AAT2856 AAT, AAT2856 Datasheet - Page 15

• Manufacturer Part Number: AAT2856
• Description: High Current Charge Pump
• Manufacturer: AAT

Selection of set resistor values outside of the typical
application must be carefully evaluated to ensure
that the application's performance requirements
can still be met.
Device Power Efficiency
The AAT2856 power conversion efficiency depends
on the charge pump mode. By definition, device effi-
ciency is expressed as the output power delivered to
the LEDs divided by the total input power consumed.
When the input voltage is sufficiently greater than
the LED forward voltages, the device optimizes
efficiency by operating in 1X mode. In 1X mode,
the device is working as a bypass switch and pass-
ing the input supply directly to the output. By sim-
plifying the conditions such that the LEDs have uni-
form V
approximated by:
Due to the very low 1X mode quiescent current, the
input current nearly equals the total output current
delivered to the LEDs. Further, the low-resistance
bypass switch introduces negligible voltage drop
from input to output.
The AAT2856 further maintains optimized perform-
ance and efficiency by detecting when the input
voltage is not sufficient to sustain LED drive cur-
rent. The device automatically switches to 1.5X
2856.2007.06.1.0
Table 13: Example Output Voltages and
R2 Standard 1% Values (R1 = 120K)
F
, the power conversion efficiency can be
Corresponding Resistor Values
V
OUT
2.8
2.5
2.0
1.8
1.5
(V)
η =
V
V
η =
F
IN
· I
· I
P
LED
P
IN
OUT
IN
≈
V
V
IN
F
R2 (Ω)
79.6K
60.4K
30.1K
160K
130K
Dual LDO for BacklightApplications
mode when the input voltage drops too low in rela-
tion to the LED forward voltages.
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:
Similarly, when the input falls further, such that
1.5X mode can no longer sustain LED drive cur-
rent, 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 dou-
bling of the input current. For ideal conversion, the
2X mode efficiency is given by:
LED Selection
The AAT2856 is designed to drive high-intensity
white LEDs. It is particularly suitable for LEDs with
an operating forward voltage in the range of 1.5V to
4.2V.
The charge pump can also drive other loads that
have similar characteristics to white LEDs. For var-
ious load types, the AAT2856 provides a high-cur-
rent, programmable ideal constant current source.
Capacitor Selection
Careful selection of the four external capacitors
C
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.
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT2856. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
IN
High Current Charge Pump with
, C
1
, C
2
, and C
η =
η =
V
OUT
V
IN
V
V
F
IN
F
· 1.5I
· I
is important because they will
· I
· 2I
LED
LED
IN
IN
=
=
1.5 · V
2 · V
V
V
AAT2856
F
F
IN
IN
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