CMOS Monolithic Voltage Converter

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7

frequency eight times. In the third mode, the oscillator

frequency is lowered by connecting a capacitor

between OSC and GND. FC can still multiply the fre-

quency by eight times in this mode, but for a lower

range of frequencies (see

Typical Operating

Characteristics).

In the inverter mode, OSC may also be overdriven by an

external clock source that swings within 100mV of V+

and GND. Any standard CMOS logic output is suitable

for driving OSC. When OSC is overdriven, FC has no

effect. Also, LV must be grounded when overdriving

OSC. Do not overdrive OSC in voltage-doubling mode.

Note: In all modes, the frequency of the signal appear-

ing at CAP+ and CAP- is one-half that of the oscillator.

Also, an undesirable effect of lowering the oscillator fre-

quency is that the effective output resistance of the

charge pump increases. This can be compensated by

increasing the value of the charge-pump capacitors

(see

Capacitor Selection section and Typical Operating

Characteristics).

In some applications, the 5kHz output ripple frequency

may be low enough to interfere with other circuitry. If

desired, the oscillator frequency can then be increased

through use of the FC pin or an external oscillator as

described above. The output ripple frequency is one-

half the selected oscillator frequency. Increasing the

clock frequency increases the MAX660’s quiescent

current, but also allows smaller capacitance values to

be used for C1 and C2.

________________Capacitor Selection

Three factors (in addition to load current) affect the

MAX660 output voltage drop from its ideal value:

1) MAX660 output resistance

2) Pump (C1) and reservoir (C2) capacitor ESRs

3) C1 and C2 capacitance

The voltage drop caused by MAX660 output resistance

is the load current times the output resistance.

Similarly, the loss in C2 is the load current times C2’s

ESR. The loss in C1, however, is larger because it

handles currents that are greater than the load current

during charge-pump operation. The voltage drop due

to C1 is therefore about four times C1’s ESR multiplied

by the load current. Consequently, a low (or high) ESR

capacitor has a much greater impact on performance

for C1 than for C2.

Generally, as the pump frequency of the MAX660

increases, the capacitance values required to maintain

comparable ripple and output resistance diminish pro-

portionately. The curves of Figure 2 show the total circuit

output resistance for various capacitor values (the pump

and reservoir capacitors’ values are equal) and oscillator

frequencies. These curves assume 0.25

Ω capacitor ESR

and a 5.25

Ω MAX660 output resistance, which is why

the flat portion of the curve shows a 6.5

because the typical specification includes the effect of

the ESRs of the capacitors in the test circuit.

In addition to the curves in Figure 2, four bar graphs in

the

Typical Operating Characteristics show output cur-

rent for capacitances ranging from 0.33µF to 220µF.

Output current is plotted for inputs of 4.5V (5V-10%) and

3.0V (3.3V-10%), and allow for 10% and 20% output

droop with each input voltage. As can be seen from the

graphs, the MAX660 6.5

Ω series resistance limits

increases in output current vs. capacitance for values

much above 47µF. Larger values may still be useful,

however, to reduce ripple.

To reduce the output ripple caused by the charge

pump, increase the reservoir capacitor C2 and/or

reduce its ESR. Also, the reservoir capacitor must have

low ESR if filtering high-frequency noise at the output is

important.

Not all manufacturers guarantee capacitor ESR in the

range required by the MAX660. In general, capacitor ESR

is inversely proportional to physical size, so larger capaci-

tance values and higher voltage ratings tend to reduce

ESR.

20

0

1

6

12

CAPACITANCE (

µF)

18

16

14

10

8

4

2

2

4 6 8 10

100

1000

ESR = 0.25

Ω

FOR BOTH

C1 AND C2

MAX660 OUTPUT

SOURCE RESISTANCE

ASSUMED TO BE

5.25

Ω

Figure 2. Total Output Source Resistance vs. C1 and C2

Capacitance (C1 = C2)