9.2.1.2.1 Capacitor Selection

The output resistance and ripple voltage are dependent on the capacitance and ESR values of the external capacitors. The output voltage drop is the load current times the output resistance, and the power efficiency is

Equation 3.

where

• I_Q(V+) is the quiescent power loss of the IC device, and
• I_L²R_OUT is the conversion loss associated with the switch on-resistance, the two external capacitors and their ESRs.

Since the switching current charging and discharging C₁ is approximately twice as the output current, the effect of the ESR of the pumping capacitor C₁ is multiplied by four in the output resistance. The output capacitor C₂ is charging and discharging at a current approximately equal to the output current, therefore, its ESR only counts once in the output resistance. However, the ESR of C₂ directly affects the output voltage ripple. Therefore, low ESR capacitors (Table 2) are recommended for both capacitors to maximize efficiency, reduce the output voltage drop and voltage ripple. For convenience, C₁ and C₂ are usually chosen to be the same.

The output resistance varies with the oscillator frequency and the capacitors. In Figure 15, the output resistance vs. oscillator frequency curves are drawn for three different tantalum capacitors. At very low frequency range, capacitance plays the most important role in determining the output resistance. Once the frequency is increased to some point (such as 20 kHz for the 150 μF capacitors), the output resistance is dominated by the ON resistance of the internal switches and the ESRs of the external capacitors. A low value, smaller size capacitor usually has a higher ESR compared with a bigger size capacitor of the same type. For lower ESR, use ceramic capacitors.

Figure 15. Output Source Resistance vs Oscillator Frequency

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 C_out is needed as shown in Figure 16. The composite output resistance is:

Equation 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 V_out is (−nV_in). The effective output resistance is equal to the weighted sum of each individual device:

Equation 5.

A three-stage cascade circuit shown in Figure 18 generates −3 V_in, from V_in.

Cascading is also possible when devices are operating in doubling mode. In Figure 19, two devices are cascaded to generate 3 V_in.

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

Figure 18. Generating −3V_IN from +V_IN

Figure 19. Generating +3V_IN from +V_IN

9.2.1.2.4 Regulating V_OUT

It is possible to regulate the output of the LM2660 by use of a low dropout regulator (such as LP2951). The whole converter is depicted in Figure 20. This converter can give a regulated output from −1.5 V to −5.5 V by choosing the proper resistor ratio:

Equation 6.

where

• V_ref = 1.235 V

The error flag on pin 5 of the LP2951 goes low when the regulated output at pin 4 drops by about 5%. The LP2951 can be shutdown by taking pin 3 high.

Figure 20. Combining LM2660 With LP2951 to Make a Negative Adjustable Regulator

Also, as shown in Figure 21 by operating LM2660 in voltage doubling mode and adding a linear regulator (such as LP2981) at the output, we can get +5 V output from an input as low as +3 V.

Figure 21. Generating +5 V from +3 V Input Voltage