AAT3134
High Efficiency 1X/1.5X Fractional
Charge Pump for White LED Applications
The AAT3134 efficiency may be quantified under
half times the input voltage. Efficiency (η) for an
ideal 1.5X charge pump can typically be expressed
as the output power divided by the input power:
very specific conditions and is dependent upon the
input voltage versus the output voltage seen across
the loads applied to outputs D1 through D6 for a
given constant current setting. Depending upon VIN
being greater than the specific voltage seen across
the load on D1 (or D6), the device will operate in
load switch mode. If VIN is less than the voltage
required on the constant current source, the device
will operate in 1.5X charge pump mode. Each of
these two modes will yield different efficiency values.
One should refer to the following two sections for
explanations for each operational mode.
POUT
PIN
η =
In addition, with an ideal 1.5X charge pump, the
output current may be expressed as 2/3 of the
input current. The expression to define the ideal
efficiency (η) can be rewritten as:
POUT
PIN
VOUT × IOUT
=
VOUT
η =
=
Load Switch Mode Efficiency
VIN × 1.5IOUT 1.5VIN
The AAT3134 load switch mode is operational at all
times and functions alone to enhance device power
conversion efficiency when the condition exists
where VIN is greater than voltage across the load
connected to the constant current source outputs.
When in load switch mode, the voltage conversion
efficiency is defined as output power divided by
input power:
-or-
VOUT
⎛
⎝
⎞
⎠
η(%) = 100
1.5V
IN
For a charge pump with an output of 5V and a nom-
inal input of 3.5V, the theoretical efficiency is 95%.
Due to internal switching losses and IC quiescent
current consumption, the actual efficiency can be
measured at 93%. These figures are in close
agreement for output load conditions from 1mA to
100mA. Efficiency will decrease as load current
drops below 0.05mA or when the level of VIN
POUT
PIN
η =
The expression to define the ideal efficiency (η)
can be rewritten as:
approaches VOUT
.
Refer to the Typical
Characteristics section of this document for meas-
ured plots of efficiency versus input voltage and
output load current for the given charge pump out-
put voltage options.
POUT VOUT × IOUT VOUT
η =
=
=
PIN
VIN × IOUT
VIN
-or-
Capacitor Selection
V
⎛
⎞
⎠
OUT
Careful selection of the four external capacitors
CIN, C1, C2, and COUT 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. A value of
1µF for all four capacitors is a good starting point
when choosing capacitors. If the LED current
sources are only programmed for light current lev-
els, then the capacitor size may be decreased.
η(%) = 100
VIN
⎝
Charge Pump Section Efficiency
The AAT3134 contains a fractional charge pump
which will boost the input supply voltage when VIN is
less than the voltage required on the constant cur-
rent source outputs. The efficiency (η) can be sim-
ply defined as a linear voltage regulator with an
effective output voltage that is equal to one and one
3134.2005.12.1.1
11
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