lm2682mx National Semiconductor Corporation, lm2682mx Datasheet - Page 4

• Manufacturer Part Number: lm2682mx
• Description: Switched Capacitor Voltage Doubling Inverter
• Manufacturer: National Semiconductor Corporation

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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
circuit requires only three external capacitors and is con-
nected as shown in Figure 1. It is important to keep in mind
that the efficiency of the circuit is determined by the output
resistance. A derivation of the output resistance is shown
below:
R
OUT
= 2(R
2(R
1/(f
OSC
SW1
SW1
xC1) + 1/(f
+R
+R
SW2
SW2
+ESR
+ESR
OSC
C1
C1
xC2) + ESR
+R
+R
SW3
SW3
FIGURE 2. +5V to −5V Regulated Voltage Converter
+R
+R
SW4
SW4
C3
FIGURE 1. Doubling Voltage Inverter
+ESR
+ESR
C2
C2
) +
) +
4
Using the assumption that all four switches have the same
ON resistance our equation becomes:
Output resistance is typically 90Ω 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
reduce 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.
R
OUT
= 16R
1/(f
10099703
OSC
SW
xC1) + 1/(f
+ 4ESR
C1
OSC
+ 4ESR
xC2)
10099704
C2
+ ESR
C3
+