AAT3171 View Datasheet(PDF) - Analog Technology Inc
Part Name
Description
MFG CO.
'AAT3171' PDF : 19 Pages View PDF
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
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 introduces negligible voltage drop from input
to output.
The AAT3171 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:
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 output voltage can be boosted to twice the input
voltage. The doubling conversion ratio introduces a
corresponding doubling of the input current. For ideal
conversion, the 2X mode efficiency is given by:
η=
VF · ILED
VIN · 2IIN
=
VF
2 · VIN
LED Selection
The AAT3171 is designed to drive high-intensity white
LEDs. It is particularly suitable for LEDs with an oper-
ating forward voltage in the range of 4.2V to 1.5V.
3171.2007.03.1.1
AAT3171
High Current, High Efficiency
Charge Pump with Auto-Timer
The charge pump device can also drive other loads
that have similar characteristics to white LEDs. For
various load types, the AAT3171 provides a high-cur-
rent, programmable ideal constant current source.
Capacitor Selection
Careful selection of the four external capacitors
CIN, C1, C2, and COUT is important because they will
affect turn-on time, output ripple, and transient per-
formance. Optimum performance will be obtained
when low equivalent series resistance (ESR)
ceramic capacitors are used. In general, low ESR
may be defined as less than 100mΩ. 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 recom-
mended over all other types of capacitors for use with
the AAT3171. Ceramic capacitors offer many advan-
tages over their tantalum and aluminum electrolytic
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.
Since ceramic capacitors are non-polarized, they are
not prone to incorrect connection damage.
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.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1µF are typically
made from NPO or C0G materials. NPO and C0G
materials generally have tight tolerance and are
very stable over temperature. Larger capacitor val-
ues are usually composed of X7R, X5R, Z5U, or
Y5V dielectric materials. Large ceramic capacitors
are often available in lower-cost dielectrics, but
capacitors greater than 4.7µF are not typically
required for AAT3171 applications.
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