AAT3119 Datasheet, PDF (11/18 Pages) Advanced Analogic Technologies – High Efficiency 2X Charge Pump

In the operating state, the AAT3119 typically con-
sumes 2mA of quiescent operating current. The
enable pin (EN) is an active high input. When
pulled low, the AAT3119 is shut down, the quies-
cent current drops to less than 1μA, and the output
is disconnected from the input.
Charge Pump Efficiency
The core of the AAT3119 is a regulated output volt-
age doubling charge pump. The efficiency (η) for
an ideal voltage doubling charge pump can typical-
ly 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
=
V_OUT
2V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT
2V_IN
⎞
⎠
For a charge pump with an output of 5.0 volts and
a nominal input of 3.0 volts, the theoretical efficien-
cy is 83.3%. Due to internal switching losses and
IC quiescent current consumption, the actual effi-
ciency can be measured at approximately 82%.
Efficiency will decrease as the level of V_IN
approaches that of the regulated V_OUT. Refer to
the device typical characteristics curves for expect-
ed actual efficiency based on either input voltage or
load current.
AAT3119
High Efficiency 2X Charge Pump
Capacitor Selection
Careful selection of the three external capacitors
(C_IN, C_FLY, and C_OUT) is important because they will
affect turn-on time, output ripple, efficiency, and
load transient response. 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. A value
of 1μF for all three capacitors is a good starting
point when designing with the AAT3119. This not
only provides for a very small printed circuit board
area, but cost is further reduced by the minimized
bill of materials.
Input Capacitor
A 1μF multilayer ceramic chip capacitor is suggest-
ed for the input. This capacitor should be connect-
ed between the IN pin and ground. 1μF should be
suitable for most applications. Even though the
AAT3119 switching ripple and noise are very low,
back-injected line noise may be further reduced by
increasing the value of C_IN. Other types of capaci-
tors may be used for C_IN at the cost of compro-
mised circuit performance.
Output Capacitor
The output capacitor (C_OUT) should be connected
between the OUT pin and ground. A 1μF ceramic
capacitor is also suggested in this position.
Switching noise and ripple seen on the charge
pump output increases with load current. Typically
1μF is sufficient for minimizing output ripple seen
by the load circuit. If the load current in an appli-
cation is low, or if higher levels of switching ripple
can be tolerated, C_OUT can be reduced as low as
0.33μF. If application circuits with greater load cur-
rent demands require lower switching ripple ampli-
tudes, C_OUT may be increased to values above
1μF. Capacitor types other than ceramic capaci-
tors can be used for C_OUT. However, capacitors
comprised of materials other than ceramic will typ-
ically have a greater value of ESR, resulting in
increased output switching ripple.
3119.2007.02.1.2
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