larger than 1.8V to insure

the operation of the oscillator. During start-up, D

1 is used to

charge up the voltage at the OUT pin to start the oscillator;

also, it protects the device from turning-on its own parasitic

diode and potentially latching-up. Therefore, the Schottky di-

ode D

1 should have enough current carrying capability to

charge the output capacitor at start-up, as well as a low for-

ward voltage to prevent the internal parasitic diode from

turning-on. A Schottky diode like 1N5817 can be used for

most applications. If the input voltage ramp is less than 10V/

ms, a smaller Schottky diode like MBR0520LT1 can be used

to reduce the circuit size.

Split V+ in Half

Another interesting application shown in the Basic Applica-

tion Circuits is using the LM2665 as a precision voltage di-

vider. . This circuit can be derived from the voltage doubler

by switching the input and output connections. In the voltage

divider, the input voltage applies across the OUT pin and the

GND pin (which are the power rails for the internal oscillator),

therefore no start-up diode is needed. Also, since the

off-voltage across each switch equals V

in/2, the input voltage

can be raised to +11V.

Shutdown Mode

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

reduce the quiescent current to 1 µA. In normal operating

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

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

voltage greater than 40% 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

L

2R

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.

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 LM2665s can be paralleled to reduce the out-

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

pacitor C

1, while only one output capacitor Cout is needed as

shown in Figure 3. The composite output resistance is:

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