AAT2847IML-EE-T1 Advanced Analog Technology, Inc., AAT2847IML-EE-T1 Datasheet - Page 15

• Manufacturer Part Number: AAT2847IML-EE-T1
• Description: Four-Channel Backlight Driver with Dual LDOs
• Manufacturer: Advanced Analog Technology, Inc.

ChargePump
The result is R2 = 60kΩ. Since 60kΩ is not a standard
1% resistor value, 60.4kΩ is selected. From this exam-
ple calculation, for V
= 60.4kΩ. A table of example output voltages and cor-
responding resistor values is provided below.
Device Power Efficiency
The AAT2847’s charge pump conversion efficiency is
defined as the power delivered to the white LED load
divided by the input power:
V
I
The expression to define the estimated ideal efficiency
(η) for the AAT2847 in 1X mode is as follows:
η =
η =
η =
The AAT2847’s charge pump is a fractional charge pump
which will boost the input supply voltage in the event
where V
the white LED load. The efficiency can be simply defined
as a linear voltage regulator with an effective white LED
forward voltage that is equal to one and a half (1.5X
mode) or two (2X mode) times the input voltage.
2847.2008.05.1.2
LEDx
LEDx
4 · V
V
P
= White LED Bias Current (I
V
P
= White LED Forward Voltage (V
LEDX
LEDs
IN
IN
Table 6: Example Output Voltages and
R2 Standard 1% Values (R1 = 120kΩ)
V
V
IN
LEDX
η =
IN
LDO(A/B)
=
is less then the required output voltage across
Corresponding Resistor Values.
· I
V
· I
P
IN
2.8
2.5
1.8
1.5
LED1
R2
P
2
LEDs
LEDX
IN
(A/B)
· I
(V)
TM
=
LED1
; x = 1, 2, 3 or 4 and I
=
V
OUT
LED1
R1
V
+ ... + V
IN
= 1.8V, use R1 = 120kΩ and R2
(A/B)
· I
· I
LED1
IN
(V
LDO(A/B)
1.2V
LED4
V
+ ... + V
IN
D
· I
)
· I
IN
LED4
- 1.2V)
F
R2 (Ω)
)
LED4
IN
79.6k
60.4k
30.1k
160k
130k
= 4 · I
· I
LED4
w w w . a n a l o g i c t e c h . c o m
LEDX
Four-Channel Backlight Driver with Dual LDOs
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 AAT2847 in 1.5X
mode is as follows:
η =
η =
η =
The same calculations apply for the AAT2847 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 AAT2847 in 2X mode is
as follows:
η =
η =
η =
Capacitor Selection
Careful selection of the six external capacitors C
C2, C
affect turn on time, output ripple and transient perfor-
mance. Optimum performance will be obtained when low
ESR (<100mΩ) ceramic capacitors are used. In general,
low ESR is defined as a resistance that is less than
100mΩ.
X7R and X5R type ceramic capacitors are highly recom-
mended over all other types of capacitors for use with
the AAT2847. For the charge pump section, a 1µF or
greater capacitor is required for the fly (C1 and C2) and
output (C
or greater output capacitor. The required input capacitor
(C
Ceramic capacitors offer many advantages over their
tantalum and aluminum electrolytic counterparts. A
ceramic capacitor typically has very low ESR, is lowest
cost, has a smaller printed circuit board (PCB) footprint,
and is non-polarized. Low ESR ceramic capacitors maxi-
mize charge pump transient response.
IN
) is 2.2µF or greater.
1.5V
4 · V
4 · V
P
P
V
V
2V
P
P
LDOA
LEDs
LEDs
LEDX
LEDX
IN
IN
V
V
IN
IN
LEDX
LEDX
, C
IN
IN
=
=
OUT
· I
· I
V
V
LDOB
) capacitors. The dual LDOs require a 2.2µF
· I
IN
· I
IN
LED1
LED1
LEDX
LEDX
, and C
· I
· I
LED1
; x = 1, 2, 3 or 4 and I
LED1
; x = 1, 2, 3 or 4 and I
PRODUCT DATASHEET
OUT
V
V
+ ... + V
+ ... + V
IN
IN
are important because they will
· I
· I
IN
IN
LED4
LED4
AAT2847
· I
· I
LED4
LED4
IN
IN
= 1.5(4 · I
= 2(4 · I
LEDX
IN
LEDX
, C1,
)
15
)