AAT3194 Datasheet, PDF (8/14 Pages) Advanced Analogic Technologies – High Efficiency 1.5X Fractional Charge Pump For White LED Applications

Functional Block Diagram
AAT3194
High Efficiency 1.5X Fractional Charge
Pump For White LED Applications
Soft Start
600kHz
Oscillator
Voltage
Reference
IN
1.5X
Charge
Pump
C1+
C1-
C2+
C2-
OUT
D1
EN/SET
S²Cwire
Interface
5
32x8 bit
ROM
8
Current
Mode
DAC
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 S²Cwire 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