AAT3172
High Current LED Flash Driver Charge Pump IC
C1
C2
1µF
1µF
C1+
VIN
C1- C2+
C2-
2.7V to 5.5V
VOUT
CIN
4.7µF
COUT
2.2µF
Flash
LED
AAT3172
F1
F2
EN/SET
EN/SET
STAT1
STAT2
GND
Figure 2: Flash Plus Status Lighting.
the input supply directly to the output. The power
conversion efficiency can be approximated by:
LED Selection
The AAT3172 is designed to drive high-intensity
white LEDs. It is particularly suitable for LEDs with
an operating forward-voltage in the range of 4.2V
to 1.5V.
VF · ILED VF
≈
η =
VIN · IIN
VIN
The charge pump device can also drive other loads
that have similar characteristics to white LEDs. For
various load types, the AAT3172 provides a high-cur-
rent, programmable ideal constant current source.
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.
Device Power Efficiency
The AAT3172 power conversion efficiency
depends on the charge pump mode. By definition,
device efficiency is expressed as the output power
delivered to the LED divided by the total input
power consumed.
The AAT3172 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.
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:
POUT
PIN
η =
When the input voltage is sufficiently greater than
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
VF · ILED
VF
η =
=
VIN · 1.5IIN 1.5 · VIN
12
3172.2006.05.1.5
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
