AAT3193 Datasheet, PDF (13/18 Pages) Advanced Analogic Technologies – 3-Channel Charge-Pump LED Driver

ChargePump^TM
The AAT3193 efficiency may be quantified under very
specific conditions and is dependent upon the input volt-
age versus the output voltage across the loads applied
to outputs D1 through D3 for a given constant current
setting. Depending on the combination of V_IN and volt-
ages sensed at the current sinks, the device will operate
in load switch mode. When any one of the voltages
sensed at the current sinks 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 opera-
tional mode.
1X Mode Efficiency
The AAT3193 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:
-or-
η = ^POUT = ^{VF · ILED} ≅ ^VF
P_IN V_IN · I_OUT
V_IN
η
(%)
=
100
·
⎛
⎝
V_{F ⎞}
V_IN⎠
2X Charge Pump Mode Efficiency
The AAT3193 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
PRODUCT DATASHEET
AAT3193
3-Channel Charge-Pump LED Driver
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:
-or-
η = ^POUT = ^{VF · ILED} = ^VF
P_IN V_IN · 2I_OUT 2V_IN
η
(%)
=
100
·
⎛ ^VF ⎞
⎝^2VIN⎠
For a charge pump with an output of 5V and a nominal
input of 3.5V, the theoretical efficiency is 71%. Due to
internal switching losses and IC quiescent current con-
sumption, the actual efficiency can be measured at
51%. 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 sinks of the AAT3193 can be combined to
drive higher current levels through a single LED. As an
example, a single LED can be driven at 90mA total by
combining together the D1-D3 outputs.
C2
D1
1μF
C1
1μF
VIN
EN/SET
1 _D1
D2 ¹⁰
2 _{OUT D3} 9
3 _{C- GND} 8
4 _{C+ RSET} 7
5 _{IN EN/SET} 6
C3
1μF
AAT3193
R1
14.3k
Figure 6: Higher Current, Single LED Application.
For lower-cost applications, the flying capacitor can be
removed. This will force the AAT3193 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.
3193.2008.07.1.3
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