LMC7660IM datasheet(5/12 Pages) NSC

5 / 12 page

Application Information (Continued)

Typical Applications

Changing Oscillator Frequency

It is possible to dramatically reduce the quiescent operating

current of the LMC7660 by lowering the oscillator frequency.

The oscillator frequency can be lowered from a nominal 10

kHz to several hundred hertz, by adding a slow-down ca-

pacitor C

osc (Figure 3). As shown in the Typical Performance

Curves the supply current can be lowered to the 10 µA

range. This low current drain can be extremely useful when

used in µPower and battery back-up equipment. It must be

understood that the lower operating frequency and supply

current cause an increased impedance of C

r and Cp. The in-

creased impedance, due to a lower switching rate, can be

offset by raising C

r and Cp until ripple and load current re-

quirements are met.

Synchronizing to an External Clock

Figure 4 shows an LMC7660 synchronized to an external

clock. The CMOS gate overrides the internal oscillator when

it is necessary to switch faster or reduce power supply inter-

ference. The external clock still passes through the ÷2 circuit

in the 7660, so the pumping frequency will be 1⁄2 the external

clock frequency.

Lowering Output Impedance

Paralleling two or more LMC7660’s lowers output imped-

ance. Each device must have it’s own pumping capacitor C

but the reservoir capacitor C

r is shared as depicted in Figure

5. The composite output resistance is:

DS009136-6

FIGURE 2. Idealized Voltage Converter

DS009136-7

FIGURE 3. Reduce Supply Current by Lowering Oscillator Frequency

DS009136-8

FIGURE 4. Synchronizing to an External Clock

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