PRODUCT DATASHEET
AAT3123/23A/24
TM
ChargePump
High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications
Since the AAT3123/23A/24 outputs are pure constant
current sources and typically drive individual loads, it is
difficult to measure the output voltage for a given output
(D1 to D6) to derive an overall output power measure-
ment. For any given application, white LED forward volt-
age levels can differ, yet the output drive current will be
maintained as a constant.
-or-
V
⎛
⎝
⎞
⎠
OUT
η(%) = 100
VIN
Charge Pump Section Efficiency
The AAT3123/23A/24 contains a fractional charge pump
which 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 one 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:
This makes quantifying output power a difficult task
when taken in the context of comparing to other white
LED driver circuit topologies. A better way to quantify
total device efficiency is to observe the total input power
to the device for a given LED current drive level. The
best white LED driver for a given application should be
based on trade-offs of size, external component count,
reliability, operating range, and total energy usage...not
just % efficiency.
POUT
η =
The AAT3123/23A/24 efficiency 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 or D6 for a given con-
stant current setting. Depending upon the case of VIN
being greater than the specific voltage seen across the
load on D1 (or D5 when the AAT3124 is used) the device
will operate in load switch mode. If the voltage seen on
the constant current source output is less than VIN, then
the device will operate in 1.5X charge pump mode. Each
of these two modes will yield different efficiency values.
Refer to the following two sections for explanations of
each operational mode.
PIN
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
=
VIN × 1.5IOUT 1.5VIN
VOUT
η =
=
-or-
VOUT
⎛
⎝
⎞
⎠
η(%) = 100
1.5V
IN
Load Switch Mode Efficiency
The AAT3123/23A/24 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:
For a charge pump with an output of 5V and a nominal
input of 3.5V, the theoretical efficiency is 95%. Due to
internal switching losses and IC quiescent current con-
sumption, the actual efficiency can be measured at 93%.
These figures are in close agreement for output load con-
ditions from 1mA to 100mA. Efficiency will decrease as
load current drops below 0.05mA or when the level of VIN
approaches VOUT. Refer to the Typical Characteristics sec-
tion of this datasheet for measured plots of efficiency
versus input voltage and output load current for the given
charge pump output voltage options.
POUT
η =
PIN
The expression to define the ideal efficiency (η) can be
rewritten as:
Capacitor Selection
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 performance. Optimum
performance will be obtained when low equivalent series
POUT VOUT × IOUT
=
VOUT
VIN
η =
=
PIN
VIN × IOUT
w w w . a n a l o g i c t e c h . c o m
10
3123.2008.03.1.5
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
