AAT3111 datasheet(12/16 Pages) AAT

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AAT3111

MicroPower™ Regulated Charge Pump

3111.2006.06.1.3

There are several methods that can be employed to

reduce output ripple depending upon the require-

ments of a given application. The most simple and

straightforward technique is to increase the value of

the COUT capacitor. The nominal 10µF COUT capac-

itor can be increased to 22µF or more. Larger val-

ues for the COUT capacitor (22µF and greater) will by

nature have lower ESR and can improve both high

and low frequency components of the charge pump

output ripple response. If a higher value tantalum

capacitor is used for COUT to reduce low frequency

ripple elements, a small 1µF low ESR ceramic

capacitor should be added in parallel to the tantalum

capacitor (see Figure 1). The reason for this is tan-

talum capacitors typically have higher ESR than

equivalent value ceramic capacitors and are less

able to reduce high-frequency components of the

output ripple. The only disadvantage to using large

values for the COUT capacitor is the AAT3111 device

turn-on time and inrush current may be increased.

If additional ripple reduction is desired, an R/C filter

can be added to the charge pump output in addi-

tion to the COUT capacitor (see Figure 2). An R/C

filter will reduce output ripple by primarily attenuat-

ing high frequency components of the output ripple

waveform. The low frequency break point for the

R/C filter will significantly depend on the capacitor

value selected.

Layout Considerations

High charge pump switching frequencies and large

peak transient currents mandate careful printed cir-

cuit board layout. As a general rule for charge

pump boost converters, all external capacitors

should be located as closely as possible to the

device package with minimum length trace con-

nections. Maximize the ground plane around the

AAT3111 charge pump and make sure all external

capacitors are connected to the immediate ground

plane. A local component side ground plane is rec-

ommended. If this is not possible due the layout

design limitations, assure good ground connec-

tions by the use of large or multiple PCB vias.

Refer to the AAT3111 evaluation board for an exam-

ple of good charge pump layout design (Figures 3

through 5).

Figure 1: Application Using Tantalum Capacitor.

Figure 2: Application With Output Ripple Reduction Filter.

μF

COUT

CFLY

CIN

μF

VOUT

VIN

(3.3V)

(1.8V to 3.3V)

C+

VOUT

GND

SHDN

VIN

C-

ON/OFF

AAT3111

CFILTER

μF

RFILTER

1.5

CFLY

μF

CIN

μF

COUT1

μF

VOUT

(3.3V)

VIN

(1.8V to 3.3V)

C+

VOUT

GND

SHDN

VIN

C-

ON/OFF

AAT3111

COUT2

μF

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