AAT2847
Four-Channel Backlight Driver with Dual LDOs
R1 is chosen to be 120kΩ, resulting in a small
The AAT2847’s charge pump is a fractional charge
pump which will boost the input supply voltage in
the event where VIN 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.
feedback network bias current of 10µA (VFB(A/B)/R1
= 1.2V/120kΩ). The desired output voltage is 1.8V.
From this information, R2 is calculated from the
equation below:
R1(A/B)(VLDO(A/B) - 1.2V)
1.2V
R2(A/B)
=
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 result is R2 = 60kΩ. Since 60kΩ is not a stan-
dard 1% resistor value, 60.4kΩ is selected. From
this example calculation, for VOUT = 1.8V, use R1 =
120kΩ and R2 = 60.4kΩ. A table of example output
voltages and corresponding resistor values is pro-
vided below.
PLEDs VLED1 · ILED1 + ... + VLED4 · ILED4
=
η =
PIN
VIN · IIN
4 · VLEDX · ILEDX
VIN · IIN
R2 Standard 1% Values (R1 = 120kΩ)
η =
η =
; x = 1, 2, 3 or 4 and IIN = 1.5(4 · ILEDX)
VLDO(A/B) (V)
R2 (Ω)
160k
130k
79.6k
60.4k
30.1k
VLEDX
1.5VIN
2.8
2.5
2
1.8
1.5
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:
Table 6: Example Output Voltages and
Corresponding Resistor Values.
PLEDs VLED1 · ILED1 + ... + VLED4 · ILED4
=
η =
Device Power Efficiency
PIN
VIN · IIN
The AAT2847’s charge pump conversion efficiency
is defined as the power delivered to the white LED
load divided by the input power:
4 · VLEDX · ILEDX
VIN · IIN
η =
η =
; x = 1, 2, 3 or 4 and IIN = 2(4 · ILEDX)
VLEDX
2VIN
PLEDs VLED1 · ILED1 + ... + VLED4 · ILED4
=
η =
PIN
VIN · IIN
Capacitor Selection
VLEDx = White LED Forward Voltage (VF)
ILEDx = White LED Bias Current (ID)
Careful selection of the six external capacitors CIN,
C1, C2, CLDOA, CLDOB, and COUT are important
because they will affect turn on time, output ripple
and transient performance. 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.
The expression to define the estimated ideal effi-
ciency (η) for the AAT2847 in 1X mode is as follows:
PLEDs VLED1 · ILED1 + ... + VLED4 · ILED4
=
η =
PIN
VIN · IIN
4 · VLEDX · ILEDX
VIN · IIN
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 (COUT) capacitors. The dual
η =
η =
; x = 1, 2, 3 or 4 and IIN = 4 · ILEDX
VLEDX
VIN
2847.2007.09.1.0
15
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