Number of Devices

Rout of each LM2660

Rout =

LM2660

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SNVS135E – SEPTEMBER 1999 – REVISED DECEMBER 2014

9.2.1.2.2

Paralleling Devices

Any number of LM2660s can be paralleled to reduce the output resistance. Each device must have its own

pumping capacitor C1, while only one output capacitor Cout is needed as shown in Figure 16. The composite

output resistance is:

(4)

Figure 16. Lowering Output Resistance By Paralleling Devices

9.2.1.2.3

Cascading Devices

Cascading the LM2660s is an easy way to produce a greater negative voltage (as shown in Figure 17). If n is the

integer representing the number of devices cascaded, the unloaded output voltage Vout is (−nVin). The effective

output resistance is equal to the weighted sum of each individual device:

(5)

A three-stage cascade circuit shown in Figure 18 generates

−3 Vin, from Vin.

Cascading is also possible when devices are operating in doubling mode. In Figure 19, two devices are

cascaded to generate 3 Vin.

An example of using the circuit in Figure 18 or Figure 19 is generating +15 V or

−15 V from a +5 V input.

Note that, the number of n is practically limited since the increasing of n significantly reduces the efficiency and

increases the output resistance and output voltage ripple.

Figure 17. Increasing Output Voltage by Cascading Devices

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