AAT3172 View Datasheet(PDF) - Advanced Analogic Technologies
Part Name
Description
MFG CO.
'AAT3172' PDF : 15 Pages View PDF
ChargePumpTM
2.7V to 5.5V
EN/SET
PRODUCT DATASHEET
AAT3172
High Current LED Flash Driver Charge Pump IC
C1
C2
1µF
1µF
C1+ C1- C2+ C2-
VIN
VOUT
CIN
4.7µF
AAT 3172
EN/SET
F1
F2
GND
COUT
2.2µF
Flash
LED
STAT1
STAT2
Figure 2: Flash Plus Status Lighting.
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 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.
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.
η
=
POUT
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:
η
=
VF · ILED
VIN · IIN
≈
VF
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 intro-
duces negligible voltage drop from input to output.
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.
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:
η
=
VF · ILED
VIN · 1.5IIN
=
VF
1.5 · VIN
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:
η=
VF · ILED
VIN · 2IIN
=
VF
2 · VIN
Capacitor Selection
Careful selection of the four external capacitors (CIN, C1,
C2, and COUT) is important because they will affect turn-on
12
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3172.2008.04.1.6
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