AAT2860 Analogic Corporation, AAT2860 Datasheet - Page 18

• Manufacturer Part Number: AAT2860
• Manufacturer: Analogic Corporation

Since the AAT2860-x 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
BL5, BL6/FL1, and BL7/FL2) to derive an overall output
power measurement. For any given application, white
LED forward voltage levels can differ, yet the output
drive current will be maintained as a constant.
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”.
The AAT2860-x efficiency may be quantified under very
specific conditions and is dependent upon the input volt-
age versus the output voltage seen across the loads
applied to outputs BL1 through BL5, BL6(FL1), and
BL7(FL2) for a given constant current setting. Depending
on the combination of V
current sinks/channels, the device will operate in “Load
Switch” mode. When any one of the voltages sensed at
the current sinks/channels nears dropout the device will
operate in 1.5x or 2x charge pump mode. Each of these
modes will yield different efficiency values. One should
refer to the following two sections for explanations for
each operational mode.
Device Power Efficiency
The AAT2860’s charge pump conversion efficiency is
defined as the power delivered to the white LED load
divided by the input power:
V
I
X = Number of White LEDs
18
ChargePump
LEDx
LEDx
= White LED Bias Current (I
= White LED Forward Voltage (V
η =
P
P
LEDs
IN
=
TM
V
LED1
Backlight/Flash LED Driver and Multiple LDO Lighting Management Unit
· I
IN
LED1
and voltages sensed at the
V
+ ... + V
IN
D
· I
)
IN
F
LEDX
)
· I
LEDX
w w w . a n a l o g i c t e c h . c o m
The expression to define the estimated ideal efficiency
(η) for the AAT2860 in 1x mode is as follows:
η =
η =
η =
The AAT2860’s charge pump is a fractional charge pump
which will boost the input supply voltage in the event
where V
the backlight white LED load. The efficiency can be simply
defined as a linear voltage regulator with an effective
backlight white LED forward voltage that is equal to one
and a half (1.5x mode) times the input voltage.
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 AAT2860 in 1.5x
mode is as follows:
η =
η =
η =
The same calculations apply for the AAT2860 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 AAT2860 in 2x mode is
as follows:
η =
η =
η =
X · V
1.5V
X · V
X · V
V
V
P
P
P
V
2V
V
P
P
P
LEDX
LEDX
LEDs
LEDs
LEDs
LEDX
IN
IN
IN
IN
V
V
IN
V
IN
LEDX
IN
IN
LEDX
LEDX
IN
IN
=
=
=
· I
is less then the required output voltage across
· I
· I
· I
IN
V
V
V
· I
IN
· I
IN
LED1
LED1
LEDX
LED1
LEDX
LEDX
; X = 1, 2, 3, ..., 6 or 7 and I
· I
· I
· I
; X = 1, 2, 3,..., 6 or 7 and I
; X = 1, 2, 3, ..., 6 or 7 and I
LED1
LED1
LED1
V
PRODUCT DATASHEET
IN
V
V
+ ... + V
+ ... + V
+ V
· I
IN
IN
IN
· I
· I
LED2
IN
IN
· I
AAT2860
LEDX
LEDX
LED2
· I
· I
LEDX
LEDX
2860.2008.05.1.0
IN
= 1.5(X · I
IN
IN
= 2(X · I
= X · I
LEDX
LEDX
LEDX
)
)