AAT3194 Datasheet, PDF (10/14 Pages) Advanced Analogic Technologies – High Efficiency 1.5X Fractional Charge Pump For White LED Applications

AAT3194
High Efficiency 1.5X Fractional Charge
Pump 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 (S²Cwire) 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,
“I_DIODE Response -9dB to -10dB”). 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 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
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_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 (see Figure 1).
EN/SET
Code OFF
t_HI
t_LO
t_OFF
1
2
3
OFF
Figure 1: EN/SET Timing Diagram.
LED Selection
The AAT3194 is designed to drive white LEDs with
forward voltages to 4.2V. Since the D1:D4 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 AAT3194 uses 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 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:
η
=
P_OUT
P_IN
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 = ^POUT = ^{VOUT · IOUT} = ^VOUT
P_IN V_IN · 1.5I_OUT 1.5V_IN
-or-
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
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. Efficiency will decrease as load
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3194.2006.09.1.0