a function of oscillator frequency and the capacitor

value. Oscillator frequency, in turn, is influenced by

temperature and supply voltage. For example, with a

5V input voltage and 10µF charge-pump capacitors,

the output resistance is typically 50

Ω. Thus, the output

voltage is about -5V under light loads, and decreases

to about -4.5V with a 10mA load current.

Minor supply voltage variations that are inconsequential

to digital circuits can affect some analog circuits.

Therefore, when using the MAX1044/ICL7660 for

powering sensitive analog circuits, the power-supply

rejection ratio of those circuits must be considered.

The output ripple and output drop increase under

heavy loads. If necessary, the MAX1044/ICL7660 out-

put impedance can be reduced by paralleling devices,

increasing the capacitance of C1 and C2, or connect-

ing the MAX1044’s BOOST pin to V+ to increase the

oscillator frequency.

Inrush Current and EMI Considerations

During start-up, pump capacitors C1 and C2 must be

charged. Consequently, the MAX1044/ICL7660 devel-

op inrush currents during start-up. While operating,

short bursts of current are drawn from the supply to C1,

and then from C1 to C2 to replenish the charge drawn

by the load during each charge-pump cycle. If the

voltage converters are being powered by a high-

impedance source, the supply voltage may drop too

low during the current bursts for them to function prop-

erly. Furthermore, if the supply or ground impedance is

too high, or if the traces between the converter IC and

charge-pump capacitors are long or have large loops,

switching noise and EMI may be generated. To reduce

these effects:

1) Power the MAX1044/ICL7600 from a low-impedance

source.

2) Add a power-supply bypass capacitor with low

effective series resistance (ESR) close to the IC

between the V+ and ground pins.

3) Shorten traces between the IC and the charge-pump

capacitors.

4) Arrange the components to keep the ground pins of

the capacitors and the IC as close as possible.

5) Leave extra copper on the board around the voltage

converter as power and ground planes. This is

easily done on a double-sided PC board.

Efficiency, Output Ripple,

and Output Impedance

The power efficiency of a switched-capacitor voltage

converter is affected by the internal losses in the con-

verter IC, resistive losses of the pump capacitors, and

conversion losses during charge transfer between the

capacitors. The total power loss is:

The internal losses are associated with the IC’s internal

functions such as driving the switches, oscillator, etc.

These losses are affected by operating conditions such

as input voltage, temperature, frequency, and connec-

tions to the LV, BOOST, and OSC pins.

The next two losses are associated with the output

resistance of the voltage converter circuit. Switch losses

occur because of the on-resistances of the MOSFET

switches in the IC. Charge-pump capacitor losses

occur because of their ESR. The relationship between

these losses and the output resistance is as follows:

where:

R

1

(f

/ 2) x C1

4 2R

ESR

ESR

OUT

OSC

SWITCHES

C1

C2

≅+

+

P

P

I

x R

OUT

2

OUT

+=

= P

+P

+P

+P

Switched-Capacitor Voltage Converters

8

_______________________________________________________________________________________

MAX1044

ICL7660

4

3

10µF

10µF

V+

V+

CMOS or

TTL GATE

10k

Ω

REQUIRED

FOR TTL

2

1

5

6

7

8

Figure 7. External Clocking

LOSS

INTERNAL

LOSSES

SWITCH

LOSSES

PUMP

CAPACITOR

LOSSES

CONVERSION

LOSSES

PUMP

CAPACITOR

LOSSES

SWITCH

LOSSES