Step-Up DC/DC Converters for
White LED Backlight Applications
Functional Block Diagram
AAT1231/1231-1
PVIN
LIN
VIN
OVP
EN/SET
FB
Output
Select
SW
Control
Reference
SEL
AGND
PGND
Functional Description
The AAT1231 and the AAT1231-1 consist of a
DC/DC boost controller, an integrated slew rate
controlled input disconnect MOSFET switch, and a
high voltage MOSFET power switch. A high voltage
rectifier, power inductor, output capacitor, and
sense resistors are required to implement a DC/DC
constant current boost converter. The input discon-
nect switch is activated when a valid input voltage
is present and the EN/SET pin is pulled high. The
slew rate control on the P-channel MOSFET
ensures minimal inrush current as the output volt-
age is charged to the input voltage, prior to the
switching of the N-channel power MOSFET.
Monotonic turn-on is guaranteed by the integrated
soft-start circuitry. Soft-start eliminates output volt-
age overshoot across the full input voltage range
and all loading conditions.
The maximum current through the LED string is set
by the ballast resistor and the feedback voltage of
the IC. The output current may be programmed by
adjusting the level of the feedback reference volt-
age which is programmed through the S
2
Cwire
interface. The SEL pin selects one of two feedback
voltage ranges. For the AAT1231 and with a LOW
logic level applied to the SEL pin, the FB pin volt-
age can be programmed from 0.1V to 0.4V. With a
logic HIGH applied to the SEL pin, the FB pin volt-
age can be programmed from 0.3V to 0.6V. In the
AAT1231-1, the SEL function is inverted in that the
FB pin voltage can be programmed from 0.4V to
0.1V with a logic LOW applied to the SEL pin and
0.6V to 0.3V with a logic HIGH applied to the SEL
1231.2007.01.1.2
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