PRODUCT DATASHEET
AAT3195
TM
ChargePump
4-Channel Charge-Pump LED Driver
to the device for a given LED current drive level. The
best white LED driver for a given application should be
based on trade-offs of size, external component count,
reliability, operating range, and total energy usage...not
just % efficiency.
In addition, with an ideal 2x charge pump, the output
current may be expressed as 1/3 of the input current.
The expression to define the ideal efficiency (η) can be
rewritten as:
POUT VOUT · IOUT VOUT
The AAT3195 efficiency may be quantified under very
specific conditions and is dependent upon the input volt-
age versus the output voltage seen across the loads
applied to outputs D1 through D4 for a given constant
current setting. Depending on the combination of VIN and
voltages sensed at the current sinks, the device will
operate in load switch mode. When any one of the volt-
ages sensed at the current sinks nears dropout, the
device will operate in 2x charge pump mode. Each of
these modes will yield different efficiency values. Refer
to the following two sections for explanations for each
operational mode.
η =
=
=
PIN
VIN · 2IOUT 2VIN
-or-
VOUT
2VIN
η (%) = 100 ·
For a charge pump with an output of 5V and a nominal
input of 3.5V, the theoretical efficiency is 71%. Due to
internal switching losses and IC quiescent current con-
sumption, the actual efficiency can be measured at 51%.
These figures are in close agreement for output load
conditions from 1mA to 100mA. Efficiency will decrease
substantially as load current drops below 1mA or when
the voltage level at VIN approaches the voltage level at
1x Mode Efficiency
The AAT3195 1x mode is operational at all times and
functions alone to enhance device power conversion effi-
ciency when VIN is higher than the voltage across the
load. When in 1x mode, voltage conversion efficiency is
defined as output power divided by input power:
VOUT
.
Capacitor Selection
Careful selection of the three external capacitors CIN, CP,
and COUT is important because they will affect turn-on
time, output ripple, and transient performance. 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 all four capacitors is a good starting point when
choosing capacitors. If the constant current sinks are
only programmed for light current levels, then the
capacitor size may be decreased.
An expression for the ideal efficiency (η) in 1X charge-
pump mode can be expressed as:
POUT VOUT · IOUT VOUT
η =
=
≅
PIN
VIN · IOUT
VIN
-or-
VOUT
VIN
η (%) = 100 ·
Capacitor Characteristics
2x Charge Pump Mode Efficiency
Ceramic composition capacitors are highly recommended
over all other types of capacitors for use with the
AAT3195. 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 maximizes
charge pump transient response. Since ceramic capaci-
tors are non-polarized, they are not prone to incorrect
connection damage.
The AAT3195 contains a fractional charge pump which
will boost the input supply voltage in the event where VIN
is less than the voltage required to supply the output.
The efficiency (η) can be simply defined as a linear volt-
age regulator with an effective output voltage that is
equal to one and one half or two times the input voltage.
Efficiency (η) for an ideal 2x charge pump can typically
be expressed as the output power divided by the input
power.
POUT
PIN
η =
w w w . a n a l o g i c t e c h . c o m
12
3195.2009.06.1.3
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