AAT3340_08 Datasheet, PDF (10/14 Pages) Advanced Analogic Technologies – High Efficiency 1x/1.5x Charge Pump

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 AAT3340, it is very difficult to measure
efficiency in terms of a percent value comparing input
power over output power. Since the AAT3340 outputs
are pure constant current sinks and typically drive indi-
vidual 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 com-
paring 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 components count, reliability, operating range,
and total energy usage...not just % efficiency. The
AAT3340 efficiency may be quantified under specific
conditions and is dependent upon the input voltage and
voltage on BL outputs BL1 through BL4 for a given con-
stant current setting.
At any given current setting, if V_IN is sufficiently high to
avoid drop-out for all the BL pin in 1x mode, the device
stays in 1x mode. If in 1x mode, Vin is less than the
voltage required on any of the 4 current sink channels to
avoid drop-out, the device will operate in 1.5x charge
pump mode. Each of these two modes will yield different
efficiency values.
The AAT3340 contains a fractional charge pump which
will boost the input supply voltage when V_IN is less than
the voltage required on the constant current sink pins.
The ideal efficiency (η) can be defined as:
η
=
P_LED
P_IN
=
V_F ·
V_IN
I_LED
· I_IN
V_F is the LED forward voltage.
In 1x mode, I_IN = I_LED + I_Q.
Ignore the quiescent current, it’s much smaller than I_LED.
PRODUCT DATASHEET
AAT3340
High Efficiency 1x/1.5x Charge Pump
η
=
V_F · I_LED
V_IN · I_IN
In 1.5X mode, I_IN = 1.5 · I_LED + I_Q.
Ignore the quiescent current, it’s much smaller than I_LED.
η
=
V_F · I_LED
V_IN · 1.5 · I_IN
Please also refer to the Typical Characteristics section of
this document for measured plots of efficiency versus
input voltage and output load current for the given
charge pump output voltage options.
Capacitor Selection
Careful selection of the four external capacitors C_IN, C1,
C2, and C_OUT is important because they will affect turn-
on time, output ripple, and transient performance.
Optimum performance will be obtained when low equiv-
alent 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 LED cur-
rent sinks are only programmed for low current levels,
then the capacitor size may be decreased.
Test Current/Channel Disable
Each current sink channel is equipped with a test current
function. While it has been enabled, the AAT3340
automatically detects the presence of LEDs all the time.
Unused channels that are tied to OUT or LED loads that
have failed will be automatically disabled.
Capacitor Characteristics
Ceramic composition capacitors are highly recommend-
ed over all other types of capacitors for use with the
AAT3340. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. A ceramic capacitor has very low ESR, is lowest
cost, has a smaller PCB footprint, and is non-polarized.
Low ESR ceramic capacitors help to maximize charge
pump transient response. Since ceramic capacitors are
non-polarized, they are not prone to incorrect connec-
tion damage.
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