AAT3112 Datasheet, PDF (13/16 Pages) Advanced Analogic Technologies – 500mA High Efficiency Charge Pump for White LED Flash Applications

AAT3112
500mA High Efficiency Charge Pump
for White LED Flash Applications
Applications Information
Charge Pump Efficiency
The AAT3112 is a regulated output voltage dou-
bling charge pump. The efficiency (η) can simply
be defined as a linear voltage regulator with an
effective output voltage that is equal to two times
the input voltage. Efficiency (η) for an ideal voltage
doubler can typically be expressed as the output
power divided by the input power:
η
=
P_OUT
P_IN
In addition, with an ideal voltage doubling charge
pump, the output current may be expressed as half
the input current. The expression to define the
ideal efficiency (η) can be rewritten as:
η = ^POUT =
P_IN
V_OUT × I_OUT
V_IN × 2I_OUT
= ^VOUT
2V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT
2V_IN
⎞
⎠
For a charge pump with an output of 4.5 volts and
a nominal input of 2.8 volts, the theoretical efficien-
cy is 80.4%. Due to internal switching losses and
IC quiescent current consumption, the actual effi-
ciency can be measured at 79.6%. These figures
are in close agreement across a wide range of out-
put load conditions. Efficiency will decrease as
load current drops below 0.05mA or when the level
of V_IN approaches V_OUT.
Capacitor Selection
Careful selection of the four external capacitors
C_IN, C₁, C₂, and C_OUT is important because they will
affect turn-on time, output ripple, and transient per-
formance. Optimum performance will be obtained
when low equivalent series resistance (ESR)
ceramic capacitors are used. In general, low ESR
may be defined as less than 100mΩ.
3112.2007.01.1.5
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use with
the AAT3112. Ceramic capacitors offer many advan-
tages over their tantalum and aluminum electrolytic
counterparts. A ceramic capacitor typically has very
low ESR, is lowest cost, has a smaller PCB footprint,
and is non-polarized. Low ESR ceramic capacitors
help to maximize charge pump transient response.
Since ceramic capacitors are non-polarized, they are
not prone to incorrect connection damage.
Equivalent Series Resistance: ESR is an impor-
tant characteristic to consider when selecting a
capacitor. ESR is a resistance internal to a capac-
itor that is caused by the leads, internal connec-
tions, size or area, material composition, and ambi-
ent temperature. Capacitor ESR is typically meas-
ured in milliohms for ceramic capacitors and can
range to more than several ohms for tantalum or
aluminum electrolytic capacitors.
Ceramic Capacitor Materials: Ceramic capacitors
less than 0.1μF are typically made from NPO or
C0G materials. NPO and C0G materials typically
have tight tolerance and are stable over tempera-
ture. Large capacitor values are typically com-
posed of X7R, X5R, Z5U, or Y5V dielectric materi-
als. Large ceramic capacitors, typically greater
than 2.2μF, are often available in low-cost Y5V and
Z5U dielectrics, but large capacitors are not
required in AAT3112 applications.
Capacitor area is another contributor to ESR.
Capacitors that are physically large will have a lower
ESR when compared to an equivalent material
smaller capacitor. These larger devices can improve
circuit transient response when compared to an
equal value capacitor in a smaller package size.
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 pack-
age with minimum length trace connections.
Maximize the ground plane around the AAT3112
charge pump and make sure all external capacitors
are connected to the immediate ground plane. A local
component side ground plane is recommended.
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