AAT3103 Datasheet, PDF (12/17 Pages) Advanced Analog Technology, Inc. – 3-Channel Charge-Pump LED Driver

ChargePump^TM
Power Efficiency and Device Evaluation
The charge pump efficiency discussion in the following
sections accounts only for efficiency of the charge pump
section itself. Due to the unique circuit architecture and
design of the AAT3103, it is very difficult to measure
efficiency in terms of a percent value comparing input
power over output power.
Since the AAT3103 outputs are pure constant current
sources and typically drive individual loads, it is difficult
to measure the output voltage for a given output pin to
derive an overall output power measurement. For any
given application, WLED forward voltage levels can dif-
fer, yet the output drive current will be maintained as a
constant.
This makes quantifying output power a difficult task
when taken in the context of comparing to other WLED
driver circuit topologies. A better way to quantify total
device efficiency is to observe the total input power to
the device for a given LED current drive level. The best
WLED driver for a given application should be based on
trade-offs of size, external component count, reliability,
operating range, and total energy usage...not just %
efficiency.
The AAT3103 efficiency may be quantified under very
specific conditions and is dependent upon the input volt-
age versus the outputs of D1, D2 and D3 for a given
constant current setting. 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 drop-
out, 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 AAT3103 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.
η
=
P_F
P_IN
PRODUCT DATASHEET
AAT3103
3-Channel Charge-Pump LED Driver
The ideal efficiency (η) in 1x charge-pump mode can be
expressed as:
-or-
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
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 AAT3103 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 half of 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 · I_LED
V_IN · 2 · I_OUT
≈
2
V_F
· V_IN
-or-
η
(%)
=
2
V_F
· V_IN
·
100
For a charge pump LED 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 mea-
sured at 57%. Efficiency will decrease substantially as
load current drops below 1mA or when the voltage level
at V_IN approaches the voltage level at V_OUT.
12
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