AAT2847_08 Datasheet, PDF (15/20 Pages) Advanced Analogic Technologies – Four-Channel Backlight Driver with Dual LDOs

ChargePump^TM
PRODUCT DATASHEET
AAT2847
Four-Channel Backlight Driver with Dual LDOs
R2_(A/B)
=
R1_(A/B)(V_LDO(A/B)
1.2V
-
1.2V)
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_OUT = 1.8V, use R1 = 120kΩ and R2
= 60.4kΩ. A table of example output voltages and cor-
responding resistor values is provided below.
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
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:
η
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ...
V_IN ·
+ V_LED4
I_IN
·
I_LED4
V_LEDx = White LED Forward Voltage (V_F)
I_LEDx = White LED Bias Current (I_D)
The expression to define the estimated ideal efficiency
(η) for the AAT2847 in 1X mode is as follows:
η
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ... + V_LED4
V_IN · I_IN
·
I_LED4
η
=
4
·
V_LEDX · I_LEDX
V_IN · I_IN
;
x
=
1,
2,
3
or
4
and
I_IN
=
4
·
I_LEDX
η
=
V_LEDX
V_IN
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:
η
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ... + V_LED4
V_IN · I_IN
·
I_LED4
η
=
4
·
V_LEDX · I_LEDX
V_IN · I_IN
;
x
=
1,
2,
3
or
4
and
I_IN
=
1.5(4
·
I_LEDX)
η
=
V_LEDX
1.5V_IN
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
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ... + V_LED4
V_IN · I_IN
·
I_LED4
η
=
4
·
V_LEDX · I_LEDX
V_IN · I_IN
;
x
=
1,
2,
3
or
4
and
I_IN
=
2(4
·
I_LEDX)
η=
V_LEDX
2V_IN
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 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_OUT) capacitors. The dual LDOs require a 2.2µF
or greater output capacitor. The required input capacitor
(C_IN) is 2.2µF or greater.
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.
2847.2008.05.1.2
www.analogictech.com
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