AAT1239-1_0810 Datasheet, PDF (10/23 Pages) Advanced Analogic Technologies – 40V Step-Up Converter for 4 to 10 White LEDs

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PRODUCT DATASHEET
AAT1239-1
40V Step-Up Converter for 4 to 10 White LEDs
required inductor current in order to maintain the desired
LED current.
The switching cycle initiates when the N-channel MOSFET
is turned ON and current ramps up in the inductor. The
ON interval is terminated when the inductor current
reaches the programmed peak current level. During the
OFF interval, the input current decays until the lower
threshold, or zero inductor current, is reached. The lower
current is equal to the peak current minus a preset hys-
teresis threshold, which determines the inductor ripple
current. The peak current is adjusted by the controller
until the LED output current requirement is met.
The magnitude of the feedback error signal determines
the average input current. Therefore, the AAT1239-1
controller implements a programmed current source
connected to the output capacitor, parallel with the LED
string and ballast resistor. There is no right-half plane
zero, and loop stability is achieved with no additional
compensation components.
An increase in the feedback voltage (V_FB) results in an
increased error signal sensed across the ballast resistor
(R1). The controller responds by increasing the peak
inductor current, resulting in higher average current in
the inductor and LED string(s). Alternatively, when the
V_FB is reduced, the controller responds by decreasing the
peak inductor current, resulting in lower average current
in the inductor and LED string(s).
Under light load conditions, the inductor OFF interval
current goes below zero and the boost converter enters
discontinuous mode operation. Further reduction in the
load current results in a corresponding reduction in the
switching frequency. The AAT1239-1 provides pulsed
frequency operation which reduces switching losses and
maintains high efficiency under light load conditions.
Operating frequency varies with changes in the input volt-
age, output voltage, and inductor size. Once the boost
converter has reached continuous mode, further increases
in the LED current will not significantly change the operat-
ing frequency. A small 2.2μH (±20%) inductor is selected
to maintain high frequency switching (up to 2MHz) and
high efficiency operation for outputs up to 40V.
Soft Start / Enable
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 current as the output voltage is
charged to the input voltage, prior to switching of the
N-channel power MOSFET. Monotonic turn-on is guaran-
teed by the built-in soft-start circuitry. Soft start elimi-
nates output current overshoot across the full input volt-
age range and all loading conditions.
After the soft start sequence has terminated, the initial
LED current is determined by the internal, default FB
voltage across the external ballast resistor at the FB pin.
Additionally, the AAT1239-1 has been designed to offer
the system designer two choices for the default FB volt-
age based on the state of the SEL pin. Changing the LED
current from its initial default setting is easy by using the
S²Cwire single wire serial interface; the FB voltage can
be decreased (as in the AAT1239-1; see Table 2) relative
to the default FB voltage.
Current Limit and
Over-Temperature Protection
The switching of the N-channel MOSFET terminates when
a current limit of 2.5A (typical) is exceeded. This mini-
mizes power dissipation and component stresses under
overload and short-circuit conditions. Switching resumes
when the current decays below the current limit.
Thermal protection disables the AAT1239-1 when inter-
nal dissipation becomes excessive. Thermal protection
disables both MOSFETs. The junction over-temperature
threshold is 140°C with 15°C of temperature hysteresis.
The output voltage automatically recovers when the
over-temperature fault condition is removed.
Over-Voltage Protection
Over-voltage protection prevents damage to the
AAT1239-1 during open-circuit or high output voltage
conditions. An over-voltage event is defined as a condi-
tion where the voltage on the OVP pin exceeds the over-
voltage threshold limit (V_OVP = 1.2V typical). When the
voltage on the OVP pin has reached the threshold limit,
the converter stops switching and the output voltage
decays. Switching resumes when the voltage on the
OVP pin drops below the lower hysteresis limit, main-
taining an average output voltage between the upper
and lower OVP thresholds multiplied by the resistor
divider scaling factor.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN input.
Under-voltage lockout (UVLO) guarantees sufficient V_IN
bias and proper operation of all internal circuitry prior to
soft start.
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1239-1.2008.10.1.2