Pin Description

Pin

Name

Function

Voltage Doubler

Voltage Split

1

V+

Power supply positive voltage input

Positive voltage output

2

GND

Power supply ground input

Same as doubler

3

CAP−

Connect this pin to the negative terminal of the

charge-pump capacitor

Same as doubler

4

GND

Power supply ground input

Same as doubler

5

OUT

Positive voltage output

Power supply positive voltage

input

6

CAP+

Connect this pin to the positive terminal of the

charge-pump capacitor

Same as doubler

Circuit Description

The LM2681 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 LM2681 is to double the input

voltage. The range of the input supply voltage is 2.5V 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, the capacitance and ESR

of C

charging 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.

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

oscillator frequency, the capacitance and ESR of the output

capacitor C

High capacitance, low ESR capacitors can reduce both the

output reslistance and the voltage ripple.

The Schottky diode D

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

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

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 LM2681 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

can be raised to +11V.

DS100965-14

FIGURE 2. Voltage Doubling Principle

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