AAT3170
High Current LED Flash Driver Charge Pump IC
When the input voltage is sufficiently greater than
Capacitor Selection
the LED forward voltage, the device optimizes effi-
ciency 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. The power
conversion efficiency can be approximated by:
Careful selection of the four external capacitors
(CIN, C1, C2, and COUT) is important because they
will affect turn-on time, output ripple, and transient
performance. Optimum performance will be
obtained when low equivalent series resistance
(ESR) (<100mΩ) ceramic capacitors are used. A
value of 1µF for the flying capacitors is a good
starting point when choosing capacitors. If the LED
current sinks are only programmed for light current
levels, then the capacitor size may be decreased.
VF · ILED VF
≈
η =
VIN · IIN
VIN
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.
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3170. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
trolytic counterparts. 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 maximize charge pump
transient response.
The AAT3170 further maintains optimized perform-
ance and efficiency by detecting when the input
voltage is not sufficient to sustain LED current. The
device automatically switches to 1.5X mode when
the input voltage drops too low in relation to the
LED forward voltage.
Equivalent Series Resistance
In 1.5X mode, the output voltage can be boosted to
3/2 the input voltage. The 3/2 conversion ratio
introduces a corresponding 1/2 increase in input
current. For ideal conversion, the 1.5X mode effi-
ciency is given by:
ESR is an important characteristic to consider
when selecting a capacitor. ESR is a resistance
internal to a capacitor that is caused by the leads,
internal connections, size or area, material compo-
sition, and ambient temperature. Capacitor ESR is
typically measured in milliohms for ceramic capac-
itors and can range to more than several ohms for
tantalum or aluminum electrolytic capacitors.
VF · ILED
VF
η =
=
VIN · 1.5IIN 1.5 · VIN
Ceramic Capacitor Materials
Similarly, when the input falls further, such that
1.5X mode can no longer sustain LED current, the
device will automatically switch to 2X mode. In 2X
mode, the output voltage can be boosted to twice
the input voltage. The doubling conversion ratio
introduces a corresponding doubling of the input
current. For ideal conversion, the 2X mode effi-
ciency is given by:
Ceramic capacitors less than 0.1µF are typically
made from NPO or C0G materials. NPO and C0G
materials typically have tight tolerance and are sta-
ble over temperature. Large capacitor values are
typically composed of X7R, X5R, Z5U, or Y5V
dielectric materials. Large ceramic capacitors are
often available in lower-cost dielectrics, but capac-
itors greater than 4.7µF are not typically required
for AAT3170 applications.
VF · ILED
VIN · 2IIN
VF
η =
=
Capacitor area is another contributor to ESR.
Capacitors that are physically large will have a lower
ESR when compared to an equivalent material,
smaller capacitor. These larger devices can improve
circuit transient response when compared to an
equal value capacitor in a smaller package size.
2 · VIN
3170.2006.10.1.0
13
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