AAT2805 Datasheet, PDF (15/17 Pages) Advanced Analogic Technologies – Dual High Efficiency Charge Pump for White LED and Flash Applications

AAT2805
Dual High Efficiency Charge Pump
for White LED and Flash Applications
Efficiency of the AAT2805 may be quantified under
very specific conditions and is dependent upon the
input voltage versus the output voltage seen across
the loads applied to outputs D1 through D4 for a given
constant current setting. Depending on the combina-
tion of V_IN and voltages sensed at the current sinks,
the device will operate in load switch mode. When
any one of the voltages sensed at the current sinks
nears dropout, the device will operate in 1.5X or 2X
charge pump mode. Each of these modes will yield
different efficiency values. Refer to the following two
sections for explanations for each operational mode.
Load Switch Mode Efficiency: The load switch
mode is operational at all times and functions alone
to enhance device power conversion efficiency when
V_IN is greater than the voltage across the load. When
in load switch mode, the voltage conversion efficien-
cy is defined as output power divided by input power:
η
=
P_OUT
P_IN
The expression to define the ideal efficiency (η) can
be rewritten as:
η
=
P_OUT
P_IN
=
V_OUT × I_OUT
V_IN × I_OUT
=
V_OUT
V_IN
-or-
η(%)
=
100
⎛
⎝
V_{OUT ⎞}
V_IN ⎠
Charge Pump Mode Efficiency: Fractional charge
pumps will boost the input supply voltage in the event
where V_IN is less than the voltage required on the con-
stant current source outputs. The efficiency (η) can be
simply defined as a linear voltage regulator with an
effective output voltage that is equal to one and a half
or two 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:
η
=
P_OUT
P_IN
2805.2005.05.1.2
In addition, with an ideal 1.5X charge pump, the out-
put current may be expressed as 2/3 of the input cur-
rent. The expression to define the ideal efficiency (η)
can be rewritten as:
η = ^POUT =
P_IN
V_OUT × I_OUT
V_IN × 1.5I_OUT
=
V_OUT
1.5V_IN
-or-
η(%)
=
100
⎛
⎝
V_{OUT ⎞}
1.5V_IN⎠
For a charge pump with an output of 5 volts and a
nominal input of 3.5 volts, the theoretical efficiency is
95%. Due to internal switching losses and IC quies-
cent 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 level of V_IN approach-
es V_OUT.
Flash Charge Pump: The flash charge pump is a
regulated output voltage doubling charge pump. The
efficiency is defined as a linear voltage regulator with
an effective output voltage that is equal to two times
the input voltage. The expression to define the ideal
efficiency can be written as:
η = ^POUT =
P_IN
V_OUT × I_OUT
V_IN × 2.0I_OUT
=
V_OUT
2.0V_IN
-or-
η(%)
=
100
⎛
⎝
V_{OUT ⎞}
2.0V_IN⎠
For a charge pump with an output of 5 volts and a
nominal input of 3.0 volts, the theoretical efficiency is
83.3%. Due to internal switching losses and IC qui-
escent current consumption, the actual efficiency
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 expected actual efficien-
cy based on either input voltage or load current.
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