AAT2806
Dual High Efficiency Charge Pump
for White LED and Flash Applications
The expression to define the ideal efficiency (η) can
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
be rewritten as:
POUT VOUT × IOUT VOUT
η =
=
=
PIN
VIN × IOUT
VIN
-or-
0.05mA or when the level of VIN approaches VOUT.
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:
V
⎛
⎝
⎞
⎠
OUT
η(%) = 100
VIN
Charge Pump Mode Efficiency: Fractional charge
pumps will boost the input supply voltage in the event
where VIN 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:
POUT
PIN
VOUT × IOUT
=
VIN × 2.0IOUT 2.0VIN
VOUT
η =
=
-or-
VOUT
⎛
⎝
⎞
⎠
η(%) = 100
2.0V
IN
POUT
η =
PIN
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 VIN approaches that of the
regulated VOUT. Refer to the device typical charac-
teristics curves for expected actual efficiency
based on either input voltage or load current.
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:
POUT
PIN
VOUT × IOUT
=
VIN × 1.5IOUT 1.5VIN
VOUT
η =
=
-or-
VOUT
⎛
⎝
⎞
⎠
η(%) = 100
1.5V
IN
14
2806.2006.04.1.8
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
