參數(shù)資料
型號(hào)： TQFN34-20
廠商： ANADIGICS, Inc.
英文描述： Four-Channel Backlight Driver with Dual LDOs
中文描述： 四通道雙LDO的背光驅(qū)動(dòng)器
文件頁(yè)數(shù)： 15/20頁(yè)
文件大?。?/td> 392K
代理商： TQFN34-20
AAT2847
Four-Channel Backlight Driver with Dual LDOs
2847.2007.09.1.0
15
R1 is chosen to be 120k, resulting in a small
feedback network bias current of 10μA (V
FB(A/B)
/R1
= 1.2V/120k). The desired output voltage is 1.8V.
From this information, R2 is calculated from the
equation below:
The result is R2 = 60k. Since 60k is not a stan-
dard 1% resistor value, 60.4k is selected. From
this example calculation, for V
OUT
= 1.8V, use R1 =
120k and R2 = 60.4k. A table of example output
voltages and corresponding resistor values is pro-
vided below.
Table 6: Example Output Voltages and
Corresponding Resistor Values.
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
LEDx
= White LED Forward Voltage (V
F
)
I
LEDx
= White LED Bias Current (I
D
)
The expression to define the estimated ideal effi-
ciency (η) for the AAT2847 in 1X mode is as follows:
The AAT2847’s charge pump is a fractional charge
pump which will boost the input supply voltage in
the event where V
IN
is less then the required output
voltage across 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.
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:
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 estimated ideal efficiency (η) for the
AAT2847 in 2X mode is as follows:
Capacitor Selection
Careful selection of the six external capacitors C
IN
,
C1, C2, C
LDOA
, C
LDOB
, and C
OUT
are important
because they will affect turn on time, output ripple
and transient performance. Optimum performance
will be obtained when low ESR (<100m) ceramic
capacitors are used. In general, low ESR is defined
as a resistance that is less than 100m.
X7R and X5R type ceramic capacitors are highly
recommended over all other types of capacitors for
use with the AAT2847. For the charge pump sec-
tion, a 1μF or greater capacitor is required for the fly
(C1 and C2) and output (C
OUT
) capacitors. The dual
η
= P
IN
P
LEDs
V
LED1
· I
LED1
+ ... + V
LED4
· I
LED4
V
IN
· I
IN
η
= P
IN
P
LEDs
V
LED1
· I
LED1
+ ... + V
LED4
· I
LED4
V
IN
· I
IN
η
= P
IN
P
LEDs
V
LED1
· I
LED1
+ ... + V
LED4
· I
LED4
V
IN
· I
IN
η
= V
IN
= 1.5(4 · I
LEDX
)
IN
· I
IN
η
=1.5V
IN
4 · V
LEDX
· I
LEDX
V
η
= V
IN
= 2(4 · I
LEDX
)
IN
· I
IN
η
= 2V
IN
4 · V
LEDX
· I
LEDX
V
LEDX
R2
(A/B)
= R1
(A/B)
(V
LDO(A/B)
- 1.2V)
1.2V
η
= V
IN
= 4 · I
LEDX
IN
· I
IN
η
= V
4 · V
LEDX
· I
LEDX
V
IN
η
= P
IN
P
LEDs
V
LED1
· I
LED1
+ ... + V
LED4
· I
LED4
V
IN
· I
IN
R2 Standard 1% Values (R1 = 120kΩ)
V
LDO(A/B)
(V)
2.8
2.5
2
1.8
1.5
R2 (Ω)
160k
130k
79.6k
60.4k
30.1k
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