LM2682MMX datasheet(4/7 Pages) NSC

4 / 7 page

Basic Application Circuits

Application Information

Voltage Doubling Inverter

The main application of the LM2682 is to generate a nega-

tive voltage that is twice the positive input voltage. This cir-

cuit requires only three external capacitors and is connected

as shown in

Figure 1. It is important to keep in mind that the

efficiency of the circuit is determined by the output resis-

tance. A derivation of the output resistance is shown below:

OUT =

2(R

SW1+RSW2+ESRC1+RSW3+RSW4+ESRC2)+

2(R

SW1+RSW2+ESRC1+RSW3+RSW4+ESRC2)+

1/(f

OSCxC1) + 1/(fOSCxC2) + ESRC3

Using the assumption that all four switches have the same

ON resistance our equation becomes:

OUT =

16R

SW + 4ESRC1 + 4ESRC2 + ESRC3 +

1/(f

OSCxC1) + 1/(fOSCxC2)

Output resistance is typically 90

Ω with an input voltage of

+5V, an operating temperature of 25˚C, and using low ESR

3.3 µF capacitors. This equation shows the importance of

capacitor selection. Large value, low ESR capacitors will re-

duce the output resistance significantly but will also require a

larger overall circuit. Smaller capacitors will take up less

space but can lower efficiency greatly if the ESR is large.

Also to be considered is that C1 must be rated at 6 VDC or

greater while C2 and C3 must be rated at 12 VDC or greater.

The amount of output voltage ripple is determined by the out-

put capacitor C3 and the output current as shown in this

equation:

RIPPLE P-P = IOUT x (2xESRC3 + 1/[2x(fOSCxC3)])

Once again a larger capacitor with smaller ESR will give bet-

ter results.

DS100997-3

FIGURE 1. Doubling Voltage Inverter

DS100997-4

FIGURE 2. +5V to −5V Regulated Voltage Converter

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