AAT3105 Datasheet, PDF (10/15 Pages) Advanced Analogic Technologies – Low Cost 1x/2x 4 Channel Charge Pump WLED Driver

ChargePump^TM
PRODUCT DATASHEET
AAT3105178
Low Cost 1x/2x 4 Channel Charge Pump WLED Driver
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 AAT3105, it is very difficult to measure
efficiency in terms of a percent value comparing input
power over output power.
Since the AAT3105 outputs are pure constant current
sources and typically drive individual loads, it is difficult
to measure the output voltage for a given output to
derive an overall output power measurement. For any
given application, white LED forward voltage levels can
differ, 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 white
LED 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 white LED 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 output power over input power efficiency.
The AAT3105 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 D4 for a given constant current
setting. Depending on the combination of V_IN and volt-
ages sensed at the current sources, the device will oper-
ate 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 AAT3105 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_OUT
P_IN
=
V_F · I_LED
V_IN · I_IN
=
V_F
V_IN
·
·
I_LED
I_OUT
≈
V_F
V_IN
An expression for the ideal efficiency (η) in 1x charge-
pump mode can be expressed as:
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 AAT3105 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
current drops below 1mA or when the voltage level at V_IN
approaches the voltage level at V_OUT.
10
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