AAT3104 Datasheet, PDF (11/15 Pages) Advanced Analogic Technologies – Low Cost 1x/2x 4 Channel Charge Pump WLED Driver

ChargePump^TM
PRODUCT DATASHEET
AAT3104178
Low Cost 1x/2x 4 Channel Charge Pump WLED Driver
to outputs D1 through for a given constant current set-
ting. Depending on the combination of V_IN and voltages
sensed at the current sources, the device will operate in
load switch mode. When any one of the voltages sensed
at the current sources nears dropout, the device will
operate in 2X charge pump mode.
Each of these modes will yield different efficiency values.
Refer to the following two sections for explanations for
each operational mode.
1X Mode Efficiency
The AAT3104 1X mode is operational at all times and
functions alone to enhance device power conversion effi-
ciency when V_IN is higher than the voltage across the
load. When in 1X mode, voltage conversion efficiency is
defined as output power divided by input power.
An expression for the ideal efficiency (η) in 1X charge-
pump mode can be expressed as:
η
=
P_OUT
P_IN
=
V_F · I_LED
V_IN · I_IN
=
V_F
V_IN
·
·
I_LED
I_OUT
≈
V_F
V_IN
-or-
η
(%)
=
V_F
V_IN
·
100
For a charge pump led driver with V_F of 3.2V and 4.2V
input voltage, the theoretical efficiency is 76%. Due to
internal switching losses and IC quiescent current con-
sumption, the actual efficiency can be measured at 73%.
2X Charge Pump Mode Efficiency
The AAT3104 contains a charge pump which will boost
the input supply voltage in the event where V_IN is less
than the voltage required to supply the output. The effi-
ciency (η) can be simply defined as a linear voltage
regulator with an effective output voltage that is equal to
one and two times the input voltage. Efficiency (η) for an
ideal 2X charge pump can typically be expressed as the
output power divided by the input power.
η=
P_F
P_IN
In addition, with an ideal 2X charge pump, the output
current may be expressed as 1/3 of the input current.
The expression to define the ideal efficiency (η) can be
rewritten as
η
=
P_OUT
P_IN
=
V_F · I_LED
V_IN · I_IN
=
V_F
V_IN ·
· I_LED
2 · I_OUT
≈
2
V_F
· V_IN
-or-
η
(%)
=
2
V_F
· V_IN
·
100
For a charge pump current source driver with V_F of 3.2V
and 2.7V input voltage, the theoretical efficiency is 59%.
Due to internal switching losses and IC quiescent current
consumption, the actual efficiency can be measured at
57%. Efficiency will decrease substantially as load cur-
rent drops below 1mA or when the voltage level at V_IN
approaches the voltage level at V_OUT.
Additional Applications
The current sources of the AAT3104 can be combined
freely to drive higher current levels through one LED. As
an example, a single LED can be driven at 120mA by
combining together D1 through D4 outputs.
For lower-cost applications, the flying capacitor can be
removed; C+ and C- should be floating. This will force
AAT3104 to operate in 1X mode. To maintain regulated
LED current, the input supply voltage has to be higher
than the charge-pump's dropout voltage in 1X mode plus
the forward voltage of the LED at the preset LED
current.
Capacitor Selection
Careful selection of the three external capacitors C_IN, C_P,
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 general,
low ESR may be defined as less than 100mΩ. A value of
1μF for all four capacitors is a good starting point when
choosing capacitors. If the constant current sources are
only programmed for light current levels, then the
capacitor size may be decreased.
Capacitor Characteristics
Ceramic composition capacitors are highly recommended
over all other types of capacitors for use with the
AAT3104. 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 maximizing
charge pump transient response. Since ceramic capaci-
tors are non-polarized, they are not prone to incorrect
connection damage.
3104.2008.09.1.0
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