PRODUCT DATASHEET
AAT3172
TM
ChargePump
High Current LED Flash Driver Charge Pump IC
C1
1µF
C2
1µF
C1+
VIN
C1- C2+
C2-
2.7V to 5.5V
EN/SET
VOUT
CIN
4.7µF
COUT
2.2µF
Flash
LED
AAT3172
F1
F2
EN/SET
STAT1
STAT2
GND
Figure 2: Flash Plus Status Lighting.
LED. Further, the low-impedance bypass switch intro-
duces negligible voltage drop from input to output.
LED Selection
The AAT3172 is designed to drive high-intensity white
LEDs. It is particularly suitable for LEDs with an operat-
ing forward-voltage in the range of 4.2V to 1.5V.
The AAT3172 further maintains optimized performance
and efficiency by detecting when the input voltage is not
sufficient to sustain LED current. The device automati-
cally switches to 1.5X mode when the input voltage
drops too low in relation to the LED forward voltage.
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-current, pro-
grammable ideal constant current source.
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 efficiency is given by:
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.
VF · ILED
=
VIN · 1.5IIN 1.5 · VIN
VF
η =
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 out-
put voltage can be boosted to twice the input voltage.
The doubling conversion ratio introduces a correspond-
ing doubling of the input current. For ideal conversion,
the 2X mode efficiency is given by:
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. The power conversion efficiency can be
approximated by:
VF · ILED
VIN · 2IIN
VF
η =
=
2 · VIN
VF · ILED VF
≈
η =
VIN · IIN
VIN
Capacitor Selection
Due to the very low 1X mode quiescent current, the
input current nearly equals the current delivered to the
Careful selection of the four external capacitors (CIN, C1,
C2, and COUT) is important because they will affect turn-on
w w w . a n a l o g i c t e c h . c o m
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3172.2008.04.1.6
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
