LMC7660 Datasheet, PDF (5/12 Pages) National Semiconductor (TI) – Switched Capacitor Voltage Converter

Application Information (Continued)
DS009136-6
FIGURE 2. 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 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 C_p. The in-
creased impedance, due to a lower switching rate, can be
offset by raising C_r and C_p 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 ¹â„2 the external
clock frequency.
FIGURE 3. Reduce Supply Current by Lowering Oscillator Frequency
DS009136-7
FIGURE 4. Synchronizing to an External Clock
Lowering Output Impedance
Paralleling two or more LMC7660’s lowers output imped-
ance. Each device must have it’s own pumping capacitor C_p,
but the reservoir capacitor C_r is shared as depicted in Figure
5. The composite output resistance is:
5
DS009136-8
www.national.com