AAT3170_08 Datasheet, PDF (12/15 Pages) Advanced Analogic Technologies – High Current LED Flash Driver Charge Pump IC

ChargePump^TM
PRODUCT DATASHEET
AAT3170
High Current LED Flash Driver Charge Pump IC
When enabling constant voltage mode, the LED channels
F1 and F2 are automatically disabled so current does not
flow through the flash LED during constant voltage
mode. Because of this, very little programming is
required. The user just addresses the voltage mode reg-
ister and programs it to voltage mode.
LED Selection
The AAT3170 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 AAT3170 provides a high-current, pro-
grammable ideal constant current source.
Device Power Efficiency
The AAT3170 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
=
P_OUT
P_IN
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
=
V_F · I_LED
V_IN · I_IN
≈
V_F
V_IN
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 AAT3170 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
=
V_F · I_LED
V_IN · 1.5I_IN
=
V_F
1.5 · V_IN
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=
V_F · I_LED
V_IN · 2I_IN
=
V_F
2 · V_IN
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 equiva-
lent series resistance (ESR) (<100mΩ) ceramic capaci-
tors 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.
Ceramic composition capacitors are highly recommended
over all other types of capacitors for use with the
AAT3170. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. 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 connec-
tions, size or area, material composition, and ambient
temperature. Capacitor ESR is typically measured in
milliohms for ceramic capacitors and can range to more
than several ohms for tantalum or aluminum electrolytic
capacitors.
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3170.2007.11.1.2