Inrush Current and EMI Considerations

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

charged. Consequently, the MAX1044/ICL7660 develop

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 con-

verters are being powered by a high-impedance source,

the supply voltage may drop too low during the current

bursts for them to function properly. 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:

Σ PLOSS = P

+

INTERNAL

LOSSES

PSWITCH

LOSSES

+

+

PPUMP

CAPACITOR

LOSSES

PCONVERSION

LOSSES

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 connections to

the LV, BOOST, and OSC pins.

The next two losses are associated with the output resis-

tance 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:

P

+

PUMP

CAPACITOR

LOSSES

PSWITCH

LOSSES

= IOUT2 x ROUT

where:

(

)

OUT

OSC

SWITCHES

SRC1

SRC2

1

R

f

/ 2 x C1

4( 2R

E

) E

≅+

++

and fOSC is the oscillator frequency.

The first term is the effective resistance from the switched-

capacitor circuit.

Conversion losses occur during the transfer of charge

between capacitors C1 and C2 when there is a voltage

difference between them. The power loss is:

(

)

(

)

22

OUT

CONV.LOSS

OSC

2

RIPPLE

RIPPLE

OUT

1 C1 (V ) V

2

P

f

/2

1 C2 V

2V

V

2





+−

+





≅×









−









Figure 7. External Clocking

MAX1044

ICL7660

+

4

3

10µF

VOUT = -(V+)

10µF

V+

V+

CMOS or

TTL GATE

10kΩ

REQUIRED

FOR TTL

2

1

5

6

7

8

MAX1044/ICL7660

Switched-Capacitor Voltage Converters

www.maximintegrated.com

Maxim Integrated │ 9