AAT2869 Advanced Analogic Technologies, Inc., AAT2869 Datasheet - Page 16

• Manufacturer Part Number: AAT2869
• Description: Fade-in/fade-out 4 Channel Backlight Driver With Dual Ldos
• Manufacturer: Advanced Analogic Technologies, Inc.

Fade In and Fade Out
AAT2869 adopts several linear current change segments
to approximate the exponential LED current change to
get a fashion fade-in visual effect. The fade-in time is
mainly determined by the external capacitor C
charge current. The following formula can be used to
estimate how much capacitance is suitable for an expect-
ed fade-in time.
For example, for a 1 second fade-in time at 20mA LED
current each, a 1μF C
shows the fade-in time with C
different LED current settings.
Fade-out time is determined by the discharging time of
the C
ple, 1μF C
generates 450ms fade-out time.
Charge Pump Efficiency
1x Mode Efficiency
The AAT2869's 1x mode is operational at all times and
functions alone to enhance device power conversion effi-
ciency when V
load. When in 1x mode, voltage conversion efficiency is
defined as output power divided by input power.
16
SystemPower
C
at Different C
F
F
0.56
0.56
0.56
0.56
0.56
through an internal 200kΩ resistor R. For exam-
(μF)
1
1
1
1
1
Table 7: Fade-in Time Examples
F
generates 800ms fade-out time; 560nF C
t
FADE-OUT
IN
is higher than the voltage across the
TM
C
F
F
(s) = 4 · RC
(μF) =
I
and LED Current Settings.
F
LED
η =
capacitor can be used. Table 7
10
15
20
25
30
10
15
20
25
30
(mA)
P
48.7 · I
P
OUT
t
IN
FADE-IN
F
F
of 560nF and 1μF C
= 0.8 · C
Fade-In/Fade-Out 4 Channel Backlight Driver with Dual LDOs
LED
t
F
FADE-IN
0.273
0.409
0.545
0.682
0.818
0.5
0.7
1.0
1.2
1.5
w w w . a n a l o g i c t e c h . c o m
F
and the
(s)
F
at
F
The ideal efficiency (η) in 1X charge pump mode can be
expressed as:
-or-
1.5x Charge Pump Mode Efficiency
The AAT2869 contains a fractional charge pump which
will boost the input supply voltage in the event where V
is less than the voltage required to supply the output.
The efficiency (η) can be simply defined as a linear volt-
age 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 be cal-
culated by the following equation:
-or-
Capacitor Selection
The AAT2869 requires seven capacitors in its typical
application: C
them, C
charge pump operation. 1μF surface-mount multi-layer
ceramic capacitors with low (less than 100mΩ) equiva-
lent series resistance (ESR) are recommended. Though
ESR of the capacitors will not affect the ability of the
capacitor to store energy, it has a large effect on perfor-
mance such as equivalent output resistance, efficiency,
and output voltage ripple of the charge pump. Tantalum
and aluminum electrolytic capacitors are not recom-
mended due to their high ESR. A value of 2.2μF or above
is required for the LDOA and LDOB output capacitors for
proper load voltage regulation and stable operation.
Some recommended capacitors are listed in Table 6.
η =
IN
P
, C
P
OUT
IN
1
, C
IN
=
, C
η =
V
2
V
η(%) = 100
F
OUT
and C
IN
· I
P
η(%) = 100
P
· I
, C
OUT
PRODUCT DATASHEET
OUT
IN
IN
1
, C
=
OUT
=
V
V
V
2
F
, C
are required for 1.5x mode
IN
IN
· I
1.5 · V
V
· I
AAT2869
· 1.5 · I
OUT
FLT
F
IN
V
V
V
· I
OUT
IN
and C
F
≈
OUT
IN
V
V
OUT
IN
F
LOA
2869.2010.01.1.0
≈
, C
1.5 · V
V
LOB
F
. Among
IN
IN