AAT3111
MicroPower Regulated Charge Pump
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.
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.
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
Refer to the AAT3111 evaluation board for an exam-
ple of good charge pump layout design (Figures 3
through 5).
VOUT
(3.3V)
VOUT
C+
+
CFLY
COUT1
22µF
COUT2
1µF
AAT3111
1µF
VIN
GND
VIN
+
(1.8V to 3.3V)
CIN
10µF
SHDN
C-
ON/OFF
Figure 1: Application Using Tantalum Capacitor.
RFILTER
1.5Ω
VOUT
(3.3V)
VOUT
C+
VIN
C-
CFLY
CFILTER
33µF
COUT
10µF
AAT3111
1µF
VIN
GND
(1.8V to 3.3V)
CIN
10µF
SHDN
ON/OFF
Figure 2: Application With Output Ripple Reduction Filter.
12
3111.2005.11.1.2
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