High Efficiency 1.5X Fractional Charge
Pump For White LED Applications
Functional Block Diagram
IN
AAT3194
Soft Start
600kHz
Oscillator
Voltage
Reference
1.5X
Charge
Pump
C1+
C1-
C2+
C2-
OUT
EN/SET
S
2
Cwire
Interface
5
32x8 bit
ROM
8
Current
Mode
DAC
D1
D2
D3
D4
GND
Functional Description
The AAT3194 is a high efficiency 1.5X fractional
charge pumps intended for white LED backlight
applications. The fractional charge pump consists
of a linear regulator followed by a 1.5X charge
pump. The AAT3194 requires only four external
components: two 1µF ceramic capacitors for the
charge pump flying capacitors (C1 and C2), one
1µF ceramic capacitor for C
IN
, and one 0.33µF to
1µF ceramic capacitor for C
OUT
. The charge pump
output is converted into four constant current out-
puts (D1 to D4) to drive four individual LEDs with a
maximum of 20mA each. The current source out-
put magnitude is controlled by the EN/SET serial
data S
2
Cwire interface. The interface records rising
edges of the EN/SET pin and decodes them into 32
individual current level settings each 1dB apart (see
Table 1, Current Level Settings). Code 32 is full
scale, and Code 1 is full scale attenuated by 31dB.
The modulo 32 interface wraps states back to state
1 after the 32nd clock. With each EN/SET pulse,
the output current increases by 1dB. To decrease
the output current by 1dB, 31 EN/SET clock pulses
are required. 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.
8
3194.2006.09.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
