PRODUCT DATASHEET
AAT2861
TM
ChargePump
Backlight/Flash LED Driver and Multiple LDO Lighting Management Unit
Since the AAT2861 outputs are pure constant current
sinks and typically drive individual loads, it is difficult to
measure the output voltage for a given output (BL1 to
BL6, BL7/FL2, and FL1) to derive an overall output power
measurement. For any given application, white LED for-
ward voltage levels can differ, yet the output drive cur-
rent will be maintained as a constant.
The AAT2861 charge pump is a fractional charge pump
which will boost the input supply voltage in the event
where IN is less then the required output voltage across
the backlight white LED load. The efficiency can be sim-
ply defined as a linear voltage regulator with an effective
white LED forward voltage that is equal to one and a half
(1.5x mode) times the input voltage.
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."
With an ideal 1.5x charge pump, the input current is
1.5x of the output current. The expression to define the
estimated ideal efficiency (η) for the AAT2861 in 1.5x
mode is as follows:
PLEDs VLED1 · ILED1 + ... + VLEDX · ILEDX
η =
=
PIN
VIN · IIN
X · VLEDX · I
VIN · IIN
η =
η =
LEDX ; X = 1, 2, 3, ..., 6 or 7 and IIN = 1.5(X · ILEDX
)
The AAT2861 efficiency may be quantified under very
specific conditions and is dependant upon the input volt-
age versus the output voltage seen across the loads
applied to outputs BL1 through BL6, BL7/FL2 or FL1 for
a given constant current setting. Depending on the com-
bination of IN and voltages sensed at the current sinks,
the device will operate in "Load Switch" (1X) 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 dif-
ferent efficiency values. One should refer to the following
two sections for explanations for each operational
mode.
VLEDX
1.5VIN
The same calculations apply for the AAT2861 in 2x mode
where for an ideal 2x charge pump, the input current is
2x of the output current. The expression for the esti-
mated ideal efficiency (η) for the AAT2861 in 2x mode is
as follows:
PLEDs VLED1 · ILED1 + ... + VLEDX · ILEDX
η =
=
PIN
VIN · IIN
X · VLEDX · I
VIN · IIN
η =
η =
LEDX ; X = 1, 2, 3, ..., 6 or 7 and IIN = 2(X · ILEDX
)
AAT2861 charge pump conversion efficiency is defined
as the power delivered to the white LED load divided by
the input power:
VLEDX
2VIN
In addition, with an ideal 1.5x charge pump, the output
current may be expressed as 2/3 of the input current.
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%.
Efficiency will decrease substantially as load current
drops below 1mA or when the level of IN approaches
OUT. The same calculations apply for 2X mode where the
output current then becomes 1/2 of the input current.
PLEDs VLED1 · ILED1 + ... + VLEDX · ILEDX
η =
=
PIN
VIN · IIN
VLEDX = White LED Forward Voltage (VF)
ILEDX = White LED Bias Current (ID)
X = Number of White LEDs
The expression to define the estimated ideal efficiency
(η) for the AAT2861 in 1X mode is as follows:
PLEDs X · VLEDX · ILEDX
η =
=
PIN
VIN · IIN
Capacitor Selection
X = 1, 2, 3, ..., 6 or 7 and IIN = X · ILEDX
Careful selection of all external capacitors CIN, C1, C2,
C
LDO(A/B/C), and COUT is important because they will affect
VLEDX
VIN
turn-on time, output ripple and transient performance.
Optimum performance will be obtained when low ESR
η =
w w w . a n a l o g i c t e c h . c o m
2861.2009.02.1.2
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AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
