LM2682M National Semiconductor, LM2682M Datasheet - Page 4

• Manufacturer Part Number: LM2682M
• Description: Switched Capacitor Voltage Doubling Inverter
• Manufacturer: National Semiconductor

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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:
Using the assumption that all four switches have the same
ON resistance our equation becomes:
R
R
OUT
OUT
=
=
2(R
2(R
1/(f
16R
1/(f
OSC
OSC
SW1
SW1
SW
xC1) + 1/(f
xC1) + 1/(f
+R
+R
+ 4ESR
SW2
SW2
+ESR
+ESR
C1
OSC
OSC
+ 4ESR
C1
C1
xC2) + ESR
xC2)
+R
+R
FIGURE 2. +5V to −5V Regulated Voltage Converter
SW3
SW3
C2
+R
+R
+ ESR
FIGURE 1. Doubling Voltage Inverter
SW4
SW4
C3
+ESR
+ESR
C3
+
C2
C2
) +
) +
4
Output resistance is typically 90
+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:
Once again a larger capacitor with smaller ESR will give bet-
ter results.
V
RIPPLE P-P
DS100997-3
= I
OUT
x (2xESR
DS100997-4
C3
with an input voltage of
+ 1/[2x(f
OSC
xC3)])