PRODUCT DATASHEET
AAT3176
TM
ChargePump
500mA LED Flash Driver IC
In movie mode, maximum continuous current value is
not limited by the package thermal resistance but by the
integrated metal current density. The maximum value
should not exceed 150mA.
ing forward voltage VF in the range of 4.2V to 1.5V. Low
forward voltage (VF) flash LED may make the 1x/2x
mode automatically transfer voltage point lower and get
higher efficiency at the mode transfer input voltage.
Charge Pump Efficiency
Capacitor Selection
The power conversion efficiency of the AAT3176 depends
on the charge pump mode. By definition, device effi-
ciency is expressed as the output power delivered to the
LED divided by the total input power consumed.
Careful selection of the three external capacitors CIN, C1,
and COUT is important because they will affect turn-on
time, output ripple and transient performance. Optimum
performance is obtained when low ESR ceramic capaci-
tors are used; in general, low ESR may be defined as
less than 100mꢀ. A capacitor value of 1μF for all three
capacitors is a good starting point when choosing capac-
itors; a 2.2μF to 4.7μF value is suitable for input and
output capacitance.
POUT
η =
PIN
When the input voltage is sufficiently greater than the
LED forward voltage, the device optimizes efficiency by
operating in 1x mode. In 1x mode, the device is working
as a bypass switch and passing the input supply directly
to the output. Due to the very low 1x mode quiescent
current, the input current nearly equals the current
delivered to the LED. Further, the low-impedance bypass
switch introduces negligible voltage drop from input to
output. The power conversion efficiency can be approxi-
mated by:
Ceramic composition capacitors are highly recommended
over all other types of capacitors for use with the
AAT3176. 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 to maximize
charge pump transient response. Since ceramic capaci-
tors are non-polarized, they are not prone to incorrect
connection damage.
VF · ILED VF
≈
η =
VIN · IIN
VIN
Equivalent Series Resistance (ESR)
The AAT3176 maintains optimized performance and effi-
ciency by detecting. When the input voltage is not suf-
ficient to sustain LED current, the device automatically
switches to 2x mode. For ideal conversion, the 2x mode
efficiency is given by:
ESR is an important characteristic to consider when
selecting a capacitor. ESR is resistance internal to a
capacitor, which is caused by the leads, internal connec-
tions, size or area, material composition, and ambient
temperature. Capacitor ESR is typically measured in mil-
liohms for ceramic capacitors and can range to more
than several ohms for tantalum or aluminum electrolytic
capacitors.
VF · ILED
VIN · IIN
VF · ILED
VIN · 2 · ILED
VF
2 · VIN
η =
=
≈
LED Selection
Printed Circuit Board
A good LED lighting application circuit not only deter-
mined by the LED driver but also the LED selected. A
higher luminous efficacy LED emits a higher amount of
luminous flux (lumens) for a given power. Most LED
manufacturers' datasheets provide the luminous efficacy
by the luminous flux curve (LED forward current vs lumi-
nous flux). From the luminous flux curve, increasing the
LED forward current may increase the LED light;
increased LED forward current also increases LED for-
ward voltage with higher rating.
Layout Recommendations
When designing a PCB for the AAT3176, the key require-
ments are:
1. Place the flying capacitor CP as close to the chip as
possible; otherwise 2x mode performance will be
compromised.
2. Place input and output decoupling capacitors as
close to the chip as possible to reduce switching
noise and output ripple.
3. Connect the exposed pad to the GND plane to
achieve the best power dissipation.
The AAT3176 is designed to drive high-intensity white
LEDs. It is particularly suitable for LEDs with an operat-
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