High Efficiency 1.5X Fractional Charge
Pumps For White LED Applications
EN/SET Serial Interface
The current source output magnitude is controlled
by the EN/SET pin using AnalogicTech’s Simple
Serial Control (S
2
Cwire) interface. The interface
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 “IDIODE Response -9dB to -10dB” on page
5). The counter can be clocked at speeds up to
1Mhz, so intermediate states are not visible. The
first rising edge of EN/SET enables the IC and ini-
tially sets the output LED current to -31dB, the low-
est setting equal to 525µA. Once the final clock
cycle is input for the desired brightness level, the
EN/SET pin is held high to maintain the device out-
put 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
SS
). 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
OFF
timeout period.
AAT3113/4
EN/SET Timing
t
HI
t
OFF
t
LO
EN/SET
Code
OFF
1
2
3
OFF
LED Selection
The AAT3113/4 devices are designed to drive white
LEDs with forward voltages of less than 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.
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:
η=P
OUT
/P
IN
=(V
OUT
xI
OUT
)/(V
IN
x1.5I
OUT
)=V
OUT
/1.5V
IN
η(%)
= 100(V
OUT
/ 1.5V
IN
)
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 quies-
cent current consumption, the actual efficiency can
be measured at 93%. These figures are in close
agreement for output load conditions from 1mA to
100mA. Efficiency will decrease as load current
drops below 0.05mA or when level of V
IN
approach-
es V
OUT
. Refer to the Typical Characteristics section
for measured plots of efficiency versus input voltage
and output load current for the given charge pump
output voltage options.
9
Charge Pump Efficiency
The AAT3113/4 is a fractional charge pump. The
efficiency (η) can be simply defined as a linear volt-
age regulator with an effective output voltage that
is equal to one and a 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.
η
= P
OUT
/ P
IN
3113.2004.07.1.4
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
