AAT3174
High Current, High Efficiency Charge Pump
current. For ideal conversion, the 2X mode effi-
ciency is given by:
Device Power Efficiency
The AAT3174 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 · 2IIN
VF
η =
=
2 · VIN
POUT
PIN
LED Selection
η =
The AAT3174 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.
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,
The charge pump device can also drive other loads
that have similar characteristics to white LEDs. For
various load types, the AAT3174 provides a high-cur-
rent, programmable ideal constant current source.
Capacitor Selection
VF · ILED VF
≈
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.
η =
VIN · IIN
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 AAT3174 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.
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3174. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
trolytic 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 con-
nection damage.
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:
VF · ILED
VF
η =
=
VIN · 1.5IIN 1.5 · VIN
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-
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
12
3174.2006.05.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
