AAT3114AISN-20-T1 AAT [Advanced Analog Technology, Inc.], AAT3114AISN-20-T1 Datasheet - Page 10

• Manufacturer Part Number: AAT3114AISN-20-T1
• Description: High Efficiency 1.5X Fractional Charge Pumps For White LED Applications
• Manufacturer: AAT [Advanced Analog Technology, Inc.]

EN/SET Serial Interface
The current source output magnitude is controlled
by the EN/SET pin using AnalogicTech’s Simple
Serial Control (S
records rising edges of the EN/SET pin and
decodes them into 32 individual current level set-
tings each 1dB apart. Code 32 is full scale, and
Code 1 is full scale attenuated by 31dB. The mod-
ulo 32 interface wraps states back to state 1 after
the 32nd clock, so 1dB of attenuation is achieved by
clocking the EN/SET pin 31 times (see graph titled,
“I
be clocked at speeds up to 1MHz, so intermediate
states are not visible.
EN/SET enables the IC and initially sets the output
LED current to -31dB, the lowest setting equal to
525µA. Once the final clock cycle is input for the
desired brightness level, the EN/SET pin is held
LED Selection
AAT3113 and AAT3114/14A devices are designed
to drive white LEDs with forward voltages to 4.2V.
Since the D1:D6 output current sources are
matched with negligible voltage dependence, the
LED brightness will be matched regardless of their
forward voltage matching.
Charge Pump Efficiency
The AAT3113 and AAT3114/14A devices are frac-
tional charge pumps. The efficiency (η) can be sim-
ply defined as a linear voltage regulator with an
effective output voltage that is equal to one and one
half times the input voltage. Efficiency (η) for an
ideal 1.5X charge pump can typically be expressed
as the output power divided by the input power:
10
DIODE
Response -9dB to -10dB”). The counter can
2
Cwire) interface. The interface
η =
P
The first rising edge of
P
EN/SET
OUT
Code
IN
OFF
Figure 1: EN/SET Timing Diagram.
1
t
High Efficiency 1.5X Fractional Charge
LO
t
Pumps For White LED Applications
HI
2
high to maintain the device output current at the
programmed level. The device is disabled 500µs
after the EN/SET pin transitions to a logic low state.
The EN/SET timing is designed to accommodate a
wide range of data rates. After the first rising edge
of EN/SET, the charge pump is enabled and reach-
es full capacity after the soft-start time (T
During the soft-start time, multiple clock pulses
may be entered on the EN/SET pin to set the final
output current level with a single burst of clocks.
Alternatively, the EN/SET clock pulses may be
entered one at a time to gradually increase the
LED brightness over any desired time period. A
constant current is sourced as long as EN/SET
remains in a logic high state. The current source
outputs are switched off after EN/SET has
remained in a low state for at least the t
period (see Figure 1).
In addition, with an ideal 1.5X charge pump, the
output current may be expressed as 2/3 of the
input current. The expression to define the ideal
efficiency (η) can be rewritten as:
-or-
For a charge pump with an output of 5 volts and a
nominal input of 3.5 volts, the theoretical efficiency
is 95%. Due to internal switching losses and IC
quiescent current consumption, the actual efficien-
cy can be measured at 93%. These figures are in
close agreement for output load conditions from
1mA to 100mA.
3
η =
η(%) = 100
P
P
t
OUT
OFF
IN
=
Efficiency will decrease as load
OFF
V
V
IN
OUT
⎛
⎝
· 1.5I
1.5V
V
· I
OUT
OUT
OUT
IN
⎞
⎠
=
1.5V
V
OUT
3113.2006.07.1.9
OFF
IN
timeout
SS
).