AAT3192/93
Low-Cost, 2-/3-Channel
Charge-Pump LED Drivers
Due to internal switching losses and IC quiescent cur-
rent consumption, the actual efficiency can be meas-
ured at 90ꢀ. Efficiency will decrease substantially as
load current drops below 1mA or when the voltage
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 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.
level at VIN approaches the voltage level at VOUT
.
Additional Applications
The current sinks of the AAT3192 and the AAT3193
can be combined to drive higher current levels
through a single LED. As an example, a single LED
can be driven at 60mA total by combining together
the D1-D3 outputs.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use with
the AAT3192/93. 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 foot-
print, and is non-polarized. Low ESR ceramic capaci-
tors help maximizing charge pump transient response.
Since ceramic capacitors are non-polarized, they are
not prone to incorrect connection damage.
C2
1μF
D1
1
2
3
4
5
10
9
8
7
6
C1
1μF
D1
D2
D3
OUT
C-
VIN
GND
C+ RSET
EN/SET
IN
R1
14.3k
C3
1μF
AAT3193
EN/SET
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 con-
nections, 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.
Figure 6: Higher Current, Single LED
Application.
For lower-cost applications, the flying capacitor can
be removed. This will force AAT3192/93 to operate
in 1X mode. To maintain regulated LED current, the
input supply voltage has to be higher than the
charge-pump's dropout voltage in 1X mode.
Ceramic Capacitor Materials
C2
1μF
D1
D2
D3
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 values are
usually composed of X7R, X5R, Z5U, or Y5V dielec-
tric materials. Large ceramic capacitors (i.e., larger
than 2.2μF) are often available in low cost Y5V and
Z5U dielectrics, but capacitors larger than 1μF are
not typically required for AAT3192/93 applications.
U1
1
2
3
4
5
10
9
8
7
6
D1
D2
D3
OUT
C-
VIN
GND
C+ RSET
IN EN/SET
R1
14.3k
C3
1μF
AAT3193
EN/SET
Capacitor area is another contributor to ESR.
Capacitors that are physically large will have a lower
ESR when compared to an equivalent material
smaller capacitor. These larger devices can improve
circuit transient response when compared to an
equal value capacitor in a smaller package size.
Figure 7: Lower Cost 1X Mode Application.
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 per-
14
3193.2007.05.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
