AAT3172 Datasheet, PDF (12/14 Pages) Advanced Analogic Technologies – High Current LED Flash Driver Charge Pump IC

2.7V to 5.5V
EN/SET
AAT3172
High Current LED Flash Driver Charge Pump IC
C1
C2
1µF
1µF
C1+ C1- C2+ C2-
VIN
VOUT
C_IN
4.7µF
AAT 3172
EN/SET
F1
F2
GND
C_OUT
2.2µF
Flash
LED
STAT1
STAT2
Figure 2: Flash Plus Status Lighting.
LED Selection
The AAT3172 is designed to drive high intensity
white flash LEDs with forward voltages up to 4.4V.
Charge Pump Efficiency
η
=
P_OUT
P_IN
=
100%
×
⎛ ^VF ⎞
⎝ ^VIN⎠
1X Mode
=
100%
×
⎛ ^VF ⎞
⎝^1.5VIN⎠
1.5X Mode
=
100%
×
⎛ ^VF ⎞
⎝ ^2VIN ⎠
2X Mode
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 equivalent series resistance
(ESR) (<100mΩ) ceramic capacitors are used. A
value of 1µF for all four capacitors is a good start-
ing point when choosing capacitors. If the LED cur-
rent sinks are only programmed for light current
levels, then the capacitor size may be decreased.
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3172. 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.
Equivalent Series Resistance
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.
12
3172.2005.11.1.1