AAT2801 Datasheet, PDF (17/20 Pages) Advanced Analogic Technologies – Multi-Mode Charge Pump for White LED Backlight and Flash Applications

AAT2801
Multi-Mode Charge Pump for
White LED Backlight and Flash Applications
In addition, with an ideal 1.5X charge pump, the
output current may be expressed as 2/3 of the
input current. The expression to define the ideal
efficiency (η) can be rewritten as:
η = ^POUT =
P_IN
V_OUT × I_OUT
V_IN × 1.5I_OUT
=
V_OUT
1.5V_IN
-or-
η(%)
=
100
⎛
⎝
V_{OUT ⎞}
1.5V_IN⎠
For the voltage doubling charge pump mode, the
expression to define the ideal efficiency (η) can be
rewritten as:
η = ^POUT =
P_IN
V_OUT × I_OUT
V_IN × 2I_OUT
=
V_OUT
2V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT
2V_IN
⎞
⎠
The flash driver charge pump section utilizes a 2X
voltage doubling topology; thus, the same efficien-
cy equation may be used to calculate the flash
charge pump efficiency.
Capacitor Selection
Careful selection of the six external capacitors C_IN,
C1, C2, C3, C_CP, and C_F 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 general,
low ESR may be defined as less than 100mΩ. A
value of 1µF for all six capacitors is a good starting
point when choosing capacitors. If the LED current
sources are only programmed for light current lev-
els, then the capacitor size may be decreased.
Conversely, if the flash driver charge pump is to
provide the maximum flash current of 250mA, C_IN
may be increased to 2.2µF or more to reduce the
load transient effects of the flash driver output.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT2801. 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.
Thermal Protection
The AAT2801 has a thermal protection circuit that
will shut down the charge pump if the die tempera-
ture rises above the thermal limit.
Flash LED Driver
The AAT2801 flash driver charge pump circuit can
source 250mA for pulsed loads up to 500ms from
an input supply as low as 3.3V. This makes the
device well suited for low-cost flash LED driver
applications in portable products. Typically the
4.5V flash output (OUT_FLSH) version of the
AAT2801 should be selected for photo flash LED
applications, as it can maintain better voltage regu-
lation at higher pulsed load current levels (refer to
Figure 1). The limitation of this option is that the
greatest flash LED forward voltage (V_F) that can be
driven is 4.5V at the maximum set forward current
(I_F) for the application. Flash LEDs with forward
voltage (V_F) levels up to 5.0V can be driven by the
AAT2801 5.0V output option. However, the maxi-
mum current for a 500ms pulse will be reduced.
Refer to the Typical Characteristics flash driver
curves for peak output current levels for a given
minimum input voltage.
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