AAT3111 Datasheet, PDF (12/16 Pages) Advanced Analogic Technologies – MicroPower™ Regulated Charge Pump

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 C_OUT capacitor. The nominal 10µF C_OUT capac-
itor can be increased to 22µF or more. Larger val-
ues for the C_OUT 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 C_OUT 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 C_OUT 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 C_OUT 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).
VOUT
(3.3V)
C_OUT2
1μF
C_OUT1
+
22μF
ON/OFF
VOUT
C+
AAT3111
GND
VIN
SHDN
C-
C_FLY
1μF
+ _CIN
10μF
VIN
(1.8V to 3.3V)
Figure 1: Application Using Tantalum Capacitor.
V_OUT
(3.3V)
C_FILTER
33μF
R_FILTER
1.5Ω
C_OUT
10μF
ON/OFF
VOUT
C+
AAT3111
GND
VIN
SHDN
C-
C_FLY
1μF
V_IN
C_IN ^{(1.8V to 3.3V)}
10μF
Figure 2: Application With Output Ripple Reduction Filter.
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3111.2006.06.1.3