PRODUCT DATASHEET
AAT1239-1
SwitchReg
TM
Functional Block Diagram
40V Step-Up Converter for 4 to 10 White LEDs
PVIN
LIN
VIN
OVP
EN/SET
SW
Control
FB
Reference
Output
Select
SEL
AGND
PGND
Functional Description
The AAT1239-1 consists 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 con-
stant current boost converter. The input disconnect
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 cur-
rent as the output voltage is charged to the input volt-
age, prior to the switching of the N-channel power
MOSFET. Monotonic turn-on is guaranteed by the inte-
grated soft-start circuitry. Soft-start eliminates output
voltage 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 voltage which is pro-
grammed through the S
2
Cwire interface. The SEL pin
selects one of two feedback voltage ranges. In the
AAT1239-1, 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 pin. The feedback volt-
age can be set to any one of 16 current levels within
each FB range, providing high-resolution control of the
LED current, using the single-wire S
2
Cwire control.
For some applications requiring a short duration of
boosting current applying a low-to-high transition on the
AAT1239-1’s SEL pin, LED current can be programmed
up to 3x. The step size is determined by the programmed
voltage at the FB pin where the internal default setting
is 1.5x in the AAT1239-1.
Control Loop
The AAT1239-1 provides the benefits of current mode
control with a simple hysteretic output current loop pro-
viding exceptional stability and fast response with mini-
mal design effort. The device maintains exceptional
constant current regulation, transient response, and
cycle-by-cycle current limit without additional compen-
sation components.
The AAT1239-1 modulates the power MOSFET switching
current to maintain the programmed FB voltage. This
allows the FB voltage loop to directly program the
1239-1.2008.10.1.2
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