AAT2806 Datasheet, PDF (14/16 Pages) Advanced Analogic Technologies – Dual High Efficiency Charge Pump for White LED AND Flash Applications

AAT2806
Dual High Efficiency Charge Pump
for White LED and Flash Applications
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
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
=
P_OUT
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 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 agree-
ment for output load conditions from 1mA to 100mA.
Efficiency will decrease as load current drops below
0.05mA or when the level of V_IN approaches 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 5V and a nom-
inal input of 3V, the theoretical efficiency is 83.3%.
Due to internal switching losses and IC quiescent
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 charac-
teristics curves for expected actual efficiency
based on either input voltage or load current.
14
2806.2006.04.1.8