AAT2846 Datasheet, PDF (18/25 Pages) Advanced Analogic Technologies – High Current Charge Pump with Dual LDO for Backlight and Flash Applications

AAT2846
High Current Charge Pump with Dual LDO
for Backlight and Flash Applications
Selection of set resistor values outside of the typical
application must be carefully evaluated to ensure
that the application's performance requirements
can still be met.
Device Power Efficiency
The AAT2846 power conversion efficiency depends
on the charge pump mode. By definition, device effi-
ciency is expressed as the output power delivered to
the LEDs divided by the total input power consumed.
η
=
P_OUT
P_IN
When the input voltage is sufficiently greater than
the LED forward voltages, the device optimizes
efficiency by operating in 1X mode. In 1X mode,
the device is working as a bypass switch and pass-
ing the input supply directly to the output. By sim-
plifying the conditions such that the LEDs have uni-
form V_F, the power conversion efficiency can be
approximated by:
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 perform-
ance and efficiency by detecting when the input
voltage is not sufficient to sustain LED drive cur-
rent. The device automatically switches to 1.5X
mode when the input voltage drops too low in rela-
tion 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 effi-
ciency 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 cur-
rent, 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 dou-
bling 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 operating 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 var-
ious load types, the AAT2846 provides a high-cur-
rent, programmable 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 per-
formance. Optimum performance will be obtained
when low equivalent series resistance (ESR)
ceramic capacitors are used. In general, low ESR
may be defined as less than 100mΩ.
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT2846. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
trolytic counterparts. 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 capacitors are
non-polarized, they are not prone to incorrect con-
nection damage.
18
2846.2007.08.1.0