AAT2846_08 Datasheet, PDF (16/22 Pages) Advanced Analogic Technologies – High Current Charge Pump with Dual LDO for Backlight and Flash Applications

PRODUCT DATASHEET
AAT2846
ChargePump^TM High Current Charge Pump with Dual LDO for Backlight and Flash Applications
η=
V_F · I_LED
V_IN · I_IN
≈
V_F
V_IN
Due to the very low 1X mode quiescent current, the input
current nearly equals the total output current delivered
to the LEDs. Further, the low-resistance bypass switch
introduces negligible voltage drop from input to output.
The AAT2846 further maintains optimized performance
and efficiency by detecting when the input voltage is not
sufficient to sustain LED drive current. The device auto-
matically switches to 1.5X mode when the input voltage
drops too low in relation to the LED forward voltages.
In 1.5X mode, the output voltage can be boosted to 3/2
the input voltage. The 3/2 conversion ratio introduces a
corresponding 1/2 increase in input current. For ideal
conversion, the 1.5X mode efficiency is given by:
η=
V_F · I_LED
V_IN · 1.5I_IN
=
V_F
1.5 · V_IN
Similarly, when the input falls further, such that 1.5X
mode can no longer sustain LED drive current, the
device will automatically switch to 2X mode. In 2X
mode, the output voltage can be boosted to twice the
input voltage. The doubling conversion ratio introduces
a corresponding doubling of the input current. For ideal
conversion, the 2X mode efficiency is given by:
η=
V_F · I_LED
V_IN · 2I_IN
=
V_F
2 · V_IN
LED Selection
The AAT2846 is designed to drive high-intensity white
LEDs. It is particularly suitable for LEDs with an operat-
ing forward voltage in the range of 1.5V to 4.2V.
The charge pump can also drive other loads that have
similar characteristics to white LEDs. For various load
types, the AAT2846 provides a high-current, program-
mable ideal constant current source.
Capacitor Selection
Careful selection of the four external capacitors C_IN, C₁,
C₂, and C_OUT is important because they will affect turn-on
time, output ripple, and transient performance. Optimum
performance will be obtained when low equivalent series
resistance (ESR) ceramic capacitors are used. In gen-
eral, low ESR may be defined as less than 100mΩ.
Ceramic composition capacitors are highly recommend-
ed over all other types of capacitors for use with the
AAT2846. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. A ceramic capacitor typically has very low ESR, is
lowest cost, has a smaller PCB footprint, and is non-
polarized. Low ESR ceramic capacitors help maximize
charge pump transient response. Since ceramic capaci-
tors are non-polarized, they are not prone to incorrect
connection damage.
Equivalent Series Resistance
ESR is an important characteristic to consider when
selecting a capacitor. ESR is a resistance internal to a
capacitor that is caused by the leads, internal connec-
tions, size or area, material composition, and ambient
temperature. Capacitor ESR is typically measured in mil-
liohms for ceramic capacitors and can range to more
than several ohms for tantalum or aluminum electrolytic
capacitors.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1µF are typically made
from NPO or C0G materials. NPO and C0G materials
generally have tight tolerance and are very stable over
temperature. Larger capacitor values are usually com-
posed of X7R, X5R, Z5U, or Y5V dielectric materials.
Large ceramic capacitors are often available in lower-
cost dielectrics, but capacitors greater than 10µF are not
typically required for AAT2846 applications.
Capacitor area is another contributor to ESR. Capacitors
that are physically larger will have a lower ESR when
compared to an equivalent material smaller capacitor.
These larger devices can improve circuit performance
when compared to an equal value capacitor in a smaller
package size.
PCB Layout
To achieve adequate electrical and thermal performance,
careful attention must be given to the PCB layout. In the
worst-case operating condition, the chip must dissipate
considerable power at full load. Adequate heat-sinking
must be achieved to ensure intended operation.
Figure 5 illustrates an example PCB layout. The bottom
of the package features an exposed metal paddle. The
exposed paddle acts, thermally, to transfer heat from
the chip and, electrically, as a ground connection.
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2846.2008.03.1.1