LMC7660

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SNOSBZ9C – APRIL 1997 – REVISED APRIL 2013

Figure 11. Idealized Voltage Converter

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 capacitor Cosc (Figure 12). 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 Cr and Cp. The increased impedance, due to a lower switching rate, can be offset by

raising Cr and Cp until ripple and load current requirements are met.

SYNCHRONIZING TO AN EXTERNAL CLOCK

Figure 13 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 interference. The external clock still passes through

the ÷2 circuit in the 7660, so the pumping frequency will be ½ the external clock frequency.

Figure 12. Reduce Supply Current by Lowering Oscillator Frequency

Figure 13. Synchronizing to an External Clock

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