Datasheet

LM2682
SNVS044B NOVEMBER 1999REVISED MAY 2013
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APPLICATION INFORMATION
VOLTAGE DOUBLING INVERTER
The main application of the LM2682 is to generate a negative voltage that is twice the positive input voltage. This
circuit requires only three external capacitors and is connected as shown in Figure 6. 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
SW1
+R
SW2
+ESR
C1
+R
SW3
+R
SW4
+ESR
C2
) +2(R
SW1
+R
SW2
+ESR
C1
+R
SW3
+R
SW4
+ESR
C2
) + 1/(f
OSC
×C1) +
1/(f
OSC
×C2) + ESR
C3
Using the assumption that all four switches have the same ON resistance our equation becomes:
R
OUT
= 16R
SW
+ 4ESR
C1
+ 4ESR
C2
+ ESR
C3
+ 1/(f
OSC
×C1) + 1/(f
OSC
×C2)
Output resistance is typically 90 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.
The amount of output voltage ripple is determined by the output capacitor C3 and the output current as shown in
this equation:
V
RIPPLE P-P
= I
OUT
× (2×ESR
C3
+ 1/[2×(f
OSC
×C3)])
Once again a larger capacitor with smaller ESR will give better results.
+5V TO 5V REGULATED VOLTAGE CONVERTER
Another application in which the LM2682 can be used is for generating a 5V regulated supply from a +5V
unregulated supply. This involves using an op-amp and a reference and is connected as shown in Figure 7. The
LM358 op-amp was chosen for its low cost and versatility and the LM4040-5.0 reference was chosen for its low
bias current requirement. Of course other combinations may be used at the designer's discretion to fit accuracy,
efficiency, and cost requirements. With this configuration the circuit is well regulated and is still capable of
providing nearly 10 mA of output current. With a 9 mA load the circuit can typically maintain 5% regulation on the
output voltage with the input varying anywhere from 4.5V to the maximum of 5.5V. With less load the results are
even better. Voltage ripple concerns are reduced in this case since the ripple at the output of the LM2682 is
reduced at the output by the PSRR of the op-amp used.
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